Successful pregnancy following blastocyst cryopreservation using super-cooling ultra-rapid vitrification

Navigating the practical challenges and ethical dilemmas of surplus cryopreserved human embryos

Cryopreservation, the use of very low temperatures to preserve structurally intact living cells and tissues, has seen exponential growth in the field of in vitro fertilization (IVF). In the last decade, cryopreservation of embryos and freeze-all protocols have become an essential aspect and a prerequisite for a successful IVF outcome. Moreover, vitrification, which is a fast and safe cryopreservation method, has proved to be an effective choice for cryopreserving gametes and embryos. The increasing number of cryopreserved embryos worldwide in cryobanks and IVF clinics is an undisputable fact that raises important physiological, ethical, and moral considerations that merit careful examination and discussion. Many couples utilizing assisted reproduction will have a surplus of cryopreserved embryos, in other words they already have completed their family without exhausting all the embryos that were created and cryopreserved during the process. Additionally, the global IVF market has also experienced significant growth due to various factors, including advancements in technology, increased awareness about infertility treatments, and changing societal norms towards delayed parenthood. Thus, for the foreseeable future the number of cryopreserved embryos, and the phenomenon of surplus embryos will likely remain unresolved. In the present review, following a description of the cryopreservation method and the physiological changes during the cryopreservation of embryos, the bioethical issues raised by the surplus cryopreserved embryos will be discussed alongside possible solutions for resolving this phenomenon.

Effect of blastocyst shrinkage on assisted reproductive outcomes: a retrospective cohort study describing a new morphological evaluation of blastocyst pre-vitrification and post-warming

BACKGROUND

The failure of frozen-thawed blastocysts to re-expand adequately within a few hours after warming has been reported to have a negative impact on assisted reproductive technology (ART) outcomes. However, the extent to which this failure truly affects ART outcomes has not yet been presented in a manner that is easily understandable to medical practitioners and patients. This study aimed to assess the effects of blastocyst shrinkage on ART outcomes and determine a more effective morphological evaluation approach for use in clinical settings.

METHODS

This retrospective observational cohort study of frozen-thawed blastocyst transfer cycles was conducted from April 2017 to March 2022. Overall, 1,331 cycles were eligible for inclusion, of which 999 were good-quality blastocysts (GQB) and 332 were non-good-quality blastocysts (non-GQB). All frozen-thawed blastocyst transfer cycles performed during the specified study period were included in the study. Exclusion criteria were established to mitigate potential sources of bias as these cycles could impact implantations. We calculated rates and age-adjusted odds ratios of implantation, clinical pregnancy, ongoing pregnancy, and live birth of the re-expansion group, which showed sufficient expansion, and shrinkage group, which showed insufficient expansion. We also calculated the implantation, clinical pregnancy, ongoing pregnancy, and live birth rates of the re-expansion and shrinkage groups for each morphological scoring system parameter.

RESULTS

A reduced ART outcome was observed with use of blastocysts with shrinkage after vitrification/warming. The age-adjusted odds ratios for implantation, clinical pregnancy, ongoing pregnancy, and live birth were lower in the shrinkage group than in the re-expansion group.

CONCLUSIONS

This study examined the adverse effect of blastocyst shrinkage after warming and recovery culturing on reproductive outcomes in a clinically useful manner by retrospectively examining a substantial number of frozen-thawed embryo transfer cycles. The study findings can possibly reduce concerns regarding over- or under-estimation of blastocyst implantation by allowing providers and patients to refer to the data.

Thermally conductive graphene-based nanofluids, a novel class of cryosolutions for mouse blastocysts vitrification

Limited heating and cooling rates have long been recognized as bottlenecks in improving embryo cryopreservation. As a result, efforts to achieve higher heat transfer rates gave rise to milestones like open cryodevices and minimal media loading. A crucial but commonly ignored variable is heat conduction by cryosolutions. The low heat conductivity of the aqueous media surrounding embryos slows down cooling and heating rates of the embryo, imposing the risk of preventable damages. In this study, we introduce a novel thermally conductive cryosolution based on graphene oxide nanoparticles and test its performance against conventional sucrose-based solutions for vitrification of mouse blastocysts. Replacing sucrose with graphene oxide brought about similar re-expansion, hatching, and implantation rates of post-vitrification embryos while also preventing an array of cellular and molecular stresses. Our results showed significantly reduced oxidative stress, characterized by control-level expression of Sod1 and significant downregulation of Sod2 transcription when graphene oxide was used instead of sucrose. This molecular response was in agreement with the reduced level of reactive oxygen species produced in vitrified/warmed embryos using graphene-based solutions. The downstream impacts of this stress reduction manifested in significant downregulation of two major pro-apoptotic genes, Bax and Trp53, down to the same level as fresh embryos. Interestingly, embryos maintained their spherical shape during dehydration in graphene-based solutions and did not "collapse" when shrinking, like in sucrose-based solutions. These results provide new insights into the benefits of thermally conductive cryosolutions and showcase the potential of graphene oxide as a cryoprotectant in embryo vitrification.

Effects of vitrification and the superovulated environment on placental function and fetal growth in an IVF mouse model

In studies of human IVF, as compared to frozen embryo transfer (ET), fresh ET is associated with smaller infants and higher risk of small for gestational age infants. Recent observations suggest that ET using vitrified embryos is associated with higher pregnancy and live birth rates compared to fresh ET, but increased rates of large for gestational age infants. The mechanisms underlying these associations are largely unknown, and available evidence suggests that the influence of IVF, vitrification and the superovulated (SO) uterine environment on placental function and fetal growth is complex. This warrants further investigation given the prevalent practice in human IVF of both fresh ET into a SO uterine environment, and vitrification with ET into a more physiologic uterine environment. Using a mouse model that closely resembles human IVF, we investigated if vitrification of IVF embryos better preserves placental function and results in better pregnancy outcomes as compared to fresh ET because of transfer into a more physiologic endometrium. We found that the SO environment, independent of vitrification status, reduced implantation rates, inhibited placental mechanistic target of rapamycin signaling and induced placental stress signaling, resulting in fetal growth restriction (1.080 ± 0.05 g estrous fresh (n = 17 litters), 1.176 ± 0.05 g estrous vitrified (n = 12), 0.771 ± 0.06 g SO fresh (n = 15), 0.895 ± 0.08 g SO vitrified (n = 10), P < 0.0001). In addition, our study suggests that vitrification impairs the developmental potential of IVF blastocysts that resulted in a significantly smaller litter size (2.6 ± 2.3 fresh estrous vs 2.5 ± 2.4 fresh SO vs 1.6 ± 1.7 estrous vitrified vs 1.7 ± 1.8 SO vitrified, P = 0.019), with no effect on fetal growth or placental function at term. Our findings suggest that vitrification may negatively impact early embryonic viability, while the SO maternal uterine environment impairs both placental development and fetal growth in IVF.

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.

Birth outcomes are superior after transfer of fresh versus frozen embryos for donor oocyte recipients

STUDY QUESTION

For donor oocyte recipients, are birth outcomes superior for fresh versus frozen embryos?

SUMMARY ANSWER

Among fresh donor oocyte recipients, fresh embryos are associated with better birth outcomes when compared with frozen embryos.

WHAT IS KNOWN ALREADY

Frozen embryo transfer (ET) with vitrification has been associated with improved pregnancy rates, but also increased rates of large for gestational age infants. Donor oocyte recipients represent an attractive biological model to attempt to isolate the impact of embryo cryopreservation on IVF outcomes, yet there is a paucity of studies in this population.

STUDY DESIGN, SIZE, DURATION

A retrospective cohort of the US national registry, the Society for Assisted Reproductive Technology Clinic Outcome Reporting System, of IVF cycles of women using fresh donor oocytes resulting in ET between 2013 and 2015. Thawed oocytes were excluded.

PARTICIPANTS/MATERIALS, SETTINGS, METHODS

Good obstetric outcome (GBO), defined as a singleton, term, live birth with appropriate for gestational age birth weight, was the primary outcome measure. Secondary outcomes included live birth, clinical pregnancy, spontaneous abortion, preterm birth, multiple births and gestational age-adjusted weight. Outcomes were modeled using the generalized estimating equation approach.

MAIN RESULTS AND THE ROLE OF CHANCE

Data are from 25 387 donor oocyte cycles, in which 14 289 were fresh and 11 098 were frozen ETs. A GBO was 27% more likely in fresh ETs (26.3%) compared to frozen (20.9%) (adjusted risk ratio 1.27; 95% confidence interval (CI) 1.21-1.35; P < 0.001). Overall, fresh transfer was more likely to result in a live birth (55.7% versus 39.5%; adjusted risk ratio 1.21; 95% CI 1.18-1.26; P < 0.001). Among singleton births, there was no difference in gestational age-adjusted birth weight between groups.

LIMITATION, REASONS FOR CAUTION

Our cohort findings contrast with data from autologous oocytes. Prospective studies with this population are warranted.

WIDER IMPLICATIONS OF THE FINDINGS

Among donor oocyte recipients, fresh ETs may be associated with better birth outcomes. Reassuringly, given its prevalent use, modern embryo cryopreservation does not appear to result in phenotypically larger infants.

STUDY FUNDING/COMPETING INTEREST(S)

None.

TRIAL REGISTRATION NUMBER

N/A.

Comparison of two different oocyte vitrification methods: a prospective, paired study on the same genetic background and stimulation protocol

STUDY QUESTION

Can two different methods for oocyte vitrification, one using an open tool and the other a closed tool, result in similar oocyte survival rates?

SUMMARY ANSWER

The oocyte survival rate was found to be higher in the closed method.

WHAT IS KNOWN ALREADY

Open vitrification is performed routinely in oocyte donation cycles. Closed oocyte vitrification may result in slower cooling rates and thus it is less used, even though it has been recommended in order to avoid the risk of cross-contamination between material from different patients.

STUDY DESIGN, SIZE, DURATION

This is a prospective cohort study with sibling oocytes carried out in a fertility center between July 2014 and January 2016. The study included 83 oocyte donors each providing a minimum of 12 mature oocytes (metaphase II: MII) at oocyte retrieval. Oocyte survival rate and fertilization rate, as well as reproductive outcomes (biochemical, clinical, ongoing pregnancy and live birth rates) per embryo transfer and also cumulatively between the two methods were compared by Chi2 tests.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Donor oocytes were denuded and six MII oocytes from each donor were vitrified using an open method and later assigned to one recipient, while another six MII oocytes were vitrified using a closed method and assigned to a different recipient (paired analysis). ICSI was used in all cases and embryo transfer was performed on Day 2-3 in all cases.

MAIN RESULTS AND THE ROLE OF CHANCE

Oocyte donors were 24.8 years old on average (SD 4.7). Recipient age (average 41.2 years, SD 4.7) and BMI (mean 23.8 kg/m2, SD 4.0) were similar between recipient groups. Oocytes vitrified using the closed method had higher survival rate (94.5% versus 88.9%, P = 0.002), but lower fertilization rate (57.1% versus 69.8%, P < 0.001) compared to the open method. The number of fresh embryos transferred in the two groups was 1.8 on average (SD 0.4). Biochemical (45% closed versus 50% open), clinical (40% versus 50%) and ongoing (37.5% versus 42.5%) pregnancy rates were not different between groups (P > 0.05) and neither were live birth rates (37.5% versus 42.5%, P > 0.05). Cumulative reproductive results (obtained after the transfer of all the embryos) were also similar between groups.

LIMITATIONS, REASONS FOR CAUTION

The participants of this study were oocyte donors, i.e. young women in good health, and care should be exerted in extending our results to other populations such as infertility patients, oncofertility patients and women freezing oocytes to delay childbearing.

WIDER IMPLICATIONS OF THE FINDINGS

Our results suggest that, in spite of different survival and fertilization rates, closed and open oocyte vitrification methods should offer similar reproductive outcomes up to cumulative live birth rates.

STUDY FUNDING/COMPETING INTEREST(S)

The authors report no conflict of interest. Vitrolife provided the media and the closed method tool needed for the study at no cost.

Different Origin, Different Response: Gene Expression Pattern in Collapsed Vitrified Blastocyst

There is a large body of animal experimental data about assisted reproductive techniques that could be applied to improve clinical outcomes. The great part of this information was obtained from research on in vivo-derived embryos. But whether these results are always similar with those we expect from embryos having in vitro origin in the clinical cases is a critical question. The present study was designed to compare the effects of vitrification (VIT) and artificial collapse (AC) as two commonly used techniques on in vivo- and in vitro-derived mouse embryos. In this regard, both origins of blastocysts were produced and randomly divided into three experimental groups, including control (non-vitrified), VIT, and AC-VIT. The survival and hatching rates and the expression of development-related genes were assessed in all groups and compared with their control counterpart. According to our results, although in vivo and in vitro origins followed the same pattern in the hatching rate, the real-time PCR data showed two distinct patterns of gene expression. Compared to the control, vitrification increased the expression of pluripotency genes in in vivo group. While in vitro vitrified blastocysts showed a significant reduction in the transcripts of these genes. More interestingly, although AC resulted in a sharp decrease of Gata6 and Grb2 in post warmed in vivo blastocysts, it could not affect the vitrified IVP ones. In conclusion, it seems that vitrification and artificial collapse techniques have different effects on embryo fate depending on in vivo or in vitro origins of the embryos.

Performance of preimplantation genetic testing for aneuploidy in IVF cycles for patients with advanced maternal age, repeat implantation failure, and idiopathic recurrent miscarriage

OBJECTIVE

The primary objective of this study was to investigate whether preimplantation genetic testing for aneuploidy (PGT-A) of blastocysts through array comparative genomic hybridization (aCGH) improves live birth rates (LBR) in IVF cycles for patients with high prevalence of aneuploidy.

MATERIALS AND METHODS

This study included 1389 blastocysts with aCGH results derived from 296 PGT-A cycles in IVF patients with advanced maternal age (AMA) (n = 87, group A), those with repeated implantation failure (RIF) (n = 82, group B), those with recurrent miscarriage (RM) (n = 82, group C), and oocyte donors (OD) (n = 45, young age, as a control group). Another 61 AMA patients without PGT-A procedures were used as a control group for group A. Vitrification was performed after blastocyst biopsy, and thawed euploid embryos were transferred in a nonstimulated cycle.

RESULTS

For the AMA group, a significant increase in LBRs was found in the PGT-A group compared with the non-PGT-A group (54.1% vs. 32.8%, p = 0.018). Consistent LBRs (54.1%, 51.6%, 55.9%, and 57.1%, respectively, in group A, B, C, and young age group) were obtained for all the indications.

CONCLUSIONS

LBRs can be improved using PGT-A of blastocysts with aCGH in IVF cycles for patients with a high rate of aneuploidy, especially for patients with AMA.

The endometrium during and after ovarian hyperstimulation and the role of segmentation of infertility treatment

Controlled ovarian hyperstimulation (COH) is a crucial part of assisted reproductive technologies (ART) that resulted in a substantial increase in pregnancy rates from in vitro fertilization (IVF). However, in spite of the apparent benefit of COH on an increase in the number of follicles and the number of oocytes retrieved, allowing for extended embryo culture and enabling the selection of the best quality embryo for transfer, several reports has shown that the supraphysiologic hormonal levels may indeed have a detrimental effect at the endometrial level. The current article revises the pathophysiological mechanisms through which ovarian stimulation may negatively affect endometrial receptivity. Also, the evidence is analyzed explaining how segmentation of IVF treatment may allow us to overcome the deleterious effects of hyperstimulation on endometrial receptivity. Deferred embryo transfer may be performed in a more physiologic uterine environment in a subsequent cycle, improve endometrial receptivity, decrease uterine contractility, diminishes the impact of premature luteinization and allow individualized stimulation according to the level of response.

Detection of early cleavage embryos improves pregnancy and delivery rates of Day 3 embryo transfer during in vitro fertilization

OBJECTIVE

This study established a simple criterion for improving the pregnancy and delivery rates of Day 3 embryo transfer for in vitro fertilization (IVF) by assessing the early cleavage of two-cell stage embryos.

MATERIALS AND METHODS

In total, 258 cycle patients undergoing an IVF and Day 3 embryo transfer program were recruited. All cycles were divided into four groups containing viable Day 3 embryos and those (A) with distinct early cleavage (equal-sized blastomeres and ≤10% fragmentation: ECA grade); (B) with indistinct early cleavage (equal sized blastomeres, >2 blastomeres, or >10% fragmentation: ECB grade); (C) without early cleavage [no early cleavage (NEC grade)]; or (D) without early cleavage being assessed (control) at 25-27 after insemination.

RESULTS

The percentage of viable Day 3 embryos from ECA grade (75.1%, 507/675) was significantly higher than that from ECB grade (19.2%, 151/403) or NEC grade (27.1%, 127/469) embryos (p < 0.01). The pregnancy and delivery rates in the ECA group [65.7% (65/990) and 48.5% (48/990), respectively] were significantly higher than those in the ECB group [30.8% (4/13) and 7.7% (1/13), respectively] or NEC group [36.8% (14/38) and 23.7% (9/38), respectively; all p < 0.01]. The implantation rate in the ECA group (32.3%, 129/400) was higher than those in the ECB (6.8%, 4/59) and NEC (13.0%, 18/136) groups (p < 0.01).

CONCLUSION

Simple selection using the early cleavage morphology may improve the pregnancy and delivery rates of Day 3 embryo transfer programs.

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.

Effects of vitrification on blastomere viability and cytoskeletal integrity in mouse embryos

Vitrification is widely used to cryopreserve supernumerary embryos following in vitro fertilization (IVF). The mouse model was used to investigate the effects of vitrification on blastomere viability, using viability markers, and on the cytoskeleton, by analysing spindle/chromosome configurations, using confocal scanning microscopy. Ninety cleavage and morula stage dimethyl sulphoxide (DMSO)/EG vitrified mouse embryos were either processed immediately following warming for viability assessment by labelling with the fluorescent markers carboxyfluorescein-diacetate succinimidylester (CFSE) and propidium iodide (PI) or were cultured to the blastocyst stage and immunostained with α-tubulin antibody to visualize microtubules and DAPI or PI to visualize DNA. Sixty-five fresh embryos were also used as the control. Vitrified embryos showed high survival rates following warming, but they had a higher incidence of damaged blastomeres compared with fresh embryos. Most mitotic spindles examined in all groups were normal, but multivariable analysis revealed that the proportion of abnormal spindles was significantly higher in vitrified/warmed embryos (P < 0.05). This study is the first to examine the immediate effects of vitrification on blastomere viability, using fluorescent markers and shows that although vitrification results in a higher incidence of damaged blastomeres, vitrified embryos may compensate for this limited number of damaged/abnormal cells, as development to the blastocyst stage was not compromised.

Factors associated with birth outcomes from cryopreserved blastocysts: experience from 4,597 autologous transfers of 7,597 cryopreserved blastocysts

OBJECTIVE

To evaluate factors associated with cryopreserved blastocyst transfer birth outcomes, including age, expansion time, cryopreservation protocol, cryodamage, and number of embryos transferred.

DESIGN

Retrospective cohort study.

SETTING

Private infertility practice.

PATIENT(S)

Cryopreserved blastocyst transfer patients from January 2003 to April 2012.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Birth per transfer and children per embryo.

RESULT(S)

Overall live birth per transfer was 32%, with 17% twin births and 0.3% triplets. Live birth per transfer was significantly higher for vitrification compared with slow-freeze (day 5 cryopreservation: 47% vs. 35%; day 6 cryopreservation: 46% vs. 24%), as was live born children per transferred embryo (39% vs. 29% for day 5; 36% vs. 18% for day 6). Birth rates declined only slightly with increasing age at cryopreservation through 37 years, followed by an increasingly rapid decline in success with increasing age thereafter. Live birth rates declined rapidly (49%-18% for vitrification and 37%-10% for slow-freeze) as the percentage of intact cells after cryopreservation decreased from 95%-100% to 70%-79%, with almost no births when the percentage of intact cells was <70%. Increasing numbers of embryos per transfer were associated with significant increase in live birth per transfer but significant decrease in children per transferred embryo. Birth rates were much lower for blastocysts with delayed expansion on day 7 (10% per transfer).

CONCLUSION(S)

Birth outcomes from cryopreserved blastocyst transfer are influenced by age, timing of expansion, cryopreservation protocol, visible cryodamage, and the number of embryos transferred. Vitrification substantially improves outcomes versus slow freezing.

Supplementation With Cell-Penetrating Peptide-Conjugated Estrogen-Related Receptor β Improves the Formation of the Inner Cell Mass and the Development of Vitrified/Warmed Mouse Embryos

Estrogen-related receptor β (ESRRB), which is a member of the nuclear orphan receptor family, regulates the messenger RNA (mRNA) expression levels of the transcription factors, Oct4 and Nanog, in early embryos and germ cells, thereby maintaining the undifferentiated state and pluripotency of the relevant cells. The present study was designed to determine whether the upregulation of pluripotency-related genes by direct delivery of ESRRB protein may affect on the commitment into inner cell mass (ICM) or the development of vitrified/warmed mouse embryos. Recombinant cell-penetrating peptide (CPP) ESRRB protein was synthesized and then added into a culture medium for cryopreserved mouse embryos. Vitrified/warmed 8-cell embryos were cultured in KSOM with/without 2 μg/mL CPP-ESRRB for 48 hours and then analyzed or transferred to the uteri of foster mothers. The mRNA expression of Oct4 and Nanog was higher in CPP-ESRRB-treated blastocysts compared to the untreated controls. No difference was observed in embryonic development, but ICM:trophectoderm ratio was increased in the CPP-ESRRB-treated group compared to the untreated group, and after embryo transfer, a higher implantation rate was obtained in the CPP-ESRRB-treated group compared to the untreated group. This study shows for the first time that recombinant CPP-ESRRB can be easily integrated into vitrified/warmed mouse embryos and that it increases Oct4 expression (via a pluripotency-related gene pathway), ICM formation, and the further embryonic and full-term development of vitrified/warmed mouse embryos. This CPP-conjugated protein delivery system could therefore be a useful tool for improving assisted reproductive technology.

Effects of reactive oxygen species levels in prepared culture media on embryo development: a comparison of two media

OBJECTIVE

This study determined the correlation between the levels of reactive oxygen species (ROS) in prepared culture media and the early development of human embryos.

MATERIALS AND METHODS

This was an autocontrolled comparison study. A total of 159 patients undergoing in vitro fertilization/intracytoplasmic sperm injection treatment were recruited in this study. The pH values, osmolarity pressures, and ROS levels of 15 batches of two culture media were measured. Sibling oocytes or embryos from individual patients were randomly assigned to two culture groups with Quinn's Advantage Cleavage and Blastocyst media (QAC/QAB) or GIII series cleavage and blastocyst media (G1.3/G2.3). The difference between the two culture groups was analyzed using one-sample t test.

RESULTS

The QAC/QAB and G1.3/G2.3 media exhibited similar pH values and osmolarity pressures. However, the prepared QAC/QAB media were characterized to contain lower amounts of ROS than the G1.3/G2.3 media. Furthermore, the blastocysts that developed under the QAC/QAB media were morphologically superior to those that developed under the G1.3/G2.3 media.

CONCLUSION

The elevated ROS levels in culture media were associated with poor development of blastocyst-stage embryos. Measurement of ROS levels may be a valuable process for medium selection or modification.

Female fertility: is it safe to "freeze?"

Objective:

To evaluate the safety and risk of cryopreservation in female fertility preservation.

Data sources:

The data analyzed in this review were the English articles from 1980 to 2013 from journal databases, primarily PubMed and Google scholar. The criteria used in the literature search show as following: (1) human; embryo; cryopreservation/freezing/vitrification, (2) human; oocyte/immature oocyte; cryopreservation/ freezing/vitrification, (3) human; ovarian tissue transplantation; cryopreservation/freezing/vitrification, (4) human; aneuploidy/DNA damage/epigenetic; cryopreservation/freezing/vitrification, and (5) human; fertility preservation; maternal age.

Study selection:

The risk ratios based on survival rate, maturation rate, fertilization rate, cleavage rate, implantation rate, pregnancy rate, and clinical risk rate were acquired from relevant meta-analysis studies. These studies included randomized controlled trials or studies with one of the primary outcome measures covering cryopreservation of human mature oocytes, embryos, and ovarian tissues within the last 7 years (from 2006 to 2013, since the pregnancy rates of oocyte vitrification were significantly increased due to the improved techniques). The data involving immature oocyte cryopreservation obtained from individual studies was also reviewed by the authors.

Results:

Vitrifications of mature oocytes and embryos obtained better clinical outcomes and did not increase the risks of DNA damage, spindle configuration, embryonic aneuploidy, and genomic imprinting as compared with fresh and slow-freezing procedures, respectively.

Conclusions:

Both embryo and oocyte vitrifications are safe applications in female fertility preservation.

Effect of vitrification on the microRNA transcriptome in mouse blastocysts

Vitrification is commonly used in the cryopreservation of mammalian blastocysts to overcome the temporal and spatial limitations of embryo transfer. Previous studies have shown that the implantation ability of vitrified blastocysts is impaired and that microRNAs (miRNAs) regulate the critical genes for embryo implantation. However, little information is available about the effect of vitrification on the miRNA transcriptome in blastocysts. In the present study, the miRNA transcriptomes in fresh and vitrified mouse blastocysts were analyzed by miRNA Taqman assay based method, and the results were validated using quantitative real-time PCR (qRT-PCR). Then, the differentially expressed miRNAs were assessed using the Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases. Overall, 760 known mouse miRNAs were detected in the vitrified and fresh mouse blastocysts. Of these, the expression levels of five miRNAs differed significantly: in the vitrified blastocysts, four miRNAs (mmu-miR-199a-5p, mmu-miR-329-3p, mmu-miR-136-5p and mmu-miR-16-1-3p) were upregulated, and one (mmu-miR-212-3p) was downregulated. The expression levels of all miRNAs measured by the miRNA Taqman assay based method and qRT-PCR were consistent. The four upregulated miRNAs were predicted to regulate 877 candidate target genes, and the downregulated miRNA was predicted to regulate 231 genes. The biological analysis further showed that the differentially expressed miRNAs mainly regulated the implantation of embryos. In conclusion, the results of our study showed that vitrification significantly altered the miRNA transcriptome in mouse blastocysts, which may decrease the implantation potential of vitrified blastocysts.

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.

Supplementing culture and vitrification-warming media with l-ascorbic acid enhances survival rates and redox status of IVP porcine blastocysts via induction of GPX1 and SOD1 expression

The present study sought to determine the effect of adding l-ascorbic acid (AC) to (1) in vitro culture medium and (2) vitrification and warming solutions on redox status and developmental ability and quality of IVP porcine embryos. In both experiments, embryo quality was analysed in terms of total cell number (TCN), DNA fragmentation, intracellular peroxide levels and expression of three oxidative stress-related genes: glutathione peroxidase 1 (GPX1), superoxide dismutase 1 (SOD1) and 2 (SOD2). In the first experiment, fresh blastocysts were found to upregulate SOD1 expression when cultured with medium supplemented 100 μM AC. No differences were found between culture groups in the other analysed parameters. In the second experiment, blastocysts cultured with or without AC were divided into two groups: vitrified and warmed with solutions containing 0 or 100 μM AC. Addition of AC during culture and vitrification-warming upregulated the expression of GPX1 and SOD1 genes, enhanced survival rates and decreased peroxide levels at 24h post-warming. In addition, peroxide levels were negatively correlated with relative GPX1- and SOD1-transcript abundances, whereas GPX1 was positively correlated with embryo survival at 24h post-warming. No effects of AC-supplementation were seen for TCN, DNA fragmentation or relative SOD2-transcript abundance in vitrified blastocysts. In conclusion, the addition of AC to culture and vitrification-warming media increases gene expression of antioxidant enzymes SOD1 and GPX1. This appears to improve redox balance and is suggested to ultimately enhance embryo cryosurvival.

A randomized controlled trial comparing two vitrification methods versus slow-freezing for cryopreservation of human cleavage stage embryos

Purpose

To compare two different vitrification methods to slow freezing method for cryopreservation of human cleavage stage embryos. Design: Prospective randomised trial. Setting: University assisted reproduction centre. Patient(s): 568 patients (mean age 33.4 ± 5.2) from April 2009 to April 2011.

Methods

1798 supernumerary good-quality cleavage stage embryos in 645 IVF cycles intended to be cryopreserved were randomly allocated to three groups: slow freezing, vitrification with the Irvine® method, vitrification with the Vitrolife® method. Main Outcome Measure(s): Embryo survival and cleavage rates, implantation rate.

Results

A total of 1055 embryos were warmed, 836 (79.2 %) survived and 676 were finally transferred (64.1 %). Post-warming embryos survival rate was significantly higher after vitrification (Irvine: 89.4 %; Vitrolife: 87.6 %) than after slow freezing (63.8 %) (p < 0.001). No differences in survival rates were observed between the two vitrification methods, but a significant higher cleavage rate was observed using Irvine compared to Vitrolife method (p < 0.05). Implantation rate (IR) per embryo replaced and per embryo warmed were respectively 15.8 % (41/259) and 12.4 % (41/330) for Irvine, 17.0 % (40/235) and 12.1 % (40/330) for Vitrolife, 21.4 % (39/182) and 9.9 % (39/395) for slow-freezing (NS).

Conclusions

Both vitrification methods (Irvine and Vitrolife) are more efficient than slow freezing for cryopreservation of human cleavage stage embryos in terms of post-warming survival rate. No significant difference in the implantation rate was observed between the three cryopreservation methods.

Ultra-high cooling rate utilizing thin film evaporation

This research introduces a cell cryopreservation method, which utilizes thin film evaporation and provides an ultra-high cooling rate. The microstructured surface forming the thin film evaporation was fabricated from copper microparticles with an average diameter of 50 μm. Experimental results showed that a cooling rate of approximately 5[Formula: see text]10(4) °C/min was achieved in a temperature range from 10 °C to -187 °C. The current investigation will give birth to a cell cryopreservation method through vitrification with relatively low concentrations of cryoprotectants.

Outcomes of day 3 embryo transfer with vitrification using Cryoleaf: a 3-year follow-up study

OBJECTIVE

To compare success rates of vitrified-warmed with fresh and frozen-thawed ETs

DESIGN

Retrospective.

SETTING

Public fertility center.

PATIENT(S)

Cryopreserved- thawed/warmed ETs were included in this study. Fresh cycles, in which supernumerary embryos were cryopreserved, were set as the fresh control group.

INTERVENTION(S)

Supernumerary day 3 embryos were cryopreserved by slow-freezing or vitrification and transferred after thawing or warming.

MAIN OUTCOME MEASURE(S)

Comparison of two cryopreservation techniques with respect to post-thaw survival of embryos, implantation and pregnancy rates, neonatal outcome, and congenital birth defects.

RESULTS

A total of 962 fresh, 151 freezing-thawed and 300 vitrified-warmed cycles were included in this study. The survival and intact cell rates in the vitrification group were significantly higher compared with those in the slow freezing group (88.5 % vs 74.5 % and 86.6 % vs 64.0 %). The implantation, clinical pregnancy and live birth rates of the vitrification group were similar to the fresh and significant higher than slow freezing group. There were no significant differences in mean gestational age, birth weight, stillbirth, birth defects and the prevalence of neonatal diseases among three groups.

CONCLUSION

Vitrified-warmed ETs yield comparable outcomes with fresh ETs and is superior to frozen-thawed ETs regarding the survival rate and clinical outcomes.

Effect of vitrification on promoter methylation and the expression of pluripotency and differentiation genes in mouse blastocysts

The present study was designed to determine the effects of vitrification on promoter methylation and the expression levels of pluripotency and differentiation genes in mouse blastocysts. Promoter region CpG methylation patterns and the expression levels of octamer-binding transcription factor (Oct4), Nanog homeobox (Nanog), caudal-type homeobox 2 (Cdx2), and heart and neural crest derivatives-expressed transcript 1 (Hand1) were analyzed in fresh and vitrified mouse blastocysts. Methylation was measured by bisulphate mutagenesis and sequencing; gene expression was determined by real-time reverse transcription-PCR. The results showed that vitrification significantly reduced the methylation levels of the Oct4 (85% vs. 62.5%), Nanog (77.5% vs. 55%), and Cdx2 promoters (4.6% vs. 1.4%; P < 0.05) in mouse blastocysts, which correlated with increased expression of Oct4 and Nanog in vitrified blastocysts. Hand1 promoter methylation was not significantly different in the fresh (17.9%) versus vitrification group (21.4%; P > 0.05). The expression levels of Cdx2 and Hand1 were not significantly different in fresh and vitrified blastocysts. In conclusion, vitrification significantly decreased Oct4, Nanog, and Cdx2 promoter methylation in mouse blastocysts, which correlated with increased expression of Oct4 and Nanog.

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.

[Outcome of embryo vitrification compared to slow freezing process at early cleavage stages. Report of the first French birth]

OBJECTIVES

Since the end of 2010, France by "l'Agence de Biomédecine" has validated the embryo vitrification procedure as an improvement of the slow freezing method. We presented here data concerning biological and clinical outcomes from a prospective observational study where early cleavage stage good quality embryos were vitrified and warmed. We compared these results to those of a retrospective series where embryos were thawed after a slow freezing procedure (SF). We report also the first French live birth following embryo vitrification.

PATIENTS AND METHODS

In all, 58 cycles of frozen-thawed embryo transfers (FET) following vitrification were prospectively included and compared with 189 FET from SF method. Primary end points were the (i) survival rate (SR) (% of embryos with ≥50% post-thaw intact blastomeres), (ii) intact survival rate (ISR) (% of embryos with 100% post-thaw intact blastomeres) and (iii) survival blastomeres index (SBI) (% of post thaw intact blastomeres per survival embryo). Secondary end point was the clinical pregnancy rate (CPR) defined as the presence of an intra-uterine gestational sac with positive foetal heart beat. We report here the first French live birth following embryo vitrification.

RESULTS

In all, 87 and 412 embryos have been thawed following vitrification and SF, respectively. We observed a highly significant increase of SR, ISR et SBI respectively when thawing concerned vitrified embryos rather than those from SF method (98.3±13.1% vs. 77.3±32.0%, P<10(-4); 88.2±28.3% vs. 47.7±41.4%, P<10(-4); 97.7±6.1% vs. 87.3±14.4%, P<10(-4)). Furthermore, CPR were of 32.7% (19/58) and of 18.5% (35/189) following FET performed after vitrification or SF and thawing (P=0.03), respectively. The live birth of two healthy girls occurred following a caesarean section after 38 weeks of amenorrhea the 8th of August 2011.

DISCUSSION AND CONCLUSION

We experienced in our study that the post-thaw survival of vitrified embryos was significantly better than those of embryos resulting from SF. Then, a better CPR per thawed embryo cycle was observed following vitrification.

Cryotop vitrification as compared to conventional slow freezing for human embryos at the cleavage stage: survival and outcomes

OBJECTIVE

This study was conducted to compare the efficacy of cryotop vitrification of human cleavage-stage embryos to that of conventional slow freezing of these embryos with respect to survival. A second objective was to compare the two cryopreservation techniques with respect to outcomes for a cohort of women.

MATERIALS AND METHODS

Cleavage-stage embryos from 102 patients were cryopreserved either by vitrification (57 patients) or by traditional slow freezing (45 patients). After thawing, rates of embryo survival, implantation, and clinical pregnancy were determined.

RESULTS

Survival of embryos was significantly higher with the vitrification procedure as compared to traditional slow freezing [287/298 (96.3%) vs. 294/446 (65.9%); p < 0.05). Rates of implantation and clinical pregnancy were also significantly higher using vitrification procedure as compared to the slow freezing procedure (24.3% vs. 7.1% and 35.6% vs. 15.6% respectively, p < 0.05).

CONCLUSION

As compared to conventional slow freezing, cryopreservation of human cleavage-stage embryo using vitrification results in higher rates of embryo survival, implantation, and clinical pregnancy. Vitrification therefore represents the superior cryopreservation technique for cleavage-stage embryos.

Cryopreservation of mammalian embryos: Advancement of putting life on hold

Rodent transgenesis and human-assisted reproductive programs involve multistep handling of preimplantation embryos. The efficacy of production and quality of results from conventionally scheduled programs are limited by temporal constraints other than the quality and quantities of embryos per se. The emergence of vitrification, a water ice-free cryopreservation technique, as a reliable way to arrest further growth of preimplantation embryos, provides an option to eliminate the time constraint. In this article, current and potential applications of cryopreservation to facilitate laboratory animal experiments, colony management, and human-assisted reproductive programs are reviewed. Carrier devices developed for vitrification in the last two decades are compared with an emphasis on their physical properties that infer cooling rate of samples and sterility assurance. Biological impacts of improved cryopreservation on preimplantation embryos are also discussed.

Human oocyte cryopreservation: comparison between slow and ultrarapid methods

The success of reproductive technologies is facilitated by the cryopreservation of embryos and gametes. In Italy, where legislation prohibits zygote and embryo cryopreservation, clinics have extensively introduced oocyte cryopreservation. Two different strategies of oocyte cryopreservation are available: slow freezing or ultrarapid cooling (vitrification). Although the results are very encouraging with both methods, there is still controversy regarding both the procedure itself and the most suitable method to use. This study reports the routine application of the two different oocyte cryopreservation methods in programmes running in two consecutive periods. The study centre carried out 286 thawing cycles for a total of 1348 thawed oocytes cryopreserved by the slow-freezing method and 59 warming cycles for a total of 285 warmed oocytes cryopreserved by vitrification. Comparison of the outcomes obtained with the slow-freezing method versus vitrification in women who underwent IVF for infertility showed survival, fertilization, pregnancy and implantation rates of 57.9% versus 78.9% (P < 0.0001), 64.6% versus 72.8% (P = 0.027), 7.6% versus 18.2% (P = 0.021) and 4.3% versus 9.3% (P = 0.043) respectively. These results suggest that oocyte vitrification is associated with a better outcome than the slow-freezing method.

Factors affecting the outcome of human blastocyst vitrification

With single blastocyst transfer practice becoming more common in ART, there is a greater demand for a convenient and reliable cryostorage of surplus blastocysts. Vitrification has emerged in the last decade as an alternative promising substitute for slow freezing. Blastocysts represent a unique challenge in cryostorage due to their size, multicellular structure and presence of blastocoele. The continuous acquisition of experience and introduction of many different technological developments has led to the improvement of vitrification as a technology and improved the results of its application in blastocyst cryostorage. The current information concerning safety and efficacy of the vitrification of blastocysts will be reviewed along with the variables that can impact the outcome of the procedure.

Vitrification versus slow freezing gives excellent survival, post warming embryo morphology and pregnancy outcomes for human cleaved embryos

PURPOSE

The objective of this retrospective study was to evaluate the efficacy of vitrification and slow freezing for the cryopreservation of human cleavage stage embryos in terms of post-warming survival rate, post-warming embryo morphology and clinical outcomes.

METHODS

The embryos of 305 patients at cleavage stages were cryopreserved either with vitrification (153 patients) or slow-freezing (152 patients) methods. After warming; the survival rate, post-warmed embryo morphology, clinical pregnancy and implantation rates were evaluated and compared between the two groups.

RESULT(S)

In the vitrification group versus slow freezing group, the survival rate (96.9% vs. 82.8%) and the post-warmed excellent morphology with all blastomeres intact (91.8% vs. 56.2%) were higher with an odds ratio of 6.607 (95% confidence interval; 4.184-10.434) and 8.769 (95% confidence interval; 6.460-11.904), respectively. In this group, the clinical pregnancy rate (40.5% vs. 21.4%) and the implantation rate (16.6% vs. 6.8%) were also higher with an odds ratio of 2.427 (95%confidence interval; 1.461-4.033) and 2.726 (95% confidence interval; 1.837-4.046), respectively.

CONCLUSION(S)

Vitrification in contrast to slow freezing is an efficient method for cryopreservation of human cleavage stage embryos. Vitrification provides a higher survival rate, minimal deleterious effects on post-warming embryo morphology and it can improve clinical outcomes.