Fresh vs. frozen: pregnancy outcomes and treatment efficacy between fresh embryo transfer vs. untested freeze-all cycles.

Zoran J. Pavlovic: Writing – review & editing, Writing – original draft, Validation, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Gabrielle E. Smotrich: Writing – review & editing, Writing – original draft, Formal analysis, Data curation. Erika P. New: Writing – review & editing, Writing – original draft, Methodology, Conceptualization. Samad Jahandideh: Writing – review & editing, Writing – original draft, Validation, Methodology. Kate Devine: Writing – review & editing, Writing – original draft, Resources, Methodology. Anthony N. Imudia: Writing – review & editing, Writing – original draft, Methodology, Data curation. Shayne Plosker: Writing – review & editing, Writing – original draft, Validation, Methodology, Investigation, Conceptualization.

As depicted in Table 1 , baseline demographic and stimulation cycle variables differed between the cohort of women whose first autologous ET was a fresh transfer and the cohort of women whose first autologous ET was a frozen transfer after embryo freeze-all. The fresh ET cohort was older by 4 months, had a higher BMI, a lower AMH, a lower AFC, and a higher basal FSH concentration than the FET cohort. During the ovarian stimulation cycle, the peak E2 and number of oocytes retrieved were lower in the fresh ET cohort compared with the freeze-all followed by FET cohort. The average progesterone level on the day of trigger in patients having fresh ET was 1.11 ng/mL, whereas this was <1.0 ng/mL on the day of final lining check in patients having FET. As also illustrated in Table 1 , more couples had male factor infertility, and fewer had ovulatory disorder, in the fresh ET cohort compared with the FET cohort. Table 1 Demographics characteristics and primary infertility diagnosis of all autologous in vitro fertilization fresh and freeze-all cycles without preimplantation genetic testing for aneuploidy from January 1, 2015 to December 31, 2020. Variables a Fresh embryo transfers (n = 6,755) FET from freeze-all without PGT-A (n = 1,564) P value Age 32.73 (3.82) 32.26 (3.50) <.001 Body mass index (kg/m 2 ) 26.70 (5.59) 25.90 (5.70) <.001 Median antimüllerian hormone (ng/mL) 2.93 (3.64) 3.96 (4.88) <.001 Median antral follicle count (AFC) 16 (13) 21 (15) <.001 Basal FSH (mIU/mL) 8.01 (3.62) 7.39 (2.60) <.001 Progesterone level on day of trigger for fresh and final lining check for FET (ng/mL) 1.11 (0.59) 0.60 (0.24) <.001 Peak estradiol during ovarian stimulation cycle 2,881 (1,923) 3,045 (2,267) <.001 Median number of oocytes retrieved during ovarian stimulation cycle 14 (12) 18 (15) <.001 Primary fertility diagnosis  Diminished ovarian reserve 413 (6.1%) 75 (4.8%) .052  Endometriosis 321 (4.8%) 75 (4.8%) .995  Male infertility 1,967 (29.1%) 386 (24.7%) <.001  Ovulation disorder 993 (14.7%) 363 (23.2%) <.001  Tubal factor 703 (10.4%) 154 (9.8%) .541  Uterine factor 108 (1.6%) 28 (1.8%) .669  Unknown 1,701 (25.2%) 357 (22.8%) .056  Other 549 (8.1%) 126 (8.1%) .967 Note: FET = frozen embryo transfer; FSH = follicle-stimulating hormone; PGT-A = preimplantation genetic testing for aneuploidy. a Numbers reported as mean (SD) or median (interquartile range) for applicable variables. Demographics characteristics and primary infertility diagnosis of all autologous in vitro fertilization fresh and freeze-all cycles without preimplantation genetic testing for aneuploidy from January 1, 2015 to December 31, 2020. Note: FET = frozen embryo transfer; FSH = follicle-stimulating hormone; PGT-A = preimplantation genetic testing for aneuploidy. Numbers reported as mean (SD) or median (interquartile range) for applicable variables. The overall LBR among the patients who had a fresh ET was 43.9% compared with 45.9% among those with FET of untested embryo after freeze-all. There was no significant difference in the overall positive pregnancy test, CPR, miscarriage rate, and LBR between the two groups after adjusting for confounders ( Fig. 2 ). To evaluate if patient’s age has any impact on the result of these clinical outcomes, the cycles were further stratified into different age groups of 40. It was found that patients’ positive pregnancy test, CPR, miscarriage rate, and LBR among the different age groups were not significantly different irrespective of whether the patient had fresh ET or FET of untested embryo ( Fig. 2 ). Figure 2 Adjusted pregnancy outcome of all autologous IVF fresh and freeze-all cycle without PGT-A from January 1, 2015 to December 31, 2020. Generalized estimating equations (GEEs) were used to adjust for demographic variables that had significant differences between groups including age, BMI, AFC, basal FSH, progesterone on day of trigger or day of final lining check, peak E2 during IVF stimulation cycle, number of oocytes retrieved during ovarian stimulation cycle, and primary diagnosis. Frozen embryo transfer without PGT-A includes all ages that are within each respective E2 cutoff group. AFC = antral follicle count; aOR = adjusted odds ratio; BMI = body mass index; CI = confidence interval; ET, embryo transfer; E2 = estradiol; FET = frozen embryo transfer; FSH = follicle-stimulating hormone; IVF = in vitro fertilization; PGT-A = preimplantation genetic testing for aneuploidy. Figure 2 was created using BioRender. Adjusted pregnancy outcome of all autologous IVF fresh and freeze-all cycle without PGT-A from January 1, 2015 to December 31, 2020. Generalized estimating equations (GEEs) were used to adjust for demographic variables that had significant differences between groups including age, BMI, AFC, basal FSH, progesterone on day of trigger or day of final lining check, peak E2 during IVF stimulation cycle, number of oocytes retrieved during ovarian stimulation cycle, and primary diagnosis. Frozen embryo transfer without PGT-A includes all ages that are within each respective E2 cutoff group. AFC = antral follicle count; aOR = adjusted odds ratio; BMI = body mass index; CI = confidence interval; ET, embryo transfer; E2 = estradiol; FET = frozen embryo transfer; FSH = follicle-stimulating hormone; IVF = in vitro fertilization; PGT-A = preimplantation genetic testing for aneuploidy. Figure 2 was created using BioRender. The LBRs in patients having fresh ET with periovulation trigger E2 level <4,000 pg/mL, 4,000–4,999 pg/mL, and ≥ 5,000 pg/mL were 43.6%, 48.2%, and 40.5%, respectively, which is clinically and statistically comparable with the overall LBR of 45.9% in patients who had FETs of untested embryo after freeze-all. As each E2 cutoff group contains a varying range of patient ages within each respective cutoff category, the corresponding “all ages” FET without PGT-A testing outcome were used for statistical analysis and comparison for the different E2 cutoffs. The positive pregnancy test, CPR, and miscarriage rate after fresh ET at these different peak stimulation E2 concentrations were also not significantly different from the clinical outcomes of those who had FETs of untested embryos ( Table 2 ). Table 2 Pregnancy outcome of all autologous in vitro fertilization fresh and freeze-all cycles without preimplantation genetic testing for aneuploidy from January 1, 2015 to December 31, 2020, based on peak E2 level during stimulation. Outcome a Fresh embryo transfers (n = 6,755) FET without PGT-A (n = 1,564) Adjusted OR (95% CI); P value Positive pregnancy test All 4,134 (61.2%) 1,101 (70.4%) 1.01 (0.94–1.08); .76 <4,000 3,144 (60.6%) 0.99 (0.92–1.07); .68 4,000–4,999 628 (65.9%) 1.03 (0.95–1.09); .84 ≥5,000 362 (58.6%) 1.04 (0.90–1.22); .64 Clinical pregnancy rate All 3,547 (52.5%) 938 (60.0%) 1.00 (0.93–1.07); .94 <4,000 2,710 (52.3%) 1.02 (0.94–1.11); .72 4,000–4,999 532 (55.8%) 0.92 (0.86–1.07); .55 ≥5,000 305 (49.4%) 0.94 (0.89–1.02); .51 Miscarriage rate All 566 (8.4%) 191 (12.2%) 0.99 (0.87–1.12); .73 <4,000 439 (8.5%) 1.02 (0.85–1.15); .78 4,000–4,999 73 (7.7%) 1.00 (0.87–1.17); .81 ≥5,000 54 (8.7%) 0.97 (0.84–1.13); .76 Live birth rate All 2,967 (43.9) 718 (45.9%) 1.00 (0.93–1.08); .97 <4,000 2,258 (43.6) 0.95 (0.83–1.03); .91 4,000–4,999 459 (48.2) 0.98 (0.89–1.11); .95 ≥5,000 250 (40.5) 1.01 (0.97–1.23); .86 Note: Generalized estimating equations were used to adjust for demographic variables that had significant differences between groups including age, BMI, AFC, basal FSH, progesterone on day of trigger/day of final lining check, peak E2 during IVF stimulation cycle, number of oocytes retrieved during ovarian stimulation cycle, and primary diagnosis. AFC = antral follicle count; BMI = body mass index; CI = confidence interval; E2 = estradiol; FET = frozen embryo transfer; FSH = follicle-stimulating hormone; IVF = in vitro fertilization; OR = odds ratio; PGT-A = preimplantation genetic testing for aneuploidy. a Numbers reported as mean (SD) or median (interquartile range) for applicable variables. Pregnancy outcome of all autologous in vitro fertilization fresh and freeze-all cycles without preimplantation genetic testing for aneuploidy from January 1, 2015 to December 31, 2020, based on peak E2 level during stimulation. Note: Generalized estimating equations were used to adjust for demographic variables that had significant differences between groups including age, BMI, AFC, basal FSH, progesterone on day of trigger/day of final lining check, peak E2 during IVF stimulation cycle, number of oocytes retrieved during ovarian stimulation cycle, and primary diagnosis. AFC = antral follicle count; BMI = body mass index; CI = confidence interval; E2 = estradiol; FET = frozen embryo transfer; FSH = follicle-stimulating hormone; IVF = in vitro fertilization; OR = odds ratio; PGT-A = preimplantation genetic testing for aneuploidy. Numbers reported as mean (SD) or median (interquartile range) for applicable variables.

After an Institutional Review Board approval for this study was obtained, a retrospective cohort analysis of pregnancy outcomes, in the absence of PGT-A, was performed in patients undergoing their first autologous single blastocyst transfer at Shady Grove Fertility Reproductive Science Centers between January 1, 2015 and December 31, 2020. A total of 8,319 embryo transfers were included in the final cohort and analyzed. Two treatment pathways were assessed. The first pathway (n = 6,755) consisted of patients whose first ET after oocyte retrieval was an elective single fresh blastocyst ET performed 5 days after egg retrieval. The second pathway (n = 1,564) consisted of patients whose first ET after retrieval was an FET of an elective single blastocyst after blastocyst embryo freeze-all. Patient selection and exclusion that lead to the final pathway numbers are listed in Figure 1 . Fresh transfers were not PGT-A tested as rapid turnaround genetic testing is not used by our practice. All transferred blastocysts had inner cell mass and trophectoderm grades of BB or better at the time of fresh ET or vitrification ( 16 ). The decision to perform a fresh ET vs. freeze-all followed by FET was at the discretion of the primary fertility provider, based on assessment of risk for ovarian hyperstimulation, elevated progesterone concentrations, and/or blastulation occurring after day 5 after retrieval, reflecting “real-life” current clinical practice ( 8 ). Although few fresh transfer cycles were cancelled because of a patient not having at least a 3BB embryo to transfer, the exact number of such cycle cancellations is unknown. All patients included in this study had more than one embryo to transfer whether their first transfer was a fresh transfer or an FET after elective freeze-all, and all underwent a single elective embryo transfer ( Fig. 1 ). Figure 1 Flowchart representation of patient selection via inclusion and exclusion criteria for fresh and freeze-all cycles. A total of 40,429 patients were initially excluded if the fresh or frozen embryo transfer (FET) was not the patient’s first transfer or was not autologous. An additional 34,957 patients were excluded if the transfer used an embryo that underwent preimplantation genetic testing. Finally, 9,230 patients were excluded from the study if the transfer was not an elective single embryo transfer (eSET), meaning the patient did not have at least two usable blastocysts available for transfer. PGT-A = preimplantation genetic testing for aneuploidy. Flowchart representation of patient selection via inclusion and exclusion criteria for fresh and freeze-all cycles. A total of 40,429 patients were initially excluded if the fresh or frozen embryo transfer (FET) was not the patient’s first transfer or was not autologous. An additional 34,957 patients were excluded if the transfer used an embryo that underwent preimplantation genetic testing. Finally, 9,230 patients were excluded from the study if the transfer was not an elective single embryo transfer (eSET), meaning the patient did not have at least two usable blastocysts available for transfer. PGT-A = preimplantation genetic testing for aneuploidy. Controlled ovarian stimulation consisted of mixed follicle-stimulating hormone (FSH)/luteinizing hormone protocols under gonadotropin-releasing hormone (GnRH) antagonist or GnRH agonist pituitary suppression. Frozen embryos underwent vitrification after stimulation followed by oocyte retrieval and insemination based on previous established methods ( 17 , 18 ). In general, oral contraceptive treatment was initiated 21 days before stimulation unless contraindicated or not tolerated. For GnRH antagonist cycles, Ganirelix acetate or Cetrorelix acetate (0.25 mg) was initiated when the lead follicle was 12–14 mm in size. For GnRH agonist cycles, 20 units of leuprolide acetate (LA) (Lupron) were initiated during the last 3 days of oral contraceptives. The LA dose was decreased to five units when ovarian suppression was confirmed with ultrasound and serum E2 <5 pg/mL. Ovarian stimulation was achieved by employing recombinant FSH and human menopausal gonadotropin. When most of the lead follicles were ≥18 mm, final oocyte maturation was triggered with 5,000–10,000 IU of human chorionic gonadotropin (hCG), or with LA (4 mg) in some of the GnRH antagonist freeze-all cycles as indicated. Leuprolide acetate triggers in fresh transfers were combined with 1,500 IU of hCG for luteinization support. Serum E2 and serum P levels were obtained on the day of trigger. Oocyte retrieval occurred 36 hours later, and fertilization was achieved with conventional IVF or intracytoplasmic sperm injection, as clinically indicated. All embryo cryopreservation was performed by vitrification at the blastocyst stage. For endometrial preparation in FET cycles, all of which were programmed/medicated/controlled cycles in this study, patients underwent ovarian and uterine suppression using combined hormonal oral contraceptive pills unless contraindicated or not tolerated. After baseline hormone assessment and a transvaginal ultrasound to document no functional ovarian cysts and a thin endometrium, the patients were started on oral micronized E2 (2 mg three times daily), or intramuscular E2 valerate (4–6 mg every 3 days) based on previously published protocols and physician preference ( 19 ). Intravaginal micronized E2 was added where required to achieve an endometrial thickness of 7 mm or greater ( 20 ). Once adequate endometrial thickness was achieved, patients were started on intramuscular progesterone (50 mg daily) in oil, or a combination of intramuscular progesterone (50 mg) in oil every 3 days with intravaginal progesterone (200 mg) (Endometrin; Ferring Pharmaceuticals Inc, Parsippany, NJ) twice daily. Embryo transfers were performed under ultrasound guidance. In patients having fresh transfer, the ET occurred on post-retrieval day 5 and daily progesterone supplementation, with intramuscular progesterone (50 mg) in oil, was initiated in the evening on the day of the retrieval. In the FET arm, previously vitrified embryos were warmed on the day of the FET, and the FET occurred 123 ± 3 hours after progesterone initiation. Serum hCG levels were assessed at 9–11 days after ET, and repeated as needed to assess trend when the initial hCG was positive, which in this practice is considered to be a hCG level >5. In fresh transfers we deferred our first hCG measurement to at least 17 days after hCG trigger to minimize the probability of having residual hCG in the serum. Ultrasound confirmation of a gestational sac and ascertainment of viability were obtained in all pregnant patients between 6 and 7 weeks estimated gestational age. Baseline characteristics including age, body mass index (BMI) (kg/m 2 ), antimüllerian hormone (AMH), antral follicle count (AFC), basal FSH, peak E2 during IVF stimulation cycle, the number of oocytes retrieved during stimulation, and the primary infertility diagnosis were examined. The primary outcome variable was live birth rate (LBR). Secondary outcome measures included positive pregnancy test, miscarriage, and clinical pregnancy rates (CPRs). Because of conflicting data on the effects of supraphysiologic E2 concentrations on implantation and pregnancy rates ( 21 , 22 , 23 ), additional analysis of outcomes was performed comparing the fresh ET cohort according to three peritrigger peak E2 strata of <4,000 pg/mL, 4,000–4,999 pg/mL, and ≥5,000 pg/mL to outcomes in the FET cohort. Although there is a no recommended peak E2 threshold to absolutely avoid fresh transfer, most fertility providers are usually reluctant to perform fresh ET in patients with levels ≥4,000 pg/mL due to concerns of OHSS and adverse obstetric outcomes with resulting pregnancy ( 9 ). However, some fresh transfers occurred with levels ≥4,000 pg/mL during this study period due to certain physician practice patterns and after an informed decision was made between the patient and their physician. Descriptive statistics were used to document the mean and SD or median and interquartile range for continuous variables, whereas categorical variables were expressed as case number and percentages. To determine the differences between the groups, parametric ( t test) and nonparametric analyses (Mann-Whitney U test) were performed after normality analysis. The differences in rates of outcomes and categorical parameters were compared with χ 2 test and Fisher’s exact test where appropriate. Clinical outcomes of interest were modeled with the use of generalized estimating equations to adjust for demographic variables that had significant differences between groups such as age, BMI, AFC, basal FSH, progesterone on day of trigger/day of final lining check, peak E2 during IVF stimulation cycle, number of oocytes retrieved during ovarian stimulation cycle, and primary diagnosis. Descriptive analysis and generalized estimating equation modeling steps were performed using the R statistical computing system (version 3.6.3), and the add-on R packages gee (v. 4.13-20), geepack (v. 1.3-1), csv (v. 0.5.5), tableone (v. 0.11.1), and tidyverse (v. 1.0.3). Statistical significance was defined as P <.05.

Results of this database study of 8,319 blastocyst embryo transfers do not support a universal elective freeze-all policy to improve LBR in the absence of PGT-A that agrees with recent prospective and randomized control trials. Therefore, after an informed discussion, patients and fertility providers can elect to pursue either fresh ET or embryo freeze-all with subsequent FET during the first autologous IVF cycle in appropriately selected patient populations. These appropriately selected patient populations for a fresh embryo transfer include those who have a progesterone level <2 ng/mL, an E2 cutoff level that takes into consideration physician practice patterns and patient desires, informed patient consent, and a minimization of patient risks, as well as patients who have more than one embryo with a grade of at least 3BB by day 5 available for elective single embryo transfer after their first retrieval providing them with the opportunity to undergo an FET if necessary, among others. Given the recent interest in potential benefits of natural cycle FET, and the methodological limitations of current studies evaluating fresh ET vs. FET, additional research is warranted to further evaluate live births after natural cycle FET compared with programmed FET and fresh ET, and to evaluate the economic burden of a freeze-all approach in different healthcare models.

It was found that live birth, positive pregnancy test, miscarriage, and CPRs were similar in non-PGT-A autologous first single blastocyst embryo transfers after fresh ET and after FET. This finding was observed within all age strata. Additionally, periovulation trigger E2 level did not affect these outcomes. Greater than 90% of the fresh ET cohort had E2 concentrations <5,000 pg/mL and it has been standard of care in our practice to avoid fresh ET when progesterone level is elevated before egg retrieval ( 7 , 8 ). Although this study showed similar pregnancy outcomes between various E2 cutoffs, and therefore the E2 level within a fresh transfer cycle needs not serve as a direct impediment to fresh transfer when considering pregnancy outcomes, the risk of moderate-to-severe OHSS has been reported to be as high as 8.6% in some studies ( 24 ). Therefore, because of this elevated risk, the decision to perform a fresh transfer should be an informed and shared decision with the patient based on minimizing adverse outcomes such as OHSS while adhering to patient goals. The retrospective nature of this study allowed for inclusion of fresh transfers at E2 levels above that which most providers at the practice would perform a fresh transfer. These findings support the continued performance of fresh ET in appropriately selected autologous IVF cycles in women not undergoing PGT. Compared with spontaneous conceptions, IVF pregnancies conceived with either fresh ET or FET are associated with several adverse outcomes through different previously proposed mechanisms ( 14 , 25 , 26 , 27 ). Universal freeze-all for all patients, followed by FET in a more physiologic environment, has been proposed as a solution to improve some of these pregnancy outcomes, yet data supporting this approach are lacking. Similar to our findings, three recent European randomized control trials have failed to demonstrate improved LBRs when comparing fresh ET with elective freeze-all followed by FET ( 28 , 29 , 30 ). Specifically, the randomized control trials by Maheshwari et al. ( 30 ) demonstrated that elective freeze-all was not preferable to a fresh transfer, and should only be used for definite clinical indications. In a single-center prospective superiority trial, Wong et al. ( 28 ) randomly assigned 102 women to elective freeze-all followed by FET and 102 women to a planned fresh ET of blastocyst embryos. The cumulative LBR was not different between the two cohorts. After the first ET, LBR, ongoing pregnancy rate, CPR, and biochemical pregnancy rate were higher in the fresh ET group. The study was limited by the small number of subjects in each treatment arm. A multicenter prospective trial of 460 women in their first three IVF cycles demonstrated no difference in LBRs between women randomly assigned to fresh ET and those randomly assigned to elective freeze-all followed by FET of blastocyst embryos ( 29 ). Again, in the multicenter trial by Maheshwari et al. ( 30 ) involving 619 patients in which >90% of embryo transfers were blastocyst ET, the investigators similarly showed no difference in the likelihood of having a healthy infant (term singleton live birth of appropriate weight), live birth, or clinical pregnancy between the fresh ET and FET randomized groups. A systematic review and meta-analysis of nine randomized controlled trials looking at LBR as a primary outcome found a significantly higher LBR after elective FET in 2,676 patients compared with fresh ET in 2,703 patients (relative risk, 1.12; 95% confidence interval, 1.01–1.24), and a substantial reduction in the risk of moderate/severe OHSS in women undergoing elective FET compared with those undergoing fresh ET ( 2 ). However, subgroup analysis found that the improved LBR was limited to women with PCOS or hyper-response retrieved for 15 oocytes or more, women who underwent blastocyst ET and women who received luteal support with intramuscular progesterone during FET ( 2 , 31 , 32 , 33 , 34 , 35 ). There was no significant difference in LBR in non-PCOS/healthy responders, or in patients who did not undergo PGT-A as a component of treatment ( 2 , 31 , 32 , 33 , 34 , 36 , 37 , 38 ). The majority of patients in the meta-analysis, 4,941 of 5,379, underwent cleavage stage ET ( 2 , 31 , 33 , 34 , 36 , 37 , 38 ). Additionally, a large Cochrane Database analysis of 15 studies comparing a conventional method of fresh transfer first vs. a freeze-all approach found moderate quality evidence that showed one strategy was not superior to the other in terms of cumulative LBR ( 39 ). Limitations to this meta-analysis include the heterogeneity of the studies included, the overall moderate quality of evidence, the inability to measure time to pregnancy, and the inclusion of studies where patients underwent a fresh transfer followed by FET to measure cumulative LBR, whereas this study only looked at a patient’s first autologous transfer involving either a fresh transfer or FET. Despite these differences, the study by Zaat et al. ( 39 ) demonstrated that a fresh transfer is a viable option for the first transfer for many patients and is relatively equivalent to a freeze-all approach, which is in line with this study’s conclusions. In the absence of compelling evidence favoring fresh ET or FET on LBR, the effects of fresh ET and FET on other outcomes require consideration. Compared with fresh ET, the risk of preterm birth, low birth weight, and small for gestational age births appear to be attenuated after programmed FET. On the other hand, programmed FET increases the risk of large for gestational age and hypertensive disorders of pregnancy ( 11 , 40 , 41 , 42 , 43 , 44 , 45 ). More recently, it has been surmised that the presence of a corpus luteum may be beneficial in diminishing the occurrence of macrosomia and hypertension, and there has been increasing interest in the potential benefits of natural cycle FET. The biological premise is that the corpus luteum secretes vasoactive substances such as relaxin and vascular endothelial growth factor, which are important for initial placentation ( 46 ). The applicability of this study to modern IVF is limited by the time interval (2006–2014), and the fact that most embryos were cryopreserved using slow freezing at cleavage stage ( 47 ). There are other considerations that may factor into the decision to perform fresh ET, or embryo freeze-all followed by FET. A retrospective cost-benefit analysis of elective blastocyst FET compared with fresh blastocyst FET within the Italian public health system demonstrated that the cost per live birth was similar in both cohorts ( 48 ). A British study within the National Health Service found that elective freeze-all followed by FET was more costly when compared with fresh ET ( 30 ). A cost-benefit analysis in a Brazilian private practice IVF program found superior pregnancy rates and a lower total cost per pregnancy in a cohort of patients who underwent freeze-all and a subsequent FET vs. fresh ET of cleavage stage embryos ( 49 ). The main strengths of our study are the large number of study subjects, elective single embryo transfer exclusively at the blastocyst stage, limiting inclusion in the study to the first ever ET at our practice and adjustments for all available potential confounders. Our statistical analysis was able to show that unadjusted rates were higher in the FET group until AMH, AFC, number of oocytes retrieved, and diagnosis of ovulation disorder were adjusted for, which provides insight into the importance of these variables. Because of prior studies being underpowered, the goal of this large retrospective study was to add additional data to the literature to strengthen conclusions similar to those found here. This study had >90% power to detect significant differences in positive (559 cycles per group) and CPRs (920 cycles per group) in all ages if such a significance existed. However, to be adequately powered to detect significant difference in live birth (44% vs. 46%), there would need to be 9,733 cycles in each group, indicating a lack of clinical significance for a 2% difference in LBR. Additionally, the results support that in appropriately patient populations, such as patients who achieved at least a 3BB embryologic grade by day 5 of incubation, that fresh transfer for a patient’s initial cycle vs. elective freeze-all followed by FET can be appropriate after an informed discussion between the patient and the provider. The main limitations include the retrospective design of the study with its inherent biases, the observed differences in median AMH, median AFC, median peak E2, and median number of oocytes retrieved that suggest that the freeze-all cohort had more robust ovarian reserve than the fresh ET group. It is possible that progesterone levels at the time of hCG or GnRH agonist trigger were higher in the freeze-all group, but this value was only available for the fresh ET group. Endometrial thickness was also not a variable that was considered when deciding whether or not to perform a fresh transfer, and therefore these data were not available nor included in statistical analysis. Because this is a limitation of this retrospective study, future prospective studies could aim to control for endometrial thickness in both fresh and frozen transfers. Despite delaying the first hCG measurement to at least 17 days after hCG trigger in fresh transfers to minimize the probability of having residual hCG in the serum, some residual hCG could have led to differences in the biochemical pregnancy rate that were not accounted for. Additionally, the decision of whether a patient underwent a fresh transfer vs. FET was at the discretion of the primary physician based on various factors as listed previously. Although this reflects real-world clinical practice, discretionary use of fresh vs. frozen transfers introduces a selection bias that is influenced by individual practice patterns and could not be controlled for in this study. Further prospective studies utilizing randomization of the transfer type would help diminish this selection bias, but risk not being adequately powered or not being truly random, because both patients and physicians can have potential insights into which type of transfer was occurring.

Z.J.P. has nothing to disclose. G.E.S. has nothing to disclose. E.P.N. has nothing to disclose. S.J. has nothing to disclose. K.D. has nothing to disclose. A.N.I. has nothing to disclose. S.P. has nothing to disclose.