Frozen-thawed Single Embryo Transfer Research Articles

EFFECT OF RE-EXPANSION IN FROZEN-THAWED EMBRYO TRANSFERS ON PREGNANCY OUTCOMES

To evaluate the effect of blastocyst re-expansion on frozen-thawed embryos at time of transfer on pregnancy outcomes. Retrospective cohort study. Patients undergoing frozen-thawed embryo transfer (FET) from January 2017 to March 2020 in our center were included. Following a standard FET protocol, vitrified blastocysts were rewarmed on the day of ET and images of the thawed embryo were taken prior to transfer. Images were reviewed by at least two embryologists and the degree of expansion at least 1 hour post thaw at time of FETs was documented and included: 1) fully re-expanded 2) partially re-expanded 3) non re-expanded. Primary outcomes included: high quality embryo, defined by minimum grades of B for both trophectoderm and inner cell mass by Gardner’s blastocyst grading scale; pregnancy; clinical pregnancy; miscarriage; and, ongoing pregnancy and live birth. Statistical analyses were done using ANOVA for continuous data and Chi Square for dichotomous outcomes. A total of 645 single FET cycles were analyzed in the study. There were 301 fully re-expanded (46.7%), 274 partially re-expanded (42.4%), and 70 non re-expanded (10.9%) embryos at the time of transfer. The mean age, BMI, and endometrial thickness was not significantly different between the groups (p=NS). There was a significantly higher rate of embryo quality (56.8% and 58.0% vs. 44.3%; p=0.034), pregnancy rate (68.4% and 68.2% vs. 55.7%; p=0.048), clinical pregnancy rate (62.1% and 62.8% vs. 55.7%; p=0.030), and ongoing or live birth rate (50.2% and 51.8% vs. 38.6%; p=0.048) in frozen-thawed embryos that were fully or partially expanded at the time of transfer versus non re-expanded, respectively. No difference was noted in miscarriage rate between the groups (10.6%, 10.9%, 10.0%; p=0.9). The majority of thawed embryos were at least partially re-expanded (89.1%). The non re-expanded group has fewer high quality embryos (56.8% and 58.0% vs. 44.3%; p=0.034). When the high quality embryos were analyzed alone, there were no significant differences (p=NS) in pregnancy rates (70.2% vs. 72.5% vs. 64.5%), clinical pregnancy rates (64.9% vs. 67.5% vs. 54.8%), ongoing or live birth rates (52.6% vs. 58.8% vs. 41.9%) or miscarriage rate (10.0% vs. 8.8% vs. 12.9%) between the fully, partially, and non re-expanded groups, respectively. Transferring a non-re-expanded blastocyst, as compared to a fully or partially expanded blastocyst, at the time of FET is associated with a lower ongoing pregnancy and live birth rate in our study. However, embryo quality appears to have a greater effect on pregnancy outcomes than re-expansion, as there were no differences in outcomes between the groups when considering only high-quality blastocysts.

Preimplantation Genetic Testing for Aneuploidy Versus Morphology as Selection Criteria for Single Frozen-Thawed Embryo Transfer in Good-Prognosis Patients: A Multicenter Randomized Clinical Trial

(Abstracted from Fertil Steril 2019;112:1071–1079.e7) The rate of embryonic aneuploidy increases exponentially in women over the age of 35 years, coinciding with a dramatic decrease in the success of in vitro fertilization (IVF) in women of advanced maternal age. Despite advancements in preimplantation genetic testing for aneuploidy (PGT-A) including the introduction of next generation sequencing (NGS), morphologic assessment remains the primary method of embryo selection in IVF.

Is Embryo Cryopreservation Causing Macrosomia-and What Else?

The number of embryos transferred during an IVF cycle is directly related to the high incidence of multiple births, which is the culprit of perinatal morbidity. Therefore, single fresh embryo transfer (ET) strategy, or freeze-all, followed by a single frozen-thawed embryo transfer (FET) cycle, may dramatically reduce the rate of multiple births, without compromising the cumulative live birth rates (LBRs). A literature review was conducted for all available evidences assessing obstetrics and perinatal outcomes associated with FET compared to fresh ET and natural conception. While studies comparing fresh and FET cycles in normal responders have yielded conflicting results for pregnancy rate, FET was associated with lower risk of prematurity and low birth weight and increased risk of large for gestational age (LGA) and/or macrosomic in singletons, when compared with fresh ET. Macrosomic/LGA births have a higher risk of fetal hypoxia, stillbirth, shoulder dystocia, perineal lacerations, cesarean section, postpartum hemorrhage and neonatal metabolic disturbances at birth. Nonetheless, it seems that other than higher risk of fetal macrosomia, there are additional obstetric complications associated with FET. The relative risk of hypertensive disorders in pregnancy, as well as perinatal mortality were also demonstrated to be increased in FET compared with singletons from fresh ET and natural conception. Therefore, when considering elective freeze-all policy, in addition to LBR and the risk of ovarian hyperstimulation syndrome, physicians should consider the aforementioned increased FET cycles' pregnancy complications, including LGA/ macrosomia, hypertensive disorders of pregnancy, as well as perinatal mortality.

Preimplantation genetic testing for aneuploidy versus morphology as selection criteria for single frozen-thawed embryo transfer in good-prognosis patients: a multicenter randomized clinical trial

To evaluate the benefit of next-generation sequencing (NGS)-based preimplantation genetic testing for aneuploidy (PGT-A) for embryo selection in frozen-thawed embryo transfer. Randomized controlled trial. Not applicable. Women aged 25-40years undergoing IVF with at least two blastocysts that could be biopsied. Randomization for single frozen-thawed embryo transfer with embryo selection based on PGT-A euploid status versus morphology. Ongoing pregnancy rate (OPR) at 20weeks' gestation per embryo transfer. A total of 661 women (average age 33.7 ± 3.6years) were randomized to PGT-A (n = 330) or morphology alone (n = 331). The OPR was equivalent between the two arms, with no significant difference per embryo transfer (50% [137/274] vs. 46% [143/313]) or per intention to treat (ITT) at randomization (41.8% [138/330] vs. 43.5% [144/331]). Post hoc analysis of women aged 35-40years showed a significant increase in OPR per embryo transfer (51% [62/122] vs. 37% [54/145]) but not per ITT. PGT-A did not improve overall pregnancy outcomes in all women, as analyzed per embryo transfer or per ITT. There was a significant increase in OPR per embryo transfer with the use of PGT-A in the subgroup of women aged 35-40years who had two or more embryos that could be biopsied, but this was not significant when analyzed by ITT. NCT02268786.

Should single embryo transfer be used in patients with any kind of infertility factor? Preliminary outcomes.

Objective:Multiple embryos have been transferred to compensate for low implantation rates, which in turn, increase the likelihood of multiple pregnancies. Despite the publication of clinical guidelines and a reduction in the number of embryos transferred, double embryo transfer still is the most common practice. There is no clear evidence of who should receive the single embryo transfer (SET), and it is more commonly indicated for patients of good prognosis. However, it is not clear how much the presence of other infertility factors can affect the SET prognosis. The aim of this study was to evaluate differences in clinical pregnancy rates (CPR) of frozen-thawed SET cycles for women presenting with different infertility factors.Methods:Retrospective cohort study evaluating 305 frozen-thawed SET cycles performed in the last 10 years in a private IVF center. We included patients undergoing ovarian stimulation cycles, using ejaculated sperm and a frozen-thawed ET. Embryos were routinely vitrified and warmed up, and the blastocysts were transferred after endometrium preparation. The cycles were categorized according to the infertility factor classified by the Society for Assisted Reproductive Technologies (SART) as anatomic female factor (n=55), endocrine female factor (n=26), endometriosis (n=37), male factor (n=60), ovarian insufficiency (n=26), unexplained (n=24), multiple factors (n=45) and other (n=32). CPR were compared between the groups and the multivariate analysis was performed to evaluate the association of each infertility factor and the CPR, adjusted for confounders.Results:The women varied in age from 18 to 44 years (35.9±3.8), presented Body Mass Index of 22.4±3.1kg/m2, baseline serum FSH of 7.4±8.3 IU/ml, and had a mean of 11.0±8.4 MII oocytes recovered and 6.4±5.3 embryos cryopreserved. The CPR, according to infertility factors were: anatomic female factor (25.9%), endocrine female factor (30.8%), endometriosis (27.8%), male factor (20.7%), ovarian insufficiency (21.7%), unexplained (9.5%), multiple factors (17.1%) and other (20.7%). Multivariate analysis did not show significant association of infertility factors and CPR adjusted for confounders.Conclusions:Patients presenting different infertility factors seem to have a satisfactory CPR for a SET cycle, except those with unexplained infertility. This is a preliminary outcome and the number of patients by category is small; in addition, the retrospective characteristics of the study are its limitations. Overall, our findings suggest that patients presenting any infertility factor, except unexplained infertility, are suitable to receive a SET with satisfactory outcomes.

Assisted reproductive technology in Latin America: the Latin American Registry, 2017.

What was the utilization, effectiveness and safety of assisted reproductive technologies (ART) carried out in Latin America during 2017. Retrospective collection of multinational data on ART carried out in 188 institutions from 15 Latin American countries. In this study, 93,600 initiated cycles, 16,976 deliveries and 20,404 births reported. Utilization of ART was 221 cycles per million inhabitants (15 to 535). Women aged 40 years and above represented 30.5% of fresh IVF and intracytoplasmic sperm injection (ICSI); however, after removing freeze-all cycles, delivery rate per oocyte retrieval was 19.9% for ICSI and 20.2% for IVF. Overall, single embryo transfer (SET) represented 26.9% of fresh transfers, with 18.2% delivery rate per transfer, increasing to 32.3% in elective SET. Delivery rate in double embryo transfers (DET) was 28.3% increasing to 37.3% with elective DET. This 5% increment in births in elective DET over elective SET resulted in a tenfold increase in twin births, gestational periods almost 3 weeks' shorter and a threefold increase in perinatal mortality. Delivery rate in frozen-thawed SET reached 25.5% increasing to 30.8% with DET, most being blastocyst transfers. Of all births, 66.9% were singletons, 31.4% twins and 1.6% triplets and higher. Overall, preterm deliveries reached 9.5% in singletons, 64.3% in twins and 97.9% in triplets; perinatal mortality was 9.4‰ in singletons, 25.3‰ in twins and 63.3‰ in high-order multiples. The number of initiated cycles has slowly increased. Frozen embryo transfers, blastocyst transfers and SET are also increasing. Our data show that, especially in young women and oocyte recipients, when there is more than one blastocyst for transfer, elective SET should be the rule.

Developments and pregnancies of direct cleavage embryos

The introduction of time-lapse imaging has enabled dynamic embryo observations such as cleavage-type and development-speed determinations in addition to the conventional morphological assessments. Direct cleavage (DC) represents a chromosomal abnormality rather than a normal cleavage (NC) and reportedly affects embryo development. This study aimed to investigate whether DC embryos influence the outcomes of embryonic development and single frozen-thawed embryo transfer (SFET). A retrospective study. We targeted 2614 embryos in 670 cycles from patients who underwent conventional in vitro fertilization or intracytoplasmic sperm injection between January and April 2017. After insemination, embryos were incubated in an EmbryoScope+ system (Vitrolife, Sweden). DC embryos were classified into four groups as follows: group A, abnormal embryos between the one- and three-cell stages; group B, abnormal embryos between the one-cell stage and four-cell or later stages in the first cleavage division; group C, abnormal embryos between the two- and five-cell stages; and group D, abnormal embryos between the two-cell stage and six-cell or later stages in the second cleavage division. We compared the day 3 good-quality embryo, day 5 blastocyst formation, day 5 good-quality blastocyst, and pregnancy and abortion rates after single-cycle, freeze-thawed blastocyst transfer between the DC and NC groups. The proportion of DC embryos was 24.8%. The day 3 good-quality embryo, day 5 blastocyst, day 5 good-quality blastocyst rates were respectively 74.3%, 70.9%, and 47.1% for the NC groups; 50.5%, 32.8%, and 11.1% for group A; 25.3%, 11.9%, and 0.7% for group B; 74.8%, 51.5%, and 15.8% for group C; and 48.6%, 23.9%, and 1.5% for group D. The day 3 good-quality embryo rate was significantly lower in groups A, B, and D than in the NC groups. The day 5 blastocyst rate was significantly lower in group B than in the other groups. The day 5 good-quality blastocyst rate was significantly lower in the 4 DC groups than in the NC groups. The pregnancy and abortion rates after single frozen-thawed blastocyst transfer were respectively 35.0% and 23.7% for the NC groups, 13.2% and 0% for group A, 0% and 0% for group B, 13.2% and 20.0% for group C, and 7.9% and 66.7% for group D. These parameters showed no significant differences. This study showed that the DC embryos had significantly lower day 5 blastocyst and good-quality blastocyst rates than the NC embryos. However, the DC embryos had the potential of blastocyst development and pregnancy. Thus, we suggest that DC embryos developing into blastocysts could be selected for transfer.

Cost-effectiveness of single versus double embryo transfer in IVF in relation to female age

ObjectiveTo evaluate the cost-effectiveness of single embryo transfer followed by an additional frozen-thawed single embryo transfer, if more embryos are available, as compared to double embryo transfer in relation to female age. Study designWe used a decision tree model to evaluate the costs from a healthcare provider perspective and the pregnancy rates of two embryo transfer policies: one fresh single embryo transfer followed by an additional frozen-thawed single embryo transfer, if more embryos are available (strategy I), and double embryo transfer (strategy II). The analysis was performed on an intention-to-treat basis. Sensitivity analyses were carried out to evaluate the robustness of our model and to identify which model parameters had the strongest impact on the results. ResultsSET followed by an additional frozen-thawed single embryo transfer if available was dominant, less costly and more effective, over DET in women under 32 years. In women aged 32 or older DET was more effective than SET followed by an additional frozen-thawed single embryo transfer if available but also more costly. ConclusionSET followed by an additional frozen-thawed single embryo transfer should be the preferred strategy in women under 32 undergoing IVF. The choice for SET followed by an additional frozen-thawed single embryo transfer or DET in women aged 32 or older depends on individual patient preferences and on how much society is willing to pay for an extra child. There is a strong need for a randomized clinical trial comparing the cost and effects of SET followed by an additional frozen-thawed single embryo transfer and DET in the latter category of women.

The optimal endometrial preparation for frozen-thawed single embryo transfer in a natural cycle: a retrospective analysis

Single embryo transfer (SET) is the best method for preventing multiple births. The purpose of this study is to evaluate pregnancy outcomes with or without luteal phase support (LPS) of frozen-thawed single blastocyst transfer (SBT) treatment performed in natural cycles. The study retrospectively analyzed 966 frozen-thawed SBT in natural cycles with or without LPS performed between the periods January 2010 to April 2013. Oral chlomadinone acetate (Lutoral®) was given daily during the luteal phase. The outcomes of frozen-thawed SBT treatment including demographic data, clinical pregnancy rates and miscarriage rates, were compared with or without LPS. Ovulation was determined by the LH surge, progesterone (P4) levels and ultrasonography. And all patients underwent ovarian stimulation with human chorionic gonadotrophin (hCG). In order to determine whether the P4 levels after ovulation are associated with clinical pregnancy rate or miscarriage rate, we analyzed P4 levels 2 days after ovulation in 383 cycles out of 966cycles with or without LPS. Post ovulation P4 levels were divided into four groups (with (+), without (-) LPS): Low (L+, L-), 1.4-4.4ng/ml; Middle Low (ML+, ML-), 4.5-5.5ng/ml; Middle High P4 (MH+, MH-), 5.6-7.5ng/ml and High (H+, H-), 7.6-14.8ng/ml. There were no significant differences in the patients’ demographic data (age, basal FSH and embryo quality), clinical pregnancy rate (42.5% vs. 38.0%, p=0.150) and miscarriage rate (24.7% vs. 20.3%, P=0.296), between the groups. On the second comparison (383 cycles), a strong tendency of increased clinical pregnancy rates were observed in the high P4 levels group compared to the low P4 levels group (51.1% (H-) vs. 31.9%(L-), p = 0.059) and (42.1% (H+) vs. 28.0%(L+), p=0.131), but no difference in the miscarriage rates were noted (26.7% (H-) vs. 25.0% (L-), p=0.908) and (37.5% (H+) vs. 23.8% (L+), p=0.367). The pregnancy outcomes of natural cycle frozen-thawed SBT were similar regardless of LPS. Our results suggest that LPS with oral chlomadinone acetate may not improve the clinical pregnancy nor decrease miscarriage rate. Patients with low levels of P4 might need additional interventions in order to achieve higher pregnancy rates. Our findings suggest that 2-day post ovulation levels ≥ 7.6 ng/ml of P4 provide an optimal endometrial environment. However, we need further studies to determine the optimal treatment for patients with low P4 levels to undergo frozen thawed SBT in a natural cycle.

Validity for assisted hatching on pregnancy rate in assisted reproductive technology: analysis based on results of Japan Assisted Reproductive Technology Registry System 2010.

The aim of this study was to assess the efficacy of assisted hatching (AH) in assisted reproductive technology (ART) treatment. In this retrospective observational study, the data of patients who were registered in the National ART Registry System of Japan between January and December 2010 were analyzed. The descriptive statistics and validity of AH in fresh embryo transfer (ET) and frozen-thawed ET were assessed by using multiple logistic regression analyses. From a total of 105,450 single ET, 46,029 (43.7%) cycles underwent AH. A total of 9737 (21.3%) and 36,292 (60.9%) cycles underwent AH from 45,818 fresh single ET and 59,632 frozen-thawed single ET, respectively. In the fresh ET patients that underwent AH, the clinical pregnancy and live birth rate were significantly decreased in patients of all ages compared with that of the non-AH group. In the frozen-thawed ET patients, there was no significant difference in pregnancy and live birth rate between the AH group and the non-AH group. AH treatment was more frequently performed in frozen-thawed ET patients than in fresh ET patients, and in the blastocyst stage than in the early cleavage stage. A significantly decreased pregnancy and live birth rate was observed in the fresh ET patients who underwent AH. In the frozen-thawed ET patients who underwent AH, improvement in the clinical pregnancy and live birth rate was not observed. Further studies on the indication and application of AH in ART treatment are required.

The influence of the type of embryo culture medium on neonatal birthweight after single embryo transfer in IVF

Does the type of medium used to culture fresh and frozen-thawed embryos influence neonatal birthweight after single embryo transfer (SET) in IVF? A comparison of two commercially available culture media showed no significant influence on mean birthweight and mean birthweight adjusted for gestational age, gender and parity (z-scores) of singletons born after a fresh or frozen-thawed SET. Furthermore, we show that embryo freezing and thawing cycles may lead to a significantly higher mean birthweight. Animal studies have shown that culture media constituents are responsible for changes in birthweight of offspring. In human IVF, there is still little knowledge of the effect of medium type on birthweight. Until now, only a small number of commercially available culture media have been investigated (Vitrolife, Cook(®) Medical and IVF online medium). Our study adds new information: it has a larger population of singleton births compared with the previously published studies, it includes outcomes of other media types (HTF and Sage(®)), not previously analysed, and it includes data on frozen-thawed SETs. This study was a retrospective analysis of birthweights of singleton newborns after fresh (Day 3) or frozen-thawed (Day 5) SET cycles, using embryos cultured in either of two different types of commercially available culture media, between 2008 and 2011. Before January 2009, a single-step culture medium was used: human tubal fluid (HTF) with 4 mg/ml human serum albumin. From January 2009 onwards, a commercially available sequential medium was introduced: Sage(®), Quinn's advantage protein plus medium. Singletons born after a fresh SET (99 embryos cultured in HTF and 259 in Sage(®)) and singletons born after a frozen-thawed SET (32 embryos cultured in HTF only, 41 in HTF and Sage(®) and 86 in Sage(®) only) were analysed. Only patients using autologous gametes without the use of a gestational carrier were considered. Also excluded were (vanishing) twins, triplets, babies with congenital or chromosomal abnormalities and babies born before 22 weeks of gestation. Analysis of 358 singletons born after a fresh SET and 159 singletons born after a frozen-thawed SET showed no significant difference between the HTF and Sage(®) groups in terms of birthweight. Gestational age, parity and gender of the baby were significantly related to birthweight in multiple linear regression analyses, and other possible confounding factors included maternal age, BMI and smoking, the number of blastomeres in the transferred embryo and the type of culture medium. Maternal age, BMI and smoking, gestational age at birth, gender of the baby and the percentage of firstborns did not differ significantly between the HTF and Sage(®) groups; however, among the fresh embryos, those cultured in Sage(®) had significantly more blastomeres at the time of embryo transfer compared with the embryos cultured in HTF. Birthweights adjusted for gestational age and gender or gestational age and parity (z-scores) were not significantly different between the HTF and Sage(®) groups for fresh or frozen-thawed SETs. Mean birthweight, as well as the mean birthweight among firstborns and the mean birthweights adjusted for gestational age and gender or parity (z-scores) were significantly higher in the cryopreservation group compared with the fresh embryo transfer group. Our study is limited by its retrospective design and only two commercially available types of culture media were tested. More research is necessary to investigate the potential influence of culture media on gene expression. Although our data do not indicate the major influences of the HTF and Sage(®) culture media on birthweight, our results cannot be extrapolated to other culture media types. Furthermore, there remains a potential influence of embryo culture environment on epigenetic variation not represented by birthweight differences but by more subtle features.

Birth after TESE-ICSI in a man with hypogonadotropic hypogonadism and congenital adrenal hypoplasia linked to a DAX-1 (NR0B1) mutation

DAX1/NR0B1 mutations are responsible for X-linked congenital adrenal hypoplasia (AHC) associated with hypogonadotropic hypogonadism (HH). Few data are available concerning testicular function and fertility in men with DAX1 mutations. Azoospermia as well as failure of gonadotrophin treatment have been reported. We induced spermatogenesis in a patient who has a DAX1 mutation (c.1210C>T), leading to a stop codon in position 404 (p.Gln404X). His endocrine testing revealed a low testosterone level at 1.2 nmol/l (N: 12–40) with low FSH and LH levels at 2.1 IU/l (N: 1–5 IU/l) and 0.1 IU/l (N: 1–4 IU/l), respectively. Baseline semen analysis revealed azoospermia. Menotropin (Menopur®:150 IU, three times weekly) and human chorionic gonadotrophin (1500 IU, twice weekly) were used. After 20 months of treatment, as azoospermia persisted, bilateral multiple site testicular biopsies were performed. Histology revealed severe hypospermatogenesis. Rare spermatozoa were extracted from the right posterior fragment and ICSI was performed. Four embryos were obtained and, after a frozen–thawed single-embryo transfer, the patient's wife became pregnant and gave birth to a healthy boy. We report the first case of paternity after TESE–ICSI in a patient with DAX1 mutation, giving potential hope to these patients to father non-affected children. Furthermore, this case illustrates the fact that patients with X-linked AHC have a primary testicular defect in addition to HH.

Randomized single versus double embryo transfer: obstetric and paediatric outcome and a cost-effectiveness analysis

Transfer of several embryos after IVF results in a high multiple birth rate associated with increased morbidity and high costs for the neonatal care. In a previous randomized trial we demonstrated that a single embryo transfer (SET) strategy, including one fresh single embryo transfer and, if no live birth, one additional frozen-thawed SET, resulted in a live-birth rate that was not substantially lower than after double embryo transfer (DET) but markedly reduced the multiple birth rate. We compared costs for maternal health care and productivity losses and paediatric costs for the SET and DET strategies. In addition, maternal and paediatric outcomes between the two groups were compared. The SET strategy resulted in lower average total costs from treatment until 6 months after delivery. There were a few more deliveries with at least one live-born child in the DET group. The incremental cost per extra delivery in the DET alternative was high, 71 940. The rates of prematurely born and low birthweight children were significantly lower with the SET strategy. There were also markedly fewer maternal and paediatric complications in the SET group. The SET strategy is superior to the DET strategy, when number of deliveries with at least one live-born child, incremental cost-effectiveness ratio and maternal and paediatric complications are taken into consideration. The findings do not support continuing transfers of two embryos in this group of patients.

Blastomere Number for Day 3 Embryos Does Not Seem Important for Prognosticating Outcome Following Frozen Embryo Transfer Using a Single Embryo Model

Background and Significance: Higher pregnancy rates (PRs) have been found following fresh embryo transfer (ET) when more blastomeres are found in day 3 embryos.Objective: To evaluate pregnancy outcome in women having a single frozen-thawed ET according to blastomere number.Materials and Methods: So that the results could be compared to other data with fresh ETs, the only women included were those who had elevated day 3 serum FSH and were treated with no or minimal gonadotropins. The reason for cryopreserving the embryos were either for inadequate peri-ovulatory endometrial parameters (n=35) or those who were interested in stockpiling embryos but were unsuccessful in doing so in 1 or more retrievals (n=31). Embryos were cryopreserved using a simplified freezing protocol and a one-step removal of the cryoprotectant 1,2 propanediol for thawing. All women in the study had serum FSH >12 mIU/mL.Results: There were 2 delivered pregnancies in 26 transfers (9.2% per transfer) with <5 cell embryos vs. 3 of 40 (7.5%) for >6 cell embryos (p=NS). The implantation rates were 7.7% vs. 10.0% respectively.Conclusions: Blastomere number does not appear to be as important in frozen ET vs. fresh ones at least based on single embryo models in a poor prognosis group. Possibly the data would be different in a eugonadotropic group. The mechanism is not clear but one possibility is that failure to have a higher rate of cleavage for a fresh embryo may be related to the intrinsic quality of the embryo. In contrast, possibly some blastomeres become damaged in the thawing process leaving less blastomeres even if the embryos had good potential for higher cell numbers in the fresh state. Possibly damaging an early blastomere by freezing does not have a significant negative impact on embryo implantation similar to embryo biopsy for pre-implantation genetic diagnosis. Background and Significance: Higher pregnancy rates (PRs) have been found following fresh embryo transfer (ET) when more blastomeres are found in day 3 embryos. Objective: To evaluate pregnancy outcome in women having a single frozen-thawed ET according to blastomere number. Materials and Methods: So that the results could be compared to other data with fresh ETs, the only women included were those who had elevated day 3 serum FSH and were treated with no or minimal gonadotropins. The reason for cryopreserving the embryos were either for inadequate peri-ovulatory endometrial parameters (n=35) or those who were interested in stockpiling embryos but were unsuccessful in doing so in 1 or more retrievals (n=31). Embryos were cryopreserved using a simplified freezing protocol and a one-step removal of the cryoprotectant 1,2 propanediol for thawing. All women in the study had serum FSH >12 mIU/mL. Results: There were 2 delivered pregnancies in 26 transfers (9.2% per transfer) with <5 cell embryos vs. 3 of 40 (7.5%) for >6 cell embryos (p=NS). The implantation rates were 7.7% vs. 10.0% respectively. Conclusions: Blastomere number does not appear to be as important in frozen ET vs. fresh ones at least based on single embryo models in a poor prognosis group. Possibly the data would be different in a eugonadotropic group. The mechanism is not clear but one possibility is that failure to have a higher rate of cleavage for a fresh embryo may be related to the intrinsic quality of the embryo. In contrast, possibly some blastomeres become damaged in the thawing process leaving less blastomeres even if the embryos had good potential for higher cell numbers in the fresh state. Possibly damaging an early blastomere by freezing does not have a significant negative impact on embryo implantation similar to embryo biopsy for pre-implantation genetic diagnosis.