Authors: Javier del Río, Noemi Díaz and Belén Gómez
What is cryopreservation?
The first successful in vitro fertilization (IVF) treatment was in 1978. Since that, there have been a remarkable number of advances in assisted reproductive technologies (ART). Initially, all available embryos were transferred in IVF treatments owing to its low success rate. However, improvements on clinical and laboratory aspects led not only to increased pregnancy rates, but also to increased risk of multiple pregnancies. To prevent this, fewer embryos are transferred and leftover embryos are cryopreserved for potential future cycles use (1).
The first pregnancy resulting from transferring a thawed cryopreserved human embryo was reported in 1983 in Australia (2), and the first live birth following embryo cryopreservation was reported in 1984 in The Netherlands (3). Subsequently, the need for an effective cryopreservation program arose from rapid development and improvements of assisted reproductive technology protocols (1).
Cryopreservation is a method that requires cells and embryos to be exposed to non-physiological ultra-low temperatures (from -20°C to -196°C) (Fig.2). It aims to achieve “cryogenic suspension of life” through multiple steps, although this puts the elements at risk of damage or “cryoinjury” during temperature changes and phase transitions. These damages could be chilling injury or ice crystal formation, for instance, as a result of the water exchange between the intra- and extracellular compartments, consequence of dramatic changes in osmotic potential (osmotic shock). Therefore, vitrification requires the use of cryoprotectants to avoid the formation of ice crystals in the cells. Two types of cryoprotectants are necessary: permeating and non-permeating. Mixing both at different relative concentrations reduces intracellular ice formation by removing water from inside the cell. Additionally, it creates an osmotic gradient that helps restrict water movement across the cell membrane, thereby preventing osmotic shock (4).
There are two typical methods used for cryopreservation: slow freezing and rapid freezing to achieve vitrification.
Vitrification is a term used to describe the transformation of a solution into glass by a dramatic increase in viscosity. This method requires to minimize the time for the sample to be exposed to temperature ranges associated with chilling injury and ice crystal formation. As slow freezing, vitrification causes cell dehydration using cryoprotectants. However, unlike that, there is no attempt to maintain equilibrium on both sides of cell membrane (4).
The time frame required to reach ultralow temperatures by vitrification is very brief, almost instantaneous. But, the main concern is the need for using high concentrations of cryoprotectant solutions. These might lead to osmotic shock and it could be toxic to cells, affecting embryo survival. Nevertheless, it is possible to limit toxicity by mixing different cryoprotectants, thereby decreasing their relative concentration and the exposure time of embryos to the solution (5).
How efficient is the vitrification?
This technique seems to be more attractive than slow freezing because it does not require expensive equipment. It uses small amount of liquid nitrogen and it is a simpler technique to perform once the embryologist has gained enough experience in it (6).
A recent research performed by Viladimoiv et al. suggests advantages arising from the freezing and thawing process; the authors hypothesize a theory about “cryo-treatment of the embryo”. According to these authors, as a result of freezing or thawing of the embryos there is a decrease in reactive oxygen species levels, in the rate of mitochondrial DNA mutation and cells detoxification is carried out. Also, the authors describe another mechanism involved in restoring the mitochondrial activity (“jumping effect”) which is part of the physiological process of implantation. However, current available data cannot confirm the hypothesis yet (7).
Advantages and disadvantages of fresh and frozen cycles
Nowadays, fresh embryo transfers (ET) are the most common choice in IVF cycles (8). Nevertheless, in the last years, controlled ovarian stimulation has increased the uncertainty on the possible adverse effects of the ovarian hyperstimulation syndrome (OHSS), and also on possible deleterious effects on the endometrium and implications in obstetric and perinatal results (9).
In spite of this, recent developments in cryopreservation of oocytes and embryos have led to substantial improvement in IVF outcomes. This resulted in a significant increase in the number of cycles with frozen embryo transfer (FET), which subsequently led to the enhancement of live births rate (10).
What are the advantages of a frozen cycle?
Ovarian hyperstimulation syndrome
The first strong argument for FET strategy is the prevention of OHSS, that results from an increase in vascular permeability (11,8). OHSS is a medical condition affecting the ovaries of some women who take fertility medication to stimulate oocyte growth. OHSS arguably remains a major cause of morbidity in IVF treatment (10).
During a fresh cycle, a woman has to undergo hormonal treatment to regulate her menstrual period, to stimulate the development of multiple oocytes (superovulation), and to encourage their maturation (11, 12). However, in a frozen cycle (FC) the patient does not have to go through ovarian stimulation or egg retrieval depending on their circumstances (13). Many people find that FETs are less stressful than fresh cycles because they do not have to worry about oocytes production or whether there will be viable embryos, since those procedures have already been done (9).
Deleterious effects on the embryo
The optimization of vitrification protocols has reduced the deleterious effects that this process may produce in embryos. Also, it have been observed similar survival and embryo development in FCs compared to fresh cycles (10). Moreover, best quality embryos, morphologywise, can be stored and transferred in a future cycle in better conditions. These data have allowed for an increment of success rates and the confidence of sanitary personnel and patients over FCs (5).
The implantation process, one of the crucial steps in the success of ART, requires a reciprocal interaction between the embryo and the endometrium during a small period of time called window of implantation. This interaction involves the embryo, along with its inherent molecular program of cell growth and differentiation, as well as differentiation of endometrial cells into an adequate uterine receptivity (11). Some patients may find easier to turn to FCs, since dealing with the whole process of medication during a normal cycle for ovarian stimulation may result psychologically and emotionally overwhelming. In this regard, FC may also provide a better outcome (3).
The importance of an adequate endometrial environment in ART is highlighted in those patients who resort to oocyte donation, where there must be a synchronization between donor and recipient in fresh cycles. Those cases that require an improvement in endometrial receptivity to stimulate implantation of these donor oocytes seem to obtain better results in frozen cycles or in the next fresh cycle (8).
Multiplet pregnancies are one of the major safety concerns of IVF due to the increased risk of neonatal and maternal complications. To achieve good results, to would be ideal to select the optimal single embryo to be transferred. Elective single embryo transfer (eSET) is the most effective way to reduce those risky pregnancies (14).
How can cryopreservation damage embryos?
Upon analyzing some ART studies and results, embryos are able to adapt and develop in a large range of culture media, showing different gene expression models in different environments. Cryopreservation causes stress in embryos and it is known as “hormesis”(5) (Fig.3).
However, if the conditions are too unfavorable or toxic, mitochondrial activity is suppressed below the threshold necessary for the development of the embryo, so that implantation in the endometrium will be affected (5).
Results of embryo transfer in fresh cycles vs. frozen cycles
The main current objective of IVF professionals is to improve pregnancy rates in both fresh and frozen-thawed cycles. It is clear that embryo and endometrial receptivity are important factors to promote pregnancy rate. Recently, many researches showed FET can enhance the embryo utilization rate and improve the success rate in contrast to other research lines (15).
In Roque et al. systematic meta‐analysis for 633 cycles in women aged 27-33 years old showed that FET resulted in a statistically significant increase in the ongoing pregnancy rate and clinical pregnancy compared with the fresh transfer group (8). Interestingly, the fresh group showed a higher miscarriage rate, but no statistical difference was found when compared with the frozen group. According to these data, it seems that the results of IVF-ICSI cycles can be improved by performing the FET especially in patients with normal or high follicular response. This advantage could be explained thanks to a more physiological preparation of endometrium. Several studies have also shown good results with cryopreservation of all embryos and subsequent FET in those patients most susceptible to develop OHSS (8, 16-19).
In contrast, Shavit et al. found lower rates of clinical pregnancy and live births in the vitrified-warmed blastocyst group. The difference in implantation and pregnancy rates could be attributed to a higher proportion of good-quality embryos in the fresh blastocysts transfer group. They suggest that in fresh cycles highest quality blastocyst is selected for transfer and the rest are usually vitrified. Thus, vitrified-warmed blastocysts may have slightly poorer grade after warming and prior to transfer (20).
In addition, it is necessary to take into account those cycles with frozen oocytes. Braga et al. found that warmed oocytes transferred in endometrial prepared cycles yield better clinical outcomes than fresh ETs. Indeed, they found that fertilization rate, embryo quality, and developmental competence was decreased in embryos derived from vitrified oocytes (12). Conversely, previous studies have suggested that the results of oocyte vitrification followed by ICSI are not inferior with regard to fertilization, embryo developmental competence, pregnancy rates, and live birth (21, 22, 23).
An interesting point found in Braga et al. research is that even with lower embryo developmental quality, warmed oocytes transferred in endometrial prepared cycles resulted in higher pregnancy and implantation rates compared with transfer in fresh cycles. This finding strongly suggests that controlled ovarian stimulation impacts endometrial receptivity, which may be a possible cause of implantation failure after ovarian stimulation (12). Indeed, some studies have suggested that pregnancy rate is inversely related to serum progesterone levels on the day of hCG administration (24-27). It has been demonstrated that elevated progesterone levels on hCG trigger day negatively influence the pregnancy, regardless of the oocyte quality. Raised concentrations of progesterone in the late follicular phase are likely to influence the secretory changes of the endometrium, leading to an asynchrony between embryo and endometrial dialogue, which may result in reduced implantation rate (12).
Another issue to consider is the obstetric and perinatal outcomes of frozen-thawed cycles. Maheshwari et al. quantified in a meta-analysis the obstetric and perinatal risks for singleton pregnancies after FET and compared it with those after fresh embryo transfer (28). They indicated better perinatal outcomes in singleton pregnancies after the transfer of frozen‐thawed embryos when compared to fresh IVF embryos. This could be explained by antepartum hemorrhage, very preterm birth (delivery at <32 weeks), preterm delivery (delivery at <37 weeks), small for gestational age, low birth weight (birth weight <2500 g), and perinatal mortality significantly lower in women who received frozen embryos than those transferred with fresh embryos (29, 28).
It is important to note that most studies comparing perinatal outcome of singleton births conceived after fresh and cryopreserved ETs included both single and multiple ETs. Therefore, part of the adverse perinatal outcome may be attributed to the vanishing twin phenomenon, which occurs in up to 10% of multiple ETs resulting in a singleton live birth (20).
What can we conclude?
Elective embryo cryopreservation followed by single FET has attracted increasing attention and has been regarded as a potential innovation of IVF treatment. Choosing the well-selected embryo could further increase the chance of live birth of a eSET, which is of high clinical significance. However, there are great gaps in the literature about the risk/benefit ratio of this strategy, which includes multiple steps of treatment (30).
The good outcomes in FC might be associated with having a well‐balanced embryo‐endometrium interaction in FC, and also with lacking controlled ovarian hyperstimulation, which may adversely affect endometrial receptivity during fresh IVF cycles. In addition, when hormone replacement cycles were applied in FETs, estrogen and progesterone were given in physiological doses to mimic natural cycles, while supraphysiological doses of gonadotropins were given in fresh cycles (31).
On the other hand, other authors find fresh cycles as the best choice, especially in patients who resort to oocyte donation. In fact, it seems that there is a higher proportion of good-quality embryos in fresh blastocysts compared to vitrified-warmed blastocysts, which may have slightly poorer grade after warming and prior to transfer. (8, 20).
In conclusion, each case must be individualized in relation to clinical characteristics of the patients and to oocyte, seminal and embryo quality. By doing so, results will be optimized in each cycle and the chances of achieving a live birth will be highly improved.
1. Wong KM, Mastenbroek S, Repping S. Cryopreservation of human embryos and its contribution to in vitro fertilization success rates. Fertil Steril. 2014;102(1):19-26.
2. Trounson A, Mohr L. Human pregnancy following cryopreservation, thawing and transfer of an eight-cell embryo. Nature. 1983;305(5936):707-9.
3. Zeilmaker GH, Alberda AT, van Gent I, Rijkmans CM, Drogendijk AC. Two pregnancies following transfer of intact frozen-thawed embryos. Fertil Steril. 1984; 42(2):293-6.
4. Sparks AE. Human embryo cryopreservation-methods, timing, and other considerations for optimizing an embryo cryopreservation program. Semin Reprod Med. 2015;33(2):128-44.
5. Konc J, Kanyó K, Kriston R, Somoskői B, Cseh S. Cryopreservation of embryos and oocytes in human assisted reproduction. Biomed Res Int. 2014;2014:307268.
6. Loutradi KE, Kolibianakis EM, Venetis CA, Papanikolaou EG, Pados G, Bontis I, et al. Cryopreservation of human embryos by vitrification or slow freezing: a systematic review and meta-analysis. Fertil Steril. 2008;90(1):186-93.
7. Vladimirov IK, Tacheva D, Diez A. Theory about the Embryo Cryo-Treatment. Reprod Med Biol. 2017;16:118–125.
8. Roque M, Lattes K, Serra S, Solá I, Geber S, Carreras R, Checa MA. Fresh embryo transfer versus frozen embryo transfer in in vitro fertilization cycles: a systematic review and meta-analysis. Fertil Steril. 2013;99(1):156-62.
9. Gurbuz AS, Gode F, Ozcimen N, Isik AZ.Gonadotrophin-releasing hormone agonist trigger and freeze-all strategy does not prevent severe ovarian hyperstimulation syndrome: a report of three cases. Reprod Biomed Online 2014;29:541-544.
10. Lattes K, Prat M, Robles A, Carreras R, Brassesco M, Checa MA. Ciclos de criopreservación y vitrificación de ovocitos y embriones: indicaciones y transferencia diferida. Guía 21 de Práctica Clínica de la SEF y de la SEGO.
11. Lessey BA. Endometrial receptivity and the window of implantation. Baillieres Best Pract Res Clin Obstet Gynaecol. 2000;14(5):775-88.
12. Braga D, Setti A, Figueira R, Azevedo M, Iaconelli A, Lo Turco E et al. Freeze-all, oocyte vitrification, or fresh embryo transfer? Lessons from an egg-sharing donation program. Fertil Steril. 2016;106(3):615-622.
13. Shapiro BS, Daneshmand ST, Garner FC, Aguirre M, Hudson C. Clinical rationale for cryopreservation of entire embryo cohorts in lieu of fresh transfer. Fertil Steril. 2014;102:3-9.
14. Tobias T, Sharara FI, Franasiak JM, Heiser PW, Pinckney-Clark E. Promoting the use of elective single embryo transfer in clinical practice. Fertil Res Pract. 2016;2(1):1-9.
15. Shen C, Shu D, Zhao X, Gao Y. Comparison of clinical outcomes between fresh embryo transfers and frozen-thawed embryo transfers. Iran J Reprod Med. 2014. Jun;12(6):409–14.
16. Griesinger G, von Otte S, Schroer A, Ludwig AK, Diedrich K, Al-Hasani S, et al. Elective cryopreservation of all pronuclear oocytes after GnRH agonist triggering of final oocyte maturation in patients at risk of developing OHSS: a prospective, observational proof-of-concept study. Hum Reprod. 2007;22(5):1348-1352.
17. D'Angelo A. Ovarian hyperstimulation syndrome prevention strategies: cryopreservation of all embryos. Semin Reprod Med. 2010;28(6):513-518.
18. Griesinger G, Schultz L, Bauer T, Broessner A, Frambach T, Kissler S. Ovarian hyperstimulation síndrome prevention by gonadotropin-releasing hormone agonist triggering of final oocyte maturation in a gonadotropin-releasing hormone antagonist protocol in combination with ‘‘freeze-all’’ strategy: a prospective multicentric study. Fertil Steril. 2011;95(6):2029-2033.
19. Devroey P, Polyzos NP, Blockeel C. An OHSS-Free Clinic by segmentation of IVF treatment. Hum Reprod. 2011;26(10):2593-2597.
20. Shavit T, Oron G, Weon-Young S, Holzer H, Tulandi T. Vitrified-warmed single-embryo transfers may be associated with increased maternal complications compared with fresh single-embryo transfers. Reprod Biomed Online. 2017;35(1):94-102.
21. Trokoudes KM, Pavlides C, Zhang X. Comparison outcome of fresh and vitri- fied donor oocytes in an egg-sharing donation program. Fertil Steril. 2011; 95:1996-2000.
22. Herrero L, Pareja S, Aragones M, Cobo A, Bronet F, Garcia-Velasco JA. Oocyte versus embryo vitrification for delayed embryo transfer: an observational study. Reprod Biomed Online. 2014;29:567-72.
23. Rienzi L, Romano S, Albricci L, Maggiulli R, Capalbo A, Baroni E, et al. Embryo development of fresh ‘versus’ vitrified metaphase II oocytes after ICSI: a prospective randomized sibling-oocyte study. Hum Reprod. 2010;25:66-73.
24. Xu, B., Li, Z., Zhang, H., Jin, L., Li, Y., Ai, J. et al, Serum progesterone level effects on the outcome of in vitro fertilization in patients with different ovarian response: an analysis of more than 10,000 cycles. Fertil Steril. 2012;97 (1321-7.e1-4).
25. Wu, Z., Li, R., Ma, Y., Deng, B., Zhang, X., Meng, Y. et al, Effect of HCG-day serum progesterone and oestradiol concentrations on pregnancy outcomes in GnRH agonist cycles. Reprod Biomed Online. 2012;24:511–520.
26. Bosch, E., Labarta, E., Crespo, J., Simon, C., Remohi, J., Jenkins, J. et al, Circulating progesterone levels and ongoing pregnancy rates in controlled ovarian stimulation cycles for in vitro fertilization: analysis of over 4000 cycles. Hum Reprod. 2010;25:2092–2100.
27. Hamdine, O., Macklon, N.S., Eijkemans, M.J., Laven, J.S., Cohlen, B.J., Verhoeff, A. et al, Elevated early follicular progesterone levels and in vitro fertilization outcomes: a prospective intervention study and meta-analysis. Fertil Steril. 2014;102:448–454.e1.
28. Maheshwari A, Pandey S, Shetty A, Hamilton M, Bhattacharya S. Obstetric and perinatal outcomes in singleton pregnancies resulting from the transfer of frozen thawed versus fresh embryos generated through in vitro fertilization treatment: a systematic review and meta-analysis. Fertil Steril. 2012;98:368–77.e1.
29. Qiao J, Zhang L, Yan L, Zhi X, Yan J. Female Fertility: Is it Safe to "Freeze?". Chin Med J (Engl). 2015;128(3):390.
30. Wei D, Sun Y, Liu J, Liang X, Zhu Y et al. Live birth after fresh versus frozen single blastocyst transfer (Frefro-blastocyst): study protocol for a randomized controlled trial. Trials 2017; 18(253): 1-7.
31. Zhang L, Yan LY, Zhi X, Yan J, Qiao J. Female Fertility: Is it Safe to “Freeze?” Chin Med J. 2015;128 (3):390-7.
Authors: Paula Brígido, Roberto de la Fuente and Javier Del Río
Assisted reproduction technology (ART) can help fertile couples to achieve successful pregnancies. Sometimes, reproductive desires of these couples are affected by the presence of a genetic disease in either partner. In such cases, couples are at a reproductive risk and find themselves in the need of assistance that only ART can provide.
Preimplantation genetic diagnosis (PGD) provides an alternative to prenatal diagnosis to detect the specific genetic condition or disease they suffer from, and allows them to avoid passing it on their offspring (2). It requires the analyses of the embryos generated by ART in the in vitro fertilization (IVF) laboratory, by means of accurate and sensitive methodologies such as embryo biopsy, genetics, single cell genomics and, of course, background on prenatal diagnosis and counselling from experts.
Clinical application of PGD dates back to the late 60’s, when blastocysts of research animals could be sexed (3) (note that this was already possible ten years before Louis Brown, the first IVF baby, was born in the UK in 1978). At the beginning of the 90’s, early human embryos were sexed before implantation and the first genetic analyses were performed to avoid children inheriting Mendelian diseases. By the end of the century, other nowadays considered basic genetic methodologies were routinely used for preimplantation diagnosis and PGD was applied as a normal procedure to guarantee healthy babies (4).
In the present post we aim to give an account of the importance of PGD and the current view of the main clinical approaches for its application.
WHEN IS PGD INDICATED?
Indications for PGD are multiple and emerge from different motivations. Firstly, the patient may have suffered from a number of terminations due to the embryo having inherited the genetic condition. It could also be motivated by the parents already having a child with a severe genetic disease. In this case they might be willing to avoid passing it on the next one or even looking for a suitable treatment, if possible. However, one of the parents (or both) may be worried about their family history, being aware of the presence of a specific genetic condition, regardless of the type of inheritance.
If the parents are carriers of any genetic disease, either an autosomal-dominant disorder like Huntington disease or an autosomal-recessive one like cystic fibrosis, they are at reproductive risk because the resulting embryo may be affected (the probability depending on the specific disorder itself and the way it is inherited) (see  for details on inherited conditions). But there are even cases in which motivation is not based on biological but in ethical or religious reasons. Certain families might have serious concerns about going on for abortion of an affected embryo. In such cases, application of PGD may circumvent this kind of ethical conflicts.
Broadly speaking, steps for PGD are as follows (2):
PGD vs. PGS
Preimplantation genetic screening (PGS) is the general term for a compound of approaches that aim to evaluate the genetic content of the cell, in contrast to genetic tests whose goals are to determine whether an embryo is affected by a specific genetic condition (PGD). Originally termed PGD-AS (preimplantation genetic diagnosis for aneuploidy screening), PGS was developed to confirm the ploidy status of the embryo, searching for possible aneuploidies. Available data suggest that most of miscarriages occurred during the first trimester are a consequence of some sort of aneuploidies (5), and that mainly selected chromosomes were involved in these structural abnormalities (6). Thus, the main approach developed for PGS was the fluorescence in situ hybridization (FISH) for such chromosomes.
Types of approaches for PGD in the laboratory
Current technical methodologies for preimplantational genetic analyses mainly lie in one of the following:
WHEN TO PERFORM BIOPSY
Typical biopsies for PGD (and PGS) are as described as follows:
DAY 3. CLEAVAGE STAGE BIOPSY
There is a controversy regarding utility of this type of biopsy. In the cleavage stage biopsy, embryos are biopsied at day 3 when individual cells can be differentiated. This technique entails aspiration of one to two blastomeres to obtain the embryonic genetic material for PGD analysis (13). Following genetic diagnosis, embryo transfer may be performed on blastocyst stage. Embryos are usually selected for biopsy based on morphological criteria. Unfortunately, these do not predict the development potential of the embryo, and so it could fail to progress until blastocyst stage. This would compromise the advantages of using the day-3 approach (14). On the other hand, performing biopsy on the cleavage stage allows embryos to be cultured in vitro until they reach the blastocyst. This means they can be fresh transferred (15), whereas embryos biopsied on day 5 must be vitrified and transferred in a subsequent cycle.
How many cells should be removed?
The number of cells to be removed in the biopsy is still a controversial issue. Aspirating one cell reduces the cellular mass extracted but it can imply the presence of mosaicism. Conversely, aspirating two cells can reduce the risk of mosaicism, but removing such cellular mass could have consequences on the implantation rate (14).
Reported data have shown a dramatic reduction of 39% in the implantation rate in cleavage stage biopsy (16). The authors related it with proportion of the embryo total cellular removed. Whereas around five cells pulled out of the embryo in the trophectoderm biopsy represent 2-3% of the total cell content (expanded blastocyst has 200-220 cells approximately), extraction of a single cell from an eight cell embryo supposes 13% of the total content (16).
What do experts say?
Cleavage stage biopsy produces different opinions among embryologists because of the presence of mosaicism and the possibility of self-correction of aneuploidies from cleavage to blastocyst stage (17). On the contrary, studies using array-comparative genomic hybridization (array-CGH) technology to analyse genetic abnormalities in day-3 blastomeres and confirming it in trophectoderm biopsy showed concordance between day 3 diagnosis and day 5 reanalysis; Treff and coauthors showed more reliable results for SNP-microarray (96% vs. 83%) and also a lower mosaicism degree (31%) for SNP-microarray samples in a study comparing array technology versus FISH technique (18). These data would support the suggestion of some authors, who proposed that the incidence of mosaicism may have been overestimated in previous studies due to technical inconsistency of the FISH technique (17, 18, 19). At present, this matter remains controversial.
Regarding pregnancy rates, in both types of biopsies higher pregnancy rates are obtained comparing with the control group, in which no biopsy was performed (14, 19).
To sum up:
DAY 5. TROPHECTODERM BIOPSY
The blastocyst stage is currently supposed to be an optimal time to perform biopsies for PGD/PGS. The combination of improved blastocyst culture, trophectoderm (TE) biopsy, refined cryopreservation techniques, and molecular assays, such as array comparative genomic hybridization that allows for 24-chromosome screening, have led to a renaissance of PGS. TE biopsy will not detect every circumstance in which the embryo is at risk of aneuploidy, but it will detect mosaicism more reliably than cleavage-stage biopsy (which cannot be relied on at all for this purpose) (20, 21).
Moreover, when diagnosing monogenic disorders in single blastomere cells using PCR-based protocols, there is a high risk of PCR failure due to either no amplification (allele dropout) or preferential amplification of one of the alleles, potentially resulting in a reduced number of unaffected embryos available for transfer. Increasing the amount of starting DNA template should in principle increase the sensitivity and reliability of genetic diagnosis. Therefore, the biopsy of multiple trophectoderm cells from the blastocyst rather than a single cell from cleavage stage embryos should potentially lead to improved PGD outcome for patients (14).
How many cells should be removed?
Research to determine the appropriate number of biopsied TE cells in blastocyst biopsies are limited. The exact number of biopsied TE cells is hard to count visually because cells are small and usually remain as a clump. In most studies using comparative genome hybridization or single-nucleotide polymorphism array technology for genetic testing, biopsied TE cells were used for genome amplification and their number was impossible to know. Moreover, some studies showed that removing four to five cells leads to better results. Therefore, the biopsied cell number should be higher in the blastocysts with better TE quality than those with worse characteristics (22, 23).
Can biopsies affect blastocyst development and its implantation?
Whereas it remains possible that biopsy of cleavage-stage embryos can critically arrest further development through reduction of cell mass, the low miscarriage rates and high term birth rates in the present series, as well as data presently under analysis, suggest that this is not the case for TE biopsy. It can be speculated that the damage to blastocyst development potential caused by TE biopsy would be less for blastocysts with a greater number of TE cells (21, 22).
Some experts assured that TE biopsy at the blastocyst stage had no meaningful impact on the developmental competence of the embryo as measured by implantation and delivery rates. This contrasts with the information above-mentioned on the significant reduction in the probability for an embryo to implant and progress up to delivery (16). When combined with TE biopsy and blastocyst vitrification, SNP microarray has resulted in high implantation and low miscarriage rates for some IVF patients (15, 16, 24).
Are there any limitations?
Owing to the limitations of genetic analysis, most of the biopsied blastocysts need to be cryopreserved by vitrification, and blastocysts with normal results would be transferred in the next frozen cycle. In addition, biopsy of numerous cells from blastocysts with grade B or C may cause damage to the embryo, leading to either its arrest or implantation failure. However, 1-5 cells may be the appropriate biopsied TE cell number to maintain the implantation potential (15, 22).
Also, the personnel experience of different embryologists is an influencing factor in this technique. The number of biopsied cells in the blastocyst biopsy is hard to quantify and largely dependent on the experience of embryologist (22).
To sum up:
WHAT CAN WE CONCLUDE?
The availability of new embryology and molecular techniques allow preimplantation genetic diagnosis laboratories to offer patients at genetic risk the transfer of developmentally competent embryos, unaffected by genetic disease. Cleavage stage biopsy allows for fresh embryo transfer after genetic diagnosis. However, there are reports of high levels of mosaicism when the biopsy is performed on day 3. Trophectoderm biopsy, in turn, provides sufficient material for an effective and more reliable diagnosis in embryos compared to those on cleavage stage. Moreover, it seems that it does not compromise embryo implantation and pregnancy rates in PGD cycles. The drawback for this option is the usual need for cryopreservation and transfer in a different cycle.
The offer of PGD in fertility centres has increased over the last decade, primarily due to the progress on the application of diagnostic methods. The choice for either development stage relates to successful outcomes in the clinic, which mainly depend on technical challenges and timing of the developing embryo. For the embryologists, both day-3 and day-5 approaches are supported by evidence, but it will be essential to consider every single aspect of them to evaluate the best option for the laboratory.
Authors: Shuyana Deba, Isabel Sánchez and Sara Sanz
Every procedure carried out in an IVF laboratory, from ovarian puncture and semen capacitation to the embryo transfer, must be performed under specific safety conditions. These standards must be followed to avoid a decrease in gamete/embryo viability (2).
From the in vitro culture, gametes and embryos are exposed to diverse artificial situations that do not take place in nature. In vivo, both fertilization and embryo development in the female reproductive tract occur in the complete absence of light. In this environment, other conditions include oxygen (O2) concentration of 2-8%, pH 7.2-7.4, 37⁰C and gradients of diverse nutrients to which the embryo is exposed (3). Changes in temperature, CO2 and O2 pressure, light exposure or volatile organic compounds may adversely affect embryo quality.
Also, manipulation of embryos by embryologists is as important as air quality and culture conditions. Each human being is covered by about 1012 bacteria (4), which could contaminate embryo cultures if embryologists do not use the necessary clothing, such as laboratory cap, footwear and mask.
Therefore, a daily quality control at different levels should be carried out to obtain good results in IVF cycles. We are going to analyze the effect of some of the elements that can affect germ cells and embryos.
In vivo, mammalian germ cells and embryos are not exposed to light, which might explain why they do not exhibit any protection mechanism against this factor (5,6). In addition, sperm do not have the capacity to repair DNA, unlike oocytes and embryos, which do present some mechanisms for DNA repair (7).
Light variables to be considered are intensity, duration and wavelength. It seems clear that photooxidation increases along with light intensity and duration. However, what type of wavelength would be the most harmful for embryos and germ cells? Energy increases when wavelength is shorter (8). Accordingly, artificial cool white fluorescent light has been demonstrated as the most stressful in mouse embryos. Incandescent light, in turn, seems to be less harmful, and the best outcomes are achieved when warm white fluorescent light is used (7).
How can light affect the quality of these cells?
Indirect effect: Culture and oil photooxidation can affect embryo development (8). In this case, modified components will damage the lipid membranes. Also, if HEPES- or riboflavin-containing media is exposed to light, it results in the formation of hydrogen peroxide, a highly cytotoxic substance (9). Additionally, light can heat up both the plasticware and the oil, resulting in more toxic and damaging components (8,9).
Direct effect: Light can potentially compromise the quality of gametes and embryos, by activating stress-related genes or by ionisation, which may also damage the DNA. This phenomenon would cause DNA fragmentation and mutation, as well as an increase in the apoptotic index and change in the number of mitochondria levels (10).
How can we avoid this effect? (7)
1) Reducing the exposure time.
2) Using warm white fluorescent light in the lab and green filters on microscopes.
3) Adding antioxidants in the media in order to mitigate damages from ROS.
4) Avoiding riboflavin, which is responsible of the phototoxicity in the media.
VOLATILE PARTICLES EFFECT
Since the 1990s, IVF laboratory indoor air quality has taken a high relevance. Thence, focused on creating an optimal environment, laboratories have become clean rooms where filtration of particles is performed by using high-efficiency particle arresting (HEPA) filters, and successful chemical air filtration is achieved by removing volatile organic compounds (VOCs) with solid-phase filtration (e.g., potassium permanganate-impregnated, carbon filters) (11).
Focusing on VOCs (hydrocarbon-based compounds that are emitted by industries, cleaning products, computers, and microscopes among others), several studies have demonstrated their harmful effect on embryos, initially reported by Boone laboratory on mouse embryo development (11). Moreover, VOCs have been shown to increase DNA fragmentation in human sperm, and they can also have detrimental effects on pregnancy rates (12).
Recently, a retrospective study by Munch et al. concluded that, without solid carbon filtration, fertilization, cleavage, and blastocyst conversion rates declined in fresh IVF cycles. Even more, results were found to be even worse in ICSI cycles, probably due to the lack of protective barrier provided by the cumulus cells (13). However, the authors did not observe the same results when embryos had been cryopreserved in an environment with carbon solid filtration but thawed in a laboratory deprived of such systems. The absence of significant changes in cleavage and blastocyst conversion rates, as well as in the proportion of good quality blastocyst developed after thawing suggests that embryos are affected in the peri-fertilization period (13).
Also, products like cosmetics emit VOCs, especially perfumes, colognes, and deodorants. They are highly toxic to embryo development in vitro, primarily due to evaporation of their solvent bases (14,15). After analyzing the results of studies determining the toxicity of VOCs, ideal levels should be below 0.2 ppm but preferably zero (12). Personnel must understand the principles of air quality control, including the function of airflows and airlocks, hygiene, dress code and the use of cleaning agents (16).
pH level depends on bicarbonate concentration of culture media and the CO2 concentration of the incubator. However, other factors like altitude and composition of culture media could affect the pH level, too (17,18). Embryos are able to develop over a range of media pH, considering that they possess an intracellular mechanism to regulate its internal level (17,18). However, it is important to control pH variations because they affect development (17). To control pH level outside the incubator some culture media contain buffers like HEPES or MOPS, but long exposure of embryos to these buffers is not recommended (17). Thawed denuded oocytes and embryos are specially sensitive to pH variations because they do not have an inner system to regulate pH (17). So, an increase in the pH of the medium can affect the physiology and development of oocytes and embryos. Thus, acidification of the medium can even affect the fetal weight and size (18).
As previously mentioned, CO2 is necessary to control pH level of culture media (17,18). The importance of CO2 was demonstrated in 1985, in a study carried out on hamsters (19). The authors cultured hamster embryos in different CO2 concentrations (5% and 10%). They found a higher rate of blastocysts in those cultured at 10% compared to 5%, which demonstrated differences in embryonic development. This results showed that CO2 level is an important factor for embryo culture (19). The capacity of CO2 to get through cell membranes allows for regulation of the inner pH levels in blastomeres. In other studies, it has been shown that the required CO2 concentration to achieve the optimal pH varies in different species. For instance, the required CO2 level in rats is 7.5%, whereas for humans it is 6.5% (19).
Some studies have compared different values of O2 concentration in the incubator and they show that a low level (5-6%) improves results when compared to an ambient level (21%). It has been shown that low O2 levels increase implantation, pregnancy and live birth rates (17,20). It seems that a low O2 level reduces ROS in the culture and the presence of volatile particles in the air, although the exact mechanism of action is still unknown (17).
Standard temperature generally used in IVF laboratories is 37⁰C (17,18). However, optimal temperature is unknown because in the female reproductive tract it could be slightly lower, about 36⁰C. On average, temperature of the Fallopian tube is about 1.5⁰C less, whereas the follicular liquid temperature can reach 2-3⁰C lower than core body temperature (17). It is important to control and prevent temperature variations because it can affect meiotic spindle stability and alter embryonic metabolism. It has been shown that an increase of 2⁰C during 20 minutes potentially alters the integrity of the meiotic spindle, which cannot be completely repaired when temperature is set back to 37⁰C. As a consequence of this increase in temperature, embryos express some stress-response genes that compromise development (18). Interestingly, a small decrease in temperature does not have any effect on oocytes, whereas a large difference can be severely harmful for the meiotic spindle (18).
CULTURE MEDIA EFFECT
Nowadays, there exist two kinds of culture media: one-step media and sequential media (with different compositions for days 0-3 and 3-6) (17,21). All culture media are similar in composition; they contain energy substrates like glucose, pyruvate or lactate, and both organic and inorganic salts, which must be balanced accordingly. Culture media also contain amino acids in different proportions. The exact composition of amino acids in culture media is unknown. One of the most important problems related to the presence of amino acids is the ammonium generated as a product of metabolism. Ammonium has negative effects on embryo and fetal development. To avoid this problem, some culture media contain glutamine, which reduces ammonium production (17,21,22). Also, culture media can be supplemented with macromolecules and other components like HSA, α and β globulins, growth factors, vitamins, lipids, nucleotides, cytokines and hormones (17,22).
What can we conclude?
There are many parameters that should be kept in mind in order to maintain the optimal conditions for both gamete and embryo development in an IVF laboratory. In vitro, cells and embryos are exposed to different stress situations that must be minimized. Therefore, a routine control at different levels needs to be performed, so that the environment in the laboratory is adapted to resemble the reproductive tract and the intrauterine conditions.
Authors: Paula Brígido, Shuyana Deba, Javier Del Río and Isabel Sánchez
Implantation is one of the key steps in human reproduction, and hatching of the blastocyst is a critical point in the sequence of physiological events that lead to implantation (2). It has been estimated that only 15-20% of embryo transfers culminate in implantation, and as a consequence clinical pregnancy and live birth rates are quite low (3, 4). The potential of an embryo to implant is related to its own nature, the quality of gametes and the endometrial receptivity. But failure in blastocyst hatching is also an important factor that prevents implantation (2, 3, 4). With the aim of solving this problem and in order to improve both implantation and pregnancy rates after Assisted Reproductive Technology (ART) procedures, scientists developed assisted hatching (AH) (2, 3, 4). AH consists of an artificial alteration of the zona pellucida (ZP) either by slimming or breakage. This technique was first described on 1988 by Cohen et al., who reported the first pregnancy after AH (4), and successive works have since shown its efficiency (2, 3).
Because AH does not seem to present clear advantages to all patients, it should not be applied, in principle, to all of them as a rule (3). Some studies have shown that AH has effectively improved ART outcomes in patients with bad prognosis, like those with a history of 2+ implantation failures, bad embryo quality, aged patients (38+), patient with frozen embryos or those who have oocytes with a thick ZP (2, 3, 4, 6).
ART have some actual consequences on gametes and embryos, one of them being hardening of the ZP by the use of certain culture media or by cryopreservation. These and other negative effects may hamper blastocyst hatching, which might be solved through AH. Overall, this approach might be useful considering the early implantation window in women treated with exogenous gonadotrophin stimulation, compared to a natural cycle. Additionally, benefits from AH include the possibility of ZP breakage improving the embryo-endometrium communication (5).
In order to help the embryo hatch through the ZP before implantation, a variety of techniques over the years have been developed (7). As a commonality for all of them, it is important to minimize the time of the embryo being outside the incubator and to optimize the methodologies to reduce both pH and temperature variations (3).
MECHANICAL ASSISTED HATCHING (PARTIAL ZONA DISSECTION)
This method does not require any new equipment, and it can be easily performed by an embryologist handling a micromanipulator (8). During this procedure, the embryo is held by the suction exerted by the holding pipette, while the microneedle is passed through the zona pellucida at the largest visible portion of the perivitelline space and introduced in a tangential trajectory to the embryo. Then, in order to tear the ZP apart, the embryo is released from the holding pipette and rubbed against it (7).
However, one of the main disadvantages of this technique is the difficulty of creating a hole of significant size without abruptly damaging the embryo. Moreover, it is not a fast procedure, which means embryos need to remain outside the incubator for a long time, and so this may affect development of the transferred embryos (7, 9).
ACID TYRODE'S ASSISTED HATCHING
For this procedure, first the embryo is stabilized with the holding pipette. Second, a pipette containing acid Tyrode is approached to the embryo, oriented at the 3 o`clock position, next to an area of empty perivitelline space. Finally, the acidic solution is gently expelled over a small area (~30 µm Ø) and immediately washed away (7). This technique allows the formation of a hole of significant size; nevertheless, the exposure to acid is problematic, since this compound may be potentially embryotoxic. Furthermore, this technique requires practice and expertise from the operator (7, 10).
Laser-assisted hatching (LAH) is designed for easy positioning of the embryos, focus and shooting. It can be executed with a single click of the mouse controller. The diameter of the drilled holes vary between 5 and 10 µm according to how many shots are performed and the irradiation time (7). From the technical point of view, this method is easier, better controlled and more precise. Without physical contact with other embryos, the procedure can be completed faster than other methods. So, time of the embryos out of the incubator is shorter than for other techniques (9).
Even though the equipment may result expensive, the laser-assisted technique seems to represent the lowest potential risk for the embryo, and it is relatively simple to perform with consistency between operators (3).
EFFICACY OF AH
Although it has been performed for more than 20 years, up to date results are still inconclusive. Ma and co-authors performed one of the first trials to determine the overall effect of this technique. They concluded that implantation rates could be enhanced by performing ICSI along with AH, but differences were not statistically significant (11).
Two recent meta-analyses evaluating potential benefits of this technique have reported significant heterogeneity among results (12, 13), suggesting that effects of AH may differ depending on specific patient features (14).
Most researchers support the hypothesis that this technique improves clinical pregnancy rates in patients with previous failed IVF cycles or poor prognosis. However, there is insufficient evidence to affirm that AH improves live-birth rates in these populations, and so it remains uncertain whether AH is beneficial to other patients (6, 14, 16, 17).
IS THERE AN INCREASE IN MULTIPLE GESTATION RATE?
Even though certain reports associate artificial manipulation of the zona pellucida with multiple pregnancies, there is actually insufficient evidence to support an increased risk of monozygotic twinning after AH. In fact, the overall rate of monozygotic twin pregnancy in IVF with AH is less than 1% (6, 16).
MISCARRIAGE AND CONGENITAL MALFORMATIONS RATES
It has been reported that this technique may enhance implantation of abnormal embryos. Thus, the lower live birth rate observed, which is related to the high number of pregnancies ending in early miscarriage, may be due to chromosomal abnormalities of the embryos (17). However, Ma et al. performed cytogenetic tests on miscarriaged embryos and umbilical cord blood from newborn infants, where they found a similar incidence of major congenital malformations in ICSI-born patients compared to the general population (11), thus ruling out any direct relationship of AH and miscarriage.
EFFECTS OF ASSISTED HATCHING ON FROZEN CYCLES
Data show that cryopreservation may induce zona hardening as well as advanced female age and in vitro culture conditions (18). The answer to this problem could be AH, but results obtained on implantation and pregnancy rates after AH for frozen embryo transfer cycles are controversial, as well as for fresh IVF cycles (19). These discrepancies may be attributed to the type of AH, the extent of ZP microdissection, the number of patients and criteria for their selection, or even the quality and stage of embryos selected for AH performance.
In an earlier study, Primi et al. (2004) were unable to show any specific advantage of LAH in cryopreserved embryos. In this study, no embryo selection was reported, so this could explain why the implantation rates observed were lower than those achieved when embryos were selected (20).
Ng et al. (2005) also found a negative effect of LAH on frozen-thawed embryos in their randomised study (implantation rates: 9.0% vs. 12.5%; pregnancy rates: 6.8% vs. 15%). Although a subgroup analysis showed a higher implantation rate, differences were not statistically significant when LAH was performed on embryos with zona thickness of 1.6 mm (21).
In contrast, other studies using similar techniques of LAH were able to show improvement in implantation and pregnancy rates in the LAH group. Such was the case in a study by Balaban and co-authors (2006), in which their data revealed that implantation rates (20.1% vs. 9.9%) and pregnancy rates (40.9% vs. 27.3%) were significantly higher in the group in which embryos had been subjected to LAH before transfer, as compared to those from the control group (22). Valojerdi et al. (2008) in turn showed that LAH increased significantly both implantation and pregnancy rates in embryo cryopreservation cycles (23). These conclusions resemble those by Kanyo and co-authors (2016), who compared clinical pregnancy rates after using LAH technique on day-3 frozen-thawed embryos, and found a higher pregnancy rate after applying the LAH procedure (24).
Reports on patient populations are found that show benefits from AH, which include patients whose embryos present a thick zona, with elevated FSH, over 38 years old and cryopreserved cycles. It seems that the laser-based method is the most used among professionals and the most effective, although differences between are found between studies. However, no significant data show any increase in clinical pregnancy rates that could be translated into an encouragement of this technique to be routinely performed.
It should also be noted that some studies do not include specific important confounding factors, such as patient socioeconomic status, parity, duration of infertility, number of repeated failed cycles, embryo quality and/or smoking and alcohol intake, which might cause residual variance in the obtained results. Additional limitations of the study include the lack of information on the type of assisted hatching (mechanical, chemical, or laser), which may have changed over time or between clinics.
To learn more about this topic, visit our ‘VIDEOS’ section, where you will find videos about mechanical, chemical and laser hatching.
Authors: Shuyana Deba, Javier Del Río, Isabel Sánchez and Sara Sanz
Fertilization is a sequence of coordinated events that results in the metabolic activation of the ootid (nearly mature oocyte) and triggers cleavage of the zygote (2).
Nowadays, in assisted reproduction laboratories cleavage can be evaluated in vitro and in real time. Once in vitro fertilization (IVF) has been accomplished, early development of the embryo can be recorded by using time-lapse systems (TLP) (3). This approach makes it possible to evaluate morphology, including dynamic parameters, based on the uninterrupted culture of the embryo, which also allows for a better embryo selection, thus rising pregnancy rates (4). Even so, there are still clinics all over the world that select embryos for transfer using light microscopy, which means the use of a conventional incubation method (5).
CRITERIA FOLLOWED FOR EMBRYO CLASSIFICATION
It is known that an international consensus is needed in the way embryos are assessed and described. The following standardized criteria is from Alpha Scientists in Reproductive Medicine and ESHRE Special Interest Group of Embryology, 2011 and includes ‘minimum standards’ for oocyte and embryo morphology scoring (6): the current expected observation for embryo development is 4 cells on day 2 and 8 cells on day 3 after fertilization (day 0). Moreover, embryos with <10% fragmentation, stage-specific cell size and not multinucleated are considered of good quality (6).
According to this consensus, scoring for day 4 (morula stage) regards as good embryos those that enter into a fourth round of cleavage, which implies evidences of compaction that virtually involve the whole volume of the embryo (6). Finally, on day 5 blastocysts are to be observed expanded with: a prominent inner cell mass (ICM) consisting of many cells, compacted and tightly adhered together; and a trophectoderm (TE), forming a cohesive epithelium (6).
Nevertheless, these parameters do not restrict laboratories to annotate further observations in order to select the best embryo for transfer (6).
BEST DAY TO PERFORM EMBRYO TRANSFER
One of the most important aspects that influence the success of ART is embryo transfer from the culture medium to the uterus (7). This has been a controversial subject that still generates quite some doubts. Morphological evaluation of embryos is sometimes a subjective process, and it provides limited information on the possible genetic abnormalities that embryos may have (8). Currently, there exists a great controversy on the optimal moment to carry out embryo transfer.
IN WHICH CASE DOES THIS TRANSFER USUALLY TAKE PLACE?
Day 2 transfer is usually indicated in cases of poorly responding patients. Indeed, it is also indicated when the sperm, oocyte and/or embryos are also of low quality and/or number (9, 10, 11).
WHAT DO EXPERTS SAY?
Several retrospective studies have compared embryo transfer on day 2, day 3 and day 5 after oocyte recovery, all of which presented conflicting results. A study performed by Mahdavi et al. among poor responder patients revealed no clinical differences between day-2 and -3 embryo transfer (10). However, this study found that pregnancy rates per oocyte retrieval and embryo transfer were significantly higher in the day-2 embryo transfer group compared to day 3 group. It is worth mentioning that other investigators did not find significant differences in pregnancy outcomes when they compared embryo transfer on day 2 and day 3 (11, 12).
Additional results from other studies have revealed higher clinical and ongoing pregnancy rates after embryo transfer on day 2 than on day 3 in poor responders. This suggests that the occurrence of miscarriage can be reduced by restricting embryo culture to only 2 days, which could also provide an alternative for managing poorly responding patients (11). That is the reason why embryo transfer on day 2 is still performed in many IVF centres; there is an actual risk of compromising the viability of embryos by prolonged in vitro culture in sub-optimal conditions, with an increased risk of obtaining no blastocysts to replace on day 5 (9, 13, 14).
Even though there seems to be a large number of benefits for these patients, certain disadvantages that may potentially occur must also be taken into account, as it can be seen below (Table 1).
PATIENTS IN WHICH TRANSFER D+3 SHOULD BE PERFORMED
There exists no criterion to select patients who should be transferred on D+3. Traditionally, embryo transfer has been performed on cleavage stage, so the chosen day was D+3 of embryo development (7). Generally speaking, embryo transfer was carried out on this day in all patients, until a culture medium was developed that allowed to keep embryos in the laboratory for 5-6 days, and with the exception of the cases previously mentioned (11).
SCIENTIFIC LITERATURE TO SUPPORT D+3 AS THE BEST DAY FOR EMBRYO TRANSFER
Many studies show contradictory results on whether it is better to transfer on D+2 or D+3. However, there are no significant differences as for implantation, clinical pregnancy or live birth rates when comparing transfer on these days. A study by Modares et al. (15) with patients under 40 years old showed a slight improvement in these results when transferring on D+3, although differences were not significant. The authors also showed embryo quality to be worse when the transfer was performed on D+3 than on D+2. Thus, implantation rate has been observed to be higher in D+3, because extending embryo culture for one day allows to discard those embryos that stop their development from D+2 to D+3 (16, 17).
Furthermore, it is necessary to consider that there are other external factors that affect embryo development and, consequently, the selection of the best day to transfer. Quinn et al. (18) determined that one of these factors is culture media. Thus, in sub-optimal lab conditions, it would be interesting to transfer on D+2 rather than D+3, in order to spend the shorter time possible in the media.
Regarding D+5 transfer, some studies have shown higher implantation rates in embryos transferred on the blastocyst stage compared to those transferred on D+3 (cleavage stage). However, it is necessary to consider that only 25% of embryos reach the blastocyst stage (15); this implies that the number of embryos transferred and vitrified in a cycle is lower than for D+2 and D+3. As a consequence, when considering cumulative pregnancy rates no significant differences are found between transferring on cleavage stage and blastocyst (7).
Again, benefits for the patients must be considered along with potential disadvantages (Table 2).
It has been observed that transfer on blastocyst stage helps to improve pregnancy rates and reduce the risk of a multiple pregnancy. Why? One reason might be that there is no method to determine whether embryos that initially seem to be of good quality are likely to develop up to blastocyst (19).
WHO ARE THE IDEAL PATIENTS?
1. Those with a large number of embryos (20).
2. Those whose day-3 embryos are of good quality (20).
3. Those in which day-1 embryos exhibit pro-nuclei and present a grading profile (20).
4. Young women with good ovarian response (21).
5. Those whose embryos display an early cleavage (22).
POTENTIAL BENEFITS OF BLASTOCYST-STAGE TRANSFER vs. CLEAVAGE-STAGE TRANSFER
First of all, the new culture media allow us to perform longer incubations in the laboratory, after which the best embryos can be selected with higher accuracy and with lower risk of aneuploidies (23). Moreover, there will exist a better synchronization between the embryo and the mother. Additionally, uterine contractility decreases during the luteal phase (24, 25). The size of these blastocysts is bigger, so some studies have found fewer cases of ectopic pregnancies in comparison to transfers on day 3 (26).
A parallel comparison of benefits vs. disadvantages for this procedure can be seen in below (Table 3).
IS IT BETTER TO TRANSFER ON DAY 5 OR ON DAY 6?
The optimal time for embryo transfer depends on a variety of factors, such as the embryo growth speed. However some studies have revealed both implantation and pregnancy rates to be more successful when embryos are transferred on day 5 compared to day 6. This is due to the fact that viability of embryos expanded on day 5 is higher than for those expanded on day 6 (30).
In conclusion, it seems difficult to define the most appropriate day for embryo transfer to be applied for each patient. Therefore, every single case must be individually analyzed. In addition, several factors should be taken into account when deciding on the day for embryo transfer, such as maternal age, sperm and oocyte quality or physiological response of the woman and/or the available embryos. By doing so, a good decision can always be made in order to improve implantation and pregnancy rates.
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