Human cleavage-stage embryo vitrification is comparable to slow-rate cryopreservation in cycles of assisted reproduction

Vitrification of human blastocysts for couples undergoing assisted reproduction: an updated review

Over the past 40 years there has been a worldwide critical change in the field of assisted reproduction technology (ART), leading to the increased application of single blastocyst transfer, which is extremely important to avoid the risks of multiple pregnancy and associated complications for both mother and babies. Indeed, advancements in ART over the last few decades have been obtained thanks to several improvements, including ovarian stimulation, embryo culture conditions and, of course, progress in cryopreservation methods, especially with the application of vitrification. The ability to cryopreserve human embryos has improved significantly with vitrification compared to the initially adopted slow-freezing procedures. Since the introduction of vitrification, it has become the gold standard method to effectively cryopreserve human blastocysts. However, some new protocols are now being explored, such as the short warming procedure and even shorter exposure to the equilibration solution before vitrification, which seem to provide optimal results. Therefore, the main aim of the current narrative review, will be to illustrate the benefit of vitrification as an effective method to cryopreserve the human blastocyst and to illustrate new protocols and variations which in future may increase the performance of vitrification protocols.

Vitrification yields higher cryo-survival rate than slow freezing in biopsied bovine in vitro produced blastocysts

Vitrification and slow freezing are the two commonly used embryo cryopreservation methods. In most studies, vitrification of intact embryos has proven superior in several respects, including cell and embryo survival and pregnancy rate. However, there is a lack of data for comparing these two methods in in vitro produced (IVP) bovine blastocysts, which have been subjected to the retrieval of trophectoderm (TE) biopsy. Day 7 IVP blastocysts were pooled and randomized into four groups: 1) non-biopsy (NB), 2) biopsy (B), 3) biopsy-vitrification (BV), 4) biopsy-slow freeze (BSF). The blastocysts in the B, BV, and BSF groups were subjected to TE biopsy. For the B group, this was followed by 5 hours (h) incubation and subsequent scoring of the biopsy-survival (re-expansion) rate before processing for further analyses. For the BV and BSF groups, the biopsy procedure was followed by 2 h incubation, allowing for a quick re-expansion, after which the blastocysts were subjected to vitrification and slow freezing, respectively. After warming and thawing, respectively, they were then incubated for 5 h followed by scoring the cryo-survival (re-expansion) rates before processing for further analyses. These included quantification of ICM and TE cells, cleaved caspase-3- and TUNEL-positive cells, quantitative PCR on cellular stress markers (SOD1 and PRDX1), and ultrastructural analysis. The biopsy-survival rate in the B group was 94% (307/326). The cryo-survival rate in BV (86%, 138/161) was higher than that in BSF (57%, 81/142; P < 0.001). No differences were noted between the average ICM, TE, and total cell numbers of the groups. The percentages of cleaved caspase-3-positive cells were higher in BV vs. NB (P < 0.05), in BSF vs. NB (P < 0.001), and in BSF vs. B (P < 0.001). The percentages of TUNEL-positive cells were higher in BV vs. NB (P < 0.05) and in BSF vs. NB (P < 0.001). The levels of mRNA abundance for SOD1 and PRDX1 in B, BV, and BSF were not different from that in NB. The ultrastructural analysis of blastocysts in the BV and BSF groups showed distension of extracellular spaces and appearance of intracellular vacuoles in the ICM, distension of mitochondria, and disorganization of mitochondrial cristae in both ICM and TE, and weakened tight junctions between adjacent TE cells. In summary, our findings demonstrate that vitrification yields a higher cryo-survival rate than slow freezing in biopsied bovine IVP blastocysts. However, biopsy-vitrification and biopsy-slow-freeze values are comparable in terms of ICM, TE, and total blastocyst cell numbers, as well as cleaved caspase-3- and TUNEL-positive cell rates. Moreover, biopsy and cryopreservation performed alone had no effect on ICM, TE, total blastocyst cell numbers, or TUNEL-positive cell rates. Biopsy and vitrification performed alone had no effect on the cleaved caspase-3 positive cell rates, whereas slow freezing resulted in an increased rate. Furthermore, double traumatization with a combination of biopsy and cryopreservation, either vitrification or slow freezing, resulted in increased rates of cleaved caspase-3- and TUNEL-positive cells.

A monocentric analysis of the efficacy of extracellular cryoprotectants in unfrozen solutions for cleavage stage embryos

BACKGROUND

In the absence of international guidelines indicating the usage of vitrification rather than slow-freezing, the study aim was to analyze a large cohort of slow-frozen/thawed embryos to produce a rationale supporting the standardization of IVF cryopreservation policy.

METHODS

This retrospective analysis included 4779 cleavage stage embryos cryopreserved by slow-freezing/thawing from September 2009 to April 2017 at a single Center. Biological and clinical outcomes of three different commercial kits adopted sequentially, i.e. Vitrolife Cleave Kit® from Vitrolife (kit 1) vs. K-SICS-5000 Kit® and K-SITS-5000 Kit® from Cook Medical (kit 2) and Freeze/Thaw 1™ Kit® from Vitrolife (kit 3) were collected and compared in the light of cryoprotectants composition.

RESULTS

Kit 3 compared to kit 1 and kit 2 showed significantly (P < 0.001) higher embryo survival (79.9% vs. 75.6 and 68.1%, respectively) and frozen embryo replacement (91.5% vs. 86.5 and 83.3%, respectively) rates, and significantly (P < 0.001) lower blastomere degeneration rate (41.5% vs. 43.6 and 52.4%, respectively). No significant difference for clinical outcomes was observed among kits. Only a slight positive trend was observed for kit 3 vs. kit 1 and kit 2 on delivery rate per thawing cycle (7.12% vs. 4.19 and 4.51%, respectively; P < 0.058) and live birth rate (3.07% vs. 2.59 and 1.93%, respectively, P < 0.069). Thawing solutions of kit 3 were similar to those of any warming protocol.

CONCLUSIONS

A defined concentration of extracellular cryoprotectants in thawing/warming solutions had a beneficial effect on the embryo cryosurvival rate. Results could provide the rationale for the adoption of a single standardized warming protocol.

Effect of vitrification on in vitro development and imprinted gene Grb10 in mouse embryos

Vitrification of embryos is a routine procedure in IVF (in vitro fertilization) laboratories. In the present study, we aimed to investigate the effect of vitrification on mouse preimplantation embryo development in vitro, and effect on the epigenetic status of imprinted gene Grb10 in mouse embryos. The blastocyst formation rate for vitrified 8-cell embryos was similar to the non-vitrified 8-cell embryos, whereas the blastocyst hatching rate was lower than that of the non-vitrified group. The expression level of Grb10 major-type transcript decreased significantly in vitrified blastocysts compared with non-vitrified and in vivo blastocysts. Moreover, the global DNA methylation level in 8-cell embryos and blastocysts, and the DNA methylation at CpG island 1 (CGI1) of Grb10 in blastocysts were also significantly decreased after vitrification. In vitro culture condition had no adverse effect, except for on the DNA methylation in Grb10 CGI1. These results suggest that vitrification may reduce the in vitro development of mouse 8-cell embryos and affect the expression and DNA methylation of imprinted gene Grb10.

The Effect of Vitrification on Expression and Histone Marks of Igf2 and Oct4 in Blastocysts Cultured from Two-Cell Mouse Embryos

OBJECTIVES

Vitrification is increasingly used in assisted reproductive technology (ART) laboratories worldwide. In this study the effect of vitrification on the expression and modifications of H3 histones of Igf2 and Oct4 was investigated in blastocysts cultured from vitrified and non-vitrified two-cell embryos.

MATERIALS AND METHODS

In this experimental study, two-cell embryos were cultured in KSOM medium to reach the blastocyst stage. Expression of Igf2 and Oct4 and modifications of H3 histones in regulatory regions of both genes were compared with in vivo blastocysts, which comprise the control group. To gene expression evaluation, reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and the ChIP assay method were carried out to assess expression and histone modifications of the two genes.

RESULTS

The expression level of Igf2 was significantly higher in both experimental groups than the control group. In the regulatory region of Igf2, H3K9 methylation decreased whereas H3K9 acetylation increased in the experimental group compared with the control group. In contrast, the expression level of Oct4 was significantly lower in experimental groups. The Oct4 gene promoter showed a significant increase in H3K9 methylation and decrease in H3K9 acetylation (P<0.05).

CONCLUSIONS

According to our results, both vitrification and cultivation conditions may lead to changes in expression level and modification of histones in Igf2 and Oct4. However, these effects were the same in vitrified and non-vitrified groups. Indeed, the embryo is most affected by culture environment and in vitro culture. Therefore, vitrification may be used as a low-risk technique for embryo cryopreservation in ART.

Effect of day 3 embryo morphometrics and morphokinetics on survival and implantation after slow freezing-thawing and after vitrification-warming: a retrospective cohort study

BACKGROUND

Morphometric and morphokinetic evaluation of in vitro cultured human embryos allows evaluation without time restriction and reduces intra- and inter-observer variability. Even though these technologies have been reported to improve the quality of cleavage stage embryo evaluation during fresh culture, possible advantages in the evaluation of cryopreserved embryos have been scarcely explored. This study aims to compare morphometric and morphokinetic parameters between slow frozen and vitrified embryos and to determine their relationship to embryo survival and implantation rate (IR) after thawing/warming.

METHODS

During fresh culture, morphometric characteristics (Total Cell Volume (TCV), symmetry, fragmentation and number of blastomeres) were measured in 286 thawed/warmed embryos. Likewise, after thawing/warming, similar morphometric characteristics were measured in 135 survived embryos. Moreover, morphokinetic parameters (time to mitosis resumption and time to compaction) were measured in 90 embryos after thawing/warming. Then, using linear regression, we investigated the differences between vitrified and slow frozen embryos and the relation of the measured characteristics to embryo survival and IR. Statistical corrections were applied to account for data clustering and for multiple testing.

RESULTS

Vitrified embryos resume mitosis and start compaction significantly earlier than slow frozen embryos. Mitosis resumption rate was 82% for vitrified and 63% for slow frozen embryos and median time to mitosis resumption was 7.6 h and 13.1 h (p = 0.02), respectively. Compaction rate was 62% in vitrified and only 23% in slow frozen embryos. Median time to compaction was 18.1 h for vitrified embryos but, for slow frozen could not be computed since less than half of the slow frozen embryos reached compaction (p = 0.0001). Moreover, intact embryos resume mitosis significantly earlier than not intact ones regardless of the freezing method (rate: 79% vs. 66%, median time: 7.6 h vs 14.6 h, respectively, p = 0.03). Regarding morphometrics, slow frozen embryos showed lower TCV and higher blastomere symmetry after thawing than vitrified embryos despite having similar blastomere number. IR was related to blastomere number at cryopreservation in slow frozen embryos, but not in vitrified ones.

CONCLUSIONS

Interestingly, vitrified/warmed embryos undergo mitosis resumption and compaction significantly earlier than slow frozen/thawed embryos. However, the clinical use of this morphokinetic parameters still remains to be investigated in larger studies.

TRIAL REGISTRATION

Retrospectively registered on December 15, 2015 NCT02639715 .

Comparison of pregnancy outcomes after vitrification at the cleavage and blastocyst stage: a meta-analysis

OBJECTIVE

This systematic review sought to evaluate the clinical outcomes of vitrification at the cleavage stage and blastocyst stage for embryo transfer in patients undergoing assisted reproductive technology (ART) treatment.

METHODS

We searched for related comparative studies published in the PubMed, EMBASE, and Cochrane Library databases up to July 2017. The primary outcomes were clinical pregnancy rate (CPR) and embryo implantation rate (IR). Secondary outcomes were multiple pregnancy rate (MPR), miscarriage rate (MR), live birth rate (LBR), and ongoing pregnancy rate (OPR). The Mantel-Haenszel fixed effects model and random effects model were used to analyze the summary risks ratios (RRs) with 95% confidence intervals (CIs).

RESULTS

Eight studies with more than 6590 cycles were included in our meta-analysis. Seven studies were observational retrospective comparative studies. One was a prospective study. Overall, the current study summarizes information from 6590 vitrification warming cycles (cleavage stage n = 4594; blastocysts n = 1996). There was no difference in the primary outcome clinical pregnancy rate (RR = 0.97, 95% CI = 0.90-1.04; fixed effects model; I ²  = 21%), whereas vitrified blastocyst transfer was significantly superior to vitrified cleavage-stage embryo transfer regarding the implantation rate (RR = 0.85, 95% CI = 0.74-0.97; random effects model; I ²  = 43). Regarding the secondary outcomes, no differences were found in the multiple pregnancy rate (RR = 1.20, 95% CI = 0.79-1.82; fixed effects model; I ²  = 22), live birth rate (RR = 1.07, 95% CI = 0.98-1.16; fixed effects model; I ²  = 0), and ongoing pregnancy rate (RR = 1.01, 95% CI = 0.92-1.120; fixed effects model; I ²  = 0), whereas a higher miscarriage rate was observed with vitrified blastocyst transfer (RR = 0.65, 95% CI = 0.45-0.93; random effects model; I ²  = 23).

CONCLUSION

In summary, this meta-analysis shows that vitrification at any stage has no detrimental effect on clinical outcome. Blastocyst transfer will still remain a favorable and promising option in ART. Due to the small sample evaluated in the pool of included studies, large-scale, prospective, and randomized controlled trials are required to determine if these small effects are clinically relevant.

Oocyte, embryo and blastocyst cryopreservation in ART: systematic review and meta-analysis comparing slow-freezing versus vitrification to produce evidence for the development of global guidance

BACKGROUND

Successful cryopreservation of oocytes and embryos is essential not only to maximize the safety and efficacy of ovarian stimulation cycles in an IVF treatment, but also to enable fertility preservation. Two cryopreservation methods are routinely used: slow-freezing or vitrification. Slow-freezing allows for freezing to occur at a sufficiently slow rate to permit adequate cellular dehydration while minimizing intracellular ice formation. Vitrification allows the solidification of the cell(s) and of the extracellular milieu into a glass-like state without the formation of ice.

OBJECTIVE AND RATIONALE

The objective of our study was to provide a systematic review and meta-analysis of clinical outcomes following slow-freezing/thawing versus vitrification/warming of oocytes and embryos and to inform the development of World Health Organization guidance on the most effective cryopreservation method.

SEARCH METHODS

A Medline search was performed from 1966 to 1 August 2016 using the following search terms: (Oocyte(s) [tiab] OR (Pronuclear[tiab] OR Embryo[tiab] OR Blastocyst[tiab]) AND (vitrification[tiab] OR freezing[tiab] OR freeze[tiab]) AND (pregnancy[tiab] OR birth[tiab] OR clinical[tiab]). Queries were limited to those involving humans. RCTs and cohort studies that were published in full-length were considered eligible. Each reference was reviewed for relevance and only primary evidence and relevant articles from the bibliographies of included articles were considered. References were included if they reported cryosurvival rate, clinical pregnancy rate (CPR), live-birth rate (LBR) or delivery rate for slow-frozen or vitrified human oocytes or embryos. A meta-analysis was performed using a random effects model to calculate relative risk ratios (RR) and 95% CI.

OUTCOMES

One RCT study comparing slow-freezing versus vitrification of oocytes was included. Vitrification was associated with increased ongoing CPR per cycle (RR = 2.81, 95% CI: 1.05-7.51; P = 0.039; 48 and 30 cycles, respectively, per transfer (RR = 1.81, 95% CI 0.71-4.67; P = 0.214; 47 and 19 transfers) and per warmed/thawed oocyte (RR = 1.14, 95% CI: 1.02-1.28; P = 0.018; 260 and 238 oocytes). One RCT comparing vitrification versus fresh oocytes was analysed. In vitrification and fresh cycles, respectively, no evidence for a difference in ongoing CPR per randomized woman (RR = 1.03, 95% CI: 0.87-1.21; P = 0.744, 300 women in each group), per cycle (RR = 1.01, 95% CI: 0.86-1.18; P = 0.934; 267 versus 259 cycles) and per oocyte utilized (RR = 1.02, 95% CI: 0.82-1.26; P = 0.873; 3286 versus 3185 oocytes) was reported. Findings were consistent with relevant cohort studies. Of the seven RCTs on embryo cryopreservation identified, three met the inclusion criteria (638 warming/thawing cycles at cleavage and blastocyst stage), none of which involved pronuclear-stage embryos. A higher CPR per cycle was noted with embryo vitrification compared with slow-freezing, though this was of borderline statistical significance (RR = 1.89, 95% CI: 1.00-3.59; P = 0.051; three RCTs; I2 = 71.9%). LBR per cycle was reported by one RCT performed with cleavage-stage embryos and was higher for vitrification (RR = 2.28; 95% CI: 1.17-4.44; P =  0.016; 216 cycles; one RCT). A secondary analysis was performed focusing on embryo cryosurvival rate. Pooled data from seven RCTs (3615 embryos) revealed a significant improvement in embryo cryosurvival following vitrification as compared with slow-freezing (RR = 1.59, 95% CI: 1.30-1.93; P < 0.001; I2 = 93%).

WIDER IMPLICATIONS

Data from available RCTs suggest that vitrification/warming is superior to slow-freezing/thawing with regard to clinical outcomes (low quality of the evidence) and cryosurvival rates (moderate quality of the evidence) for oocytes, cleavage-stage embryos and blastocysts. The results were confirmed by cohort studies. The improvements obtained with the introduction of vitrification have several important clinical implications in ART. Based on this evidence, in particular regarding cryosurvival rates, laboratories that continue to use slow-freezing should consider transitioning to the use of vitrification for cryopreservation.

Effect of embryo morphology and morphometrics on implantation of vitrified day 3 embryos after warming: a retrospective cohort study

BACKGROUND

Characteristics routinely used to evaluate embryo quality after thawing include number of blastomeres survived and presence of mitosis resumption after overnight culture. It is unknown to which extent symmetry and fragmentation affect implantation after warming and whether application of stricter criteria either before vitrification or after warming would improve implantation rate (IR) of vitrified/warmed embryos. This study aimed to find new parameters to improve selection criteria for vitrification and for transfer after warming.

METHODS

Firstly, we evaluated standard morphological characteristics (intact survival, mitosis resumption, number of blastomeres, symmetry and fragmentation) of 986 warmed day 3 embryos and, from a subset of 654, we evaluated morphometric characteristics (fragmentation, symmetry and volume change). Secondly, we tested the hypothesis that IR of day 3 vitrified/warmed embryos is influenced by morphometric characteristics. IR per embryo transferred was calculated using embryos that were transferred in a single embryo transfer (SET) or a double embryo transfer (DET) with either 0 or 100 % implantation (830/986). We investigated the significant differences in IR between the different categories of a specific characteristic. These categories were based on our standard embryo evaluation system. The statistical tests Chi-square, Fisher's exact or Cochrane-Armitage were used according to the type and/or categories of the variable.

RESULTS

The 986 embryos were transferred in 671 FET cycles with 16.9 % (167/986) IR. After exclusion of DET with 1 embryo implanted, IR per embryo transferred was 12.4 % (103/830). Embryo symmetry, fragmentation and volume change in vitrified/warmed day 3 embryos were not associated with IR. However, when mitosis resumption was present after overnight culture, intact embryos reached significantly higher IR than non-intact embryos and only when the embryo compacted after overnight culture the number of cells damaged after warming had no effect on IR. Concretely, embryos with 8 cells after warming or >9 cells after overnight culture-including compacted embryos-reached the highest IR (>15 %) while embryos with <6 cells after warming or with ≤6 cells after overnight culture had extremely low IR (<1 %).

CONCLUSIONS

IR of vitrified embryos is determined by the number of cells lost, by the occurrence of mitosis resumption, and by the specific number of blastomeres present but not by fragmentation, blastomere symmetry or volume change. Unselecting embryos for cryopreservation because of fragmentation >10 % and/or symmetry < 75 % only leads to unwanted loss of embryos with acceptable implantation potential.

TRIAL REGISTRATION

Retrospectively registered NCT02639715 .

The freezing method of cleavage stage embryos has no impact on the weight of the newborns

PURPOSE

The aim of this study was to study the effect of the embryo freezing method on the birth weight of newborns from frozen embryo transfer (FET) cycles, and the pregnancy results of cleavage stage embryos cryopreserved by slow freezing or vitrification.

METHODS

This is a retrospective cohort study undertaken in a University Hospital IVF unit using concurrently both the slow-freezing and the vitrification techniques. All frozen-thawed and vitrified-warmed day 2 and day 3 embryo transfers during the time period from 1 April 2009 to 31 November 2013 were included in the study.

RESULTS

There was no statistically significant weight difference between newborns from vitrified or slow-frozen embryos (3588 vs 3670 g). A higher post-thaw viability rate was achieved after cryopreservation by the vitrification technique compared to the slow-freezing protocol (83.4 vs 61.4%). The miscarriage rate was lower in the vitrification group (15.7 vs 29.0%). The live birth rates were similar (19.5 vs 19.1%) in the slow-freezing and vitrification groups, respectively. Among vitrified embryos, 7.4 embryos needed to be thawed to produce one delivery; in the slow-freezing group, that number was 11.9.

CONCLUSIONS

The freezing method has no impact on the weight of the newborn. With lower post-thaw survival rates and higher miscarriage rates, the slow-freezing cryopreservation protocol is inferior to the vitrification technique.

The association of serum estradiol level with outcomes of clomiphene citrate/human menopausal gonadotropin ovarian stimulation for in vitro fertilization and embryo transfer

BACKGROUND

The purpose of this study was to test the hypothesis that estradiol (E2) level on day 3 may be associated with in vitro fertilization (IVF) outcomes.

METHODS

The records of patients who received clomiphene citrate 100 mg/day plus human menopausal gonadotropin 150 IU/day from day 3 and received frozen-thawed embryo transfers were reviewed. Patients were divided into three groups: group A (E2 ≤30 pg/ml), group B (30< E2 ≤50 pg/ml), and group C (E2 >50 pg/ml). A total of 1080 cycles from 941 patients were included.

RESULTS

The number of eggs and MII oocytes were less in group C than group A (both, P = 0.001). The embryo implantation (P = 0.006) and clinical pregnancy rates (P = 0.036) were lower in group C than group B, and the rates were similar between group A and B.

CONCLUSION

Maintaining the serum E2 level from 30 to 50 pg/ml may result in a higher clinical pregnancy rate in IVF cycles.

Frozen-thawed embryo transfer in a natural or mildly hormonally stimulated cycle in women with regular ovulatory cycles: a RCT

STUDY QUESTION

Can ovarian stimulation with low dose hMG improve the implantation rate (IR) per frozen-thawed embryo transferred (FET) when compared with natural cycle in an FET programme in women with a regular ovulatory cycle?

SUMMARY ANSWER

Both IR and live birth rate (LBR) per FET were similar in the group with mild ovarian stimulation and the natural cycle group.

WHAT IS KNOWN ALREADY

Different cycle regimens for endometrial preparation are used prior to FET: spontaneous ovulatory cycles, cycles with artificial endometrial preparation using estrogen and progesterone hormones, and cycles stimulated with gonadotrophins or clomiphene citrate. At present, it is not clear which regimen results in the highest IR or LBR. More specifically, there are no RCTs in ovulatory women comparing reproductive outcome after FET during a natural cycle and during a hormonally stimulated cycle.

STUDY DESIGN, SIZE, DURATION

A total of 410 women scheduled for FET during 579 cycles (December 2003-September 2013) were enrolled in an open-label RCT to natural cycle (NC FET group, n = 291) or to a cycle hormonally stimulated with s.c. gonadotrophins (hMG FET group, 37.5-75 IU per day, n = 288). A total of 672 embryos were transferred during 434 cycles (332 embryos and 213 cycles in the NC FET group; 340 embryos and 221 cycles in the hMG FET group). Assuming a = 0.05 and 80% power, it was calculated that 219 frozen-thawed embryos were required for transfer in each group to demonstrate a difference of 10% in IR.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Women were eligible according to the following inclusion criteria: regular ovulatory cycle, female age ≥21 years and ≤45 years, informed consent. FET cycles with preimplantation genetic screening were excluded. The primary outcome was IR per embryo transferred. Secondary outcomes included IR with fetal heart beat (FHB), LBR per embryo transferred and endometrial thickness on the day of hCG administration. Statistical analysis was by intention to treat and controlled for the presence of multiple measures, as eligible women could be randomized in more than one cycle. Chi-square and independent t-test were used to compare categorical and continuous variables. The relative risk (RR) was estimated using a Poisson model with log link. Hierarchical models with random intercepts for patient and cycle were considered to account for clustering of cycles within patients and of embryos within cycles.

MAIN RESULTS AND THE ROLE OF CHANCE

The primary outcome, IR per embryo transferred, was not statistically different between the NC FET group (41/332 (12.35%)) and in the hMG FET group (55/340 (16.18%)) (RR 1.3 (95% confidence interval (CI) 0.9-2.0), P = 0.19). Similarly, the secondary outcome, IR with FHB per embryo transferred, was 34/332 (10.24%) in the NC FET group and 48/340 (14.12%) in the hMG FET group (RR 1.4 (95% CI 0.9-2.1), P = 0.15). The LBR per embryo transferred was 32/332 (9.64%) in the NC FET group and 45/340 (13.24%) in the hMG FET group (RR 1.4 (95% CI 0.9-2.2), P = 0.17). Endometrial thickness was also similar in both groups [8.9 (95% CI 8.7-9.1) in the NC FET group and 8.9 (95% CI 8.7-9.1) in the hMG FET group]. The duration of the follicular phase was significantly shorter (P < 0.001) in the hMG FET group [13.7 days (95% CI 13.2-14.2)] than in the NC FET group [15.4 days (95% CI 14.8-15.9)].

LIMITATIONS, REASONS FOR CAUTION

Randomization of cycles instead of patients; open-label design; relatively long period of recruitment.

WIDER IMPLICATIONS OF THE FINDINGS

Our observation that the IR per embryo transferred is not significantly increased after FET during natural or gonadotrophin stimulated cycle, suggests that the effect of mild hormonal stimulation with gonadotrophins is smaller than what was considered clinically relevant with respect to reproductive outcome after FET. These data suggest that endometrial receptivity is not relevantly improved, but also not impaired after hormonal stimulation with gonadotrophins. Since FET during a natural cycle is cheaper and more patient-friendly, we recommend this regimen as the treatment of choice for women with regular cycles undergoing FET.

TRIAL REGISTRATION NUMBER

clinicaltrials.gov NCT00492934.

TRIAL REGISTRATION DATE

26 June 2007.

DATE OF FIRST PATIENT'S ENROLMENT

1 December 2003.

Slow freezing should not be totally substituted by vitrification when applied to day 3 embryo cryopreservation: an analysis of 5613 frozen cycles

PURPOSE

This study aimed to compare slow freezing (SF) and vitrification (VT) techniques for day 3 embryo cryopreservation in infertile couples.

METHODS

This retrospective cohort study enrolled 5613 infertile patients, with 7862 frozen-thawed day 3 embryos and 3845 vitrified-warmed day 3 embryos, from 2010 to 2014, at a single center. The rates of embryo survival, pregnancy, implantation, miscarriage, live birth, and live birth weight were compared between the two groups.

RESULTS

A total of 5613 cycles with 5520 transfers were analyzed. Using SF, the rates of overall embryo survival and fully intact blastomeres were lower than those in VT (91.5 vs. 97.4 % and 68.7 vs. 92.3 %, respectively). The rate of good quality embryos after thawing/warming was lower in SF than in VT. In single frozen embryo transfer cycles (FETs), the pregnancy and implantation rates were similar between the two groups (35.0 vs. 40.8 % and 34.6 vs. 35.9 %, respectively). In double FETs, the pregnancy rate per cycle was also similar between the groups (58.8 vs. 58.4 %). The implantation rate per embryo transfer was significantly higher with SF than with VT (38.8 vs. 34.6 %). With adjustment for maternal age and the number of good quality embryos, differences in implantation rate remained significant (adjusted P value, SF vs. VT P < 0.05). No independent effect was found for the method of cryopreservation on the pregnancy rate. No significant differences in the rates of miscarriage, live birth, and live birth weight were observed between the two techniques.

CONCLUSIONS

Despite the significantly low embryo survival rate, fully intact blastomere rate, and good quality embryo rate in SF, the pregnancy and implantation rates were not adversely affected in single and double FETs. SF yielded an equivalent miscarriage rate, live birth rate, and live birth weight compared with VT. The SF protocol to cryopreserve day 3 embryos still should be considered.

Vitrification of cleavage stage day 3 embryos results in higher live birth rates than conventional slow freezing: a RCT

STUDY QUESTION

Is the live birth rate (LBR) per embryo thawed/warmed higher when Day 3 cleavage stage embryos are cryopreserved by vitrification compared with slow freezing?

SUMMARY ANSWER

The LBR per embryo thawed/warmed was higher after vitrification than after slow freezing on Day 3, based on better embryo survival, quality and availability of embryos in the vitrification group.

WHAT IS KNOWN ALREADY

Post-thawing survival rate of cleavage-stage embryos has been reported to be higher after vitrification than after slow freezing.

STUDY DESIGN, SIZE, DURATION

This RCT was performed in an academic tertiary center between September 2011 and March 2013. If supernumerary embryos were available on Day 3, patients were randomized at the time of cryopreservation using a computerized system to determine a simple allocation to the vitrification group or the slow freezing group and all embryos were frozen with the same technique. The primary outcome of this study was the LBR per embryo thawed/warmed. Power calculation revealed that 184 thawed embryos were needed in each group (β = 0.8, α < 0.05) to test the hypothesis that the LBR per embryo thawed/warmed was significantly higher (16%) after vitrification than after slow freezing (6%).

PARTICIPANTS/MATERIALS, SETTING, METHODS

Patients <40 years old undergoing their first oocyte retrieval (OR), with embryo transfer and with supernumerary embryos on Day 3, were randomized. Day 3 embryos with ≥6 cells, <25% fragmentation and morphologically equal blastomeres were cryopreserved by slow freezing (using 1,2-propanediol and 0.1 M sucrose as cryoprotectant) or by closed vitrification using commercially available freezing/vitrification media. Survival was defined as ≥50% cells were intact after thawing. Thawed embryos were further cultured overnight. In total, 307 patients were randomized to slow freezing (155 patients, 480 embryos) or vitrification (152 patients, 495 embryos).

MAIN RESULTS AND THE ROLE OF CHANCE

By March 2013, 200 embryos were thawed after slow freezing in 95 cycles for 79 patients and 217 embryos were warmed after vitrification in 121 cycles in 90 patients. The LBR per embryo thawed/warmed was significantly higher after vitrification (16.1% (35/217)) than after slow freezing (5.0% (10/200); P < 0.0022; relative risk (RR) 3.23; 95% confidence interval (CI) 1.64-6.35). Similarly, the implantation rate per embryo thawed/warmed was higher after vitrification (20.7% (45/217) than after slow freezing (7.5% (15/200); P = 0.0012; RR 2.76; CI 1.59-4.81). The survival rate was significantly higher after vitrification (84.3% (183/217) than after slow freezing (52.5% (105/200); P < 0.0001). Significantly more embryos were fully intact after vitrification (75.4% (138/183) than after slow freezing (28.6% (30/105); P < 0.0001). The number of transfers was significantly higher after vitrification (90.1% (109/121)) than after slow freezing (73.7% (70/95); P = 0.0024).

LIMITATIONS, REASONS FOR CAUTION

Survival rates in the slow freezing group were low in this study. Additional RCTs are needed to compare reproductive outcome after vitrification and after slow freezing with 1,2-propanediol and 0.2 M sucrose, since this method has been reported to have better survival than the method used in our study. Our findings are only applicable to the specific slow freezing cryopreservation medium used in our study, and not to any other commercially available media.

WIDER IMPLICATIONS OF THE FINDINGS

When compared with slow freezing using 1,2-propanediol and 0.1 M sucrose as cryoprotectant, vitrification of Day 3 cleavage stage embryos resulted in a higher LBR per embryo warmed, and may therefore result into a higher cumulative delivery rate after one oocyte retrieval.

STUDY FUNDING/COMPETING INTERESTS

None.

TRIAL REGISTRATION NUMBER

NCT02013024.

[Vitrification: Principles and results]

Sperm and embryos cryopreservation is a commonly applied technique for several years. Recently authorized in France, vitrification tends to replace gradually the conventional technique of slow freezing, so upsetting the practices in the management of patients. It allows from now on the cryopreservation of oocytes and opens new perspectives in egg donation either still in fertility preservation. This review thus attempted to examine the contribution of vitrification in the freezing of oocytes and human embryos at various stages of development. If obviously vitrification appears as the current method of choice for the cryopreservation of oocytes as well as blastocysts, the results are less cut as regards embryos to early stages. No increase in adverse obstetric and perinatal outcomes in children conceived from vitrified oocytes or embryos is noted in the literature.

Cryopreservation of human embryos and its contribution to in vitro fertilization success rates

Cryopreservation of human embryos is now a routine procedure in assisted reproductive technologies laboratories. There is no consensus on the superiority of any protocol, and substantial differences exist among centers in day of embryo cryopreservation, freezing method, selection criteria for which embryos to freeze, method of embryo thawing, and endometrial preparation for transfer of frozen-thawed embryos. In the past decade, the number of frozen-thawed embryo transfer cycles per started in vitro fertilization (IVF) cycle increased steadily, and at the same time the percentage of frozen-thawed embryo transfers that resulted in live births increased. Currently, cryopreservation of human embryos is more important than ever for the cumulative pregnancy rate after IVF. Interestingly, success rates after frozen-thawed embryo transfer are now nearing the success rates of fresh embryo transfer. This supports the hypothesis of so called freeze-all strategies in IVF, in which all embryos are frozen and no fresh transfer is conducted, to optimize success rates. High-quality randomized controlled trials should be pursued to find out which cryopreservation protocol is best and whether the time has come to completely abandon fresh transfers.

Human embryo cryopreservation: one-step slow freezing does it all?

PURPOSE

Previous studies have shown that a modified one-step slow freezing method with higher sucrose concentration (0.2 M) can achieve higher embryo and blastomere survival rates that are comparable to vitrification. However, no study has evaluated the efficacy of a one-step method using commercial slow freezing kit without altering its composition. This retrospective study examines the effects of using 1.5 M PROH with 0.1 M sucrose (F2 medium) alone in a one-step slow freezing method compared to the conventional two-step method.

METHODS

Cleavage stage embryos from 526 thaw cycles previously cryopreserved by either the conventional two-step slow freezing method or the modified one-step method were studied. The embryo and blastomere survival rates, cleavage rate, clinical pregnancy rate and live birth rate were compared between the two groups.

RESULTS

The results showed that the embryo survival rate was significantly higher in the modified one-step method compared to the conventional two-step method (86.9 % and 83.1 %, respectively; p = 0.04). Total blastomere survival rate was also significantly increased as a result of the modification (81.0 % versus 76.5 %; p < 0.001). However, there was no statistical difference in the cleavage rates, clinical pregnancy rates (CPR/ET) and live birth rates between the two methods.

CONCLUSIONS

Slow freezing using the one-step method is superior to the conventional two-step method in terms of embryo and blastomere survival rates without affecting cleavage rate and clinical outcomes. It can be routinely applied to cleavage stage embryo cryopreservation in IVF centres for greater workflow efficiency.

Cryopreservation of embryos and oocytes in human assisted reproduction

Both sperm and embryo cryopreservation have become routine procedures in human assisted reproduction and oocyte cryopreservation is being introduced into clinical practice and is getting more and more widely used. Embryo cryopreservation has decreased the number of fresh embryo transfers and maximized the effectiveness of the IVF cycle. The data shows that women who had transfers of fresh and frozen embryos obtained 8% additional births by using their cryopreserved embryos. Oocyte cryopreservation offers more advantages compared to embryo freezing, such as fertility preservation in women at risk of losing fertility due to oncological treatment or chronic disease, egg donation, and postponing childbirth, and eliminates religious and/or other ethical, legal, and moral concerns of embryo freezing. In this review, the basic principles, methodology, and practical experiences as well as safety and other aspects concerning slow cooling and ultrarapid cooling (vitrification) of human embryos and oocytes are summarized.

Cryopreservation of day 2-3 embryos by vitrification yields better outcome than slow freezing

OBJECTIVE

To compare the outcome of vitrification versus slow freezing cryopreservation for cleavage stage day 2-3 embryos.

DESIGN

A retrospective observational study.

SETTING

All thawed embryos assisted reproduction cycles between January 2010 and December 2012 at a single IVF laboratory of a Tertiary Medical Center.

PATIENTS

Five hundred and thirty-nine cycles of day 2-3 thawed embryos.

INTERVENTIONS

In 327 of the thawed cycles, the embryos were vitrified and in 212 of the cycles the embryos were derived from slow freezing embryos.

MAIN OUTCOMES MEASURE

Embryo survival rate, blastomere surviving rate and pregnancy rate.

RESULTS

Embryo survival rate was significantly higher after vitrification compared with slow freezing (81.6%, 647/793 versus 70.0%, 393/562 embryos, p < 0.0001). The clinical pregnancy rate per ET was significantly higher following vitrification compared to slow freezing, 20.0%, 63/314 versus 11.9%, 23/193, respectively (p = 0.02).

CONCLUSIONS

Vitrification of day 2-3 cleavage stage embryos yields better cycle outcome in all the parameters compared to slow freezing.

The new Rapid-i carrier is an effective system for human embryo vitrification at both the blastocyst and cleavage stage

BACKGROUND

The Rapid-i is a new FDA cleared closed carrier for embryo vitrification. The cooling rate of - 1220°C/min is far lower than that reported with open vitrification systems such as the cryoloop (-15,000°C/min). Little published data is currently available on this device. This study presents our initial clinical data, as well as live birth outcomes, with the Rapid-i. The efficacy of this device for the cryopreservation of cleavage, as well as blastocyst stage human embryos is also analyzed. We further compare outcomes to those achieved with the cryoloop, an "open" vitrification system routinely used in our laboratory.

METHODS

Human embryos were vitrified at either the 8-10 cell stage or else the blastocyst stage. The vitrification protocol was: 7.5% DMSO/7.5% ethylene glycol (EG) (2-3 min) followed by incubation in 15% DMSO /15% EG (45 sec) before loading on the vitrification carrier. Cryoprotectant was removed during warming by sequential washes in 0.25 M and 0.125 M sucrose in culture medium. Clinical outcome data for frozen cycles between January 2011 and August 2012 were stratified according to carrier and cell stage. The student t-test and chi square test were used to compare results. P value of < 0.05 was considered significant.

RESULTS

A total of 486 vitrified-warmed embryos were assessed and 92% of them were transferred. The clinical pregnancy rate (CPR) and implantation rate (IR) with Rapid-i vitrified blastocysts were 59% and 49%, versus 47% and 37%, respectively for cleavage stage embryos. This was not statistically different from results with the cryoloop vitrified blastocysts (CPR 46%, IR 38%) nor the cleavage stage vitrified embryos (CPR 49%, IR 35%). To date, there have been 31 deliveries of 34 healthy infants from Rapid-i vitrified embryos, with another 12 pregnancies still on-going.

CONCLUSIONS

The Rapid-i offers an excellent alternative to existing open vitrification devices for embryo cryopreservation at the 8-10 cell stage as well as the blastocyst stage. Use of this type of "closed" sealed system that prevents direct contact between the embryos and liquid nitrogen reduces the potential risk of sample cross-contamination or infection. These preliminary data and live birth outcomes have paved the way toward transitioning to a closed vitrification system in our own IVF program.

A critical appraisal of cryopreservation (slow cooling versus vitrification) of human oocytes and embryos

BACKGROUND

Vitrification is now a commonly applied technique for cryopreservation in assisted reproductive technology (ART) replacing, in many cases, conventional slow cooling methodology. This review examines evidence relevant to comparison of the two approaches applied to human oocytes and embryos at different developmental stages.

METHODS

Critical review of the published literature using PubMed with particular emphasis on studies which include data on survival and implantation rates, data from fresh control groups and evaluation of the two approaches in a single setting.

RESULTS

Slow cooling is associated with lower survival rates and compromised development relative to vitrification when applied to metaphase II (MII) oocytes, although the vitrification results have predominantly been obtained using direct contact with liquid nitrogen and there is some evidence that optimal protocols for slow cooling of MII oocytes are yet to be established. There are no prospective randomized controlled trials (RCTs) which support the use of either technique with pronuclear oocytes although vitrification has become the method of choice. Optimal slow cooling, using modifications of traditional methodology, and vitrification can result in high survival rates of early embryos, which implant at the same rate as equivalent fresh counterparts. Many studies report high survival and implantation rates following vitrification of blastocysts. Although slow cooling of blastocysts has been reported to be inferior in some studies, others comparing the two approaches in the same clinical setting have demonstrated comparable results. The variation in the extent of embryo selection applied in studies can lead to apparent differences in clinical efficiency, which may not be significant if expressed on a 'per oocyte used' basis.

CONCLUSIONS

Available evidence suggests that vitrification is the current method of choice when cryopreserving MII oocytes. Early cleavage stage embryos can be cryopreserved with equal success using slow cooling and vitrification. Successful blastocyst cryopreservation may be more consistently achieved with vitrification but optimal slow cooling can produce similar results. There are key limitations associated with the available evidence base, including a paucity of RCTs, limited reporting of live birth outcomes and limited reporting of detail which would allow assessment of the impact of differences in female age. While vitrification has a clear role in ART, we support continued research to establish optimal slow cooling methods which may assist in alleviating concerns over safety issues, such as storage, transport and the use of very high cryoprotectant concentrations.