Cumulative pregnancy rates of two strategies: Day 3 fresh embryo transfer followed by Day 3 or Day 5/6 vitrification and embryo transfer: a randomized controlled trial

Abstract STUDY QUESTION Are cumulative pregnancy rates better if supernumerary embryos are vitrified on Day 5/6 instead of Day 3? SUMMARY ANSWER The results do not show a significant difference in cumulative pregnancy rates between the Day 3 and Day 5/6 vitrification groups. WHAT IS KNOWN ALREADY Pregnancy and live birth rates following IVF or ICSI treatment are higher after extended embryo culture and blastocyst transfer (Day 5/6) compared to cleavage-stage (Day 3) transfer. Cumulative pregnancy rates from one oocyte retrieval (OR) cycle show no significant difference after fresh and frozen embryo transfers, but only one study has used vitrification for the cryopreservation of supernumerary embryos while four studies have used a slow freezing protocol. STUDY DESIGN, SIZE, DURATION Our prospective randomized controlled trial was performed in an academic centre between January 2018 and August 2020. Patients were randomized into vitrification Day 3 (n = 80) or Day 5/6 (n = 81) groups. The primary outcome was the cumulative ongoing pregnancy rate (cOPR), considering only the first pregnancy for each couple. The power calculation revealed that 75 patients were required in each group, when assuming a 50% cOPR with four embryo transfers in the vitrification Day 3 group vs two transfers in the vitrification Day 5/6 group. PARTICIPANTS/MATERIALS, SETTING, METHODS Patients <38 years undergoing their first or second OR cycles were randomized at the start of the first cycle. Up to two cycles were included in the analysis. A fresh embryo transfer was performed on Day 3. Supernumerary embryos (with ≥6 cells, <25% fragmentation, and equal blastomeres) or blastocysts (with expansion grade ≥2 with inner cell mass and trophectoderm score A/B) were vitrified on Day 3 or Day 5/6, respectively, and then transferred at a later date. A time-to-event analysis was performed with the patient's first ongoing pregnancy as the event of interest and the number of embryo transfers as the time component. The statistical comparison was performed by a Cox proportional hazards model. Cumulative costs of vitrification on Day 3 vs Day 5/6 were explored and compared using Mann–Whitney U tests. MAIN RESULTS AND THE ROLE OF CHANCE By December 2021, 233 transfers (96 fresh and 137 frozen) in 77 patients were performed in the vitrification Day 3 group and 201 transfers (88 fresh and 113 frozen) in 77 patients were performed in the vitrification Day 5/6 group. The time-to-event analysis did not show a difference between the two arms with regard to the patient's first ongoing pregnancy as the primary study outcome (hazard ratio [HR] 1.25, 95% CI 0.82; 1.92, P = 0.30). The cumulative ongoing pregnancy rate after eight transfers (from one or two ORs) was 57% in the vitrification Day 3 group vs 58% in the vitrification Day 5/6 group. The median number of embryo transfers until a pregnancy was achieved was five vs four, respectively, in the vitrification Day 3 group vs the Day 5/6 group. Similar results were found for the secondary study outcome, i.e. clinical pregnancy with foetal heart rate (HR 1.19, 95% CI 0.78; 1.80, P = 0.41). The cumulative clinical pregnancy rate (cCPR) after eight embryo transfers was 62% in the vitrification Day 3 group vs 59% in the vitrification Day 5/6 group. The median number of transfers until a pregnancy was achieved was four in both groups. The healthcare consumption pattern differed between the two groups and we observed higher costs for the vitrification Day 3 group compared to the vitrification Day 5/6 group, although these differences were not statistically significant. LIMITATIONS, REASONS FOR CAUTION Although our power calculation revealed that only 75 patients were needed in each study group (β = 0.87, α < 0.05), the numbers were low. Also, different numbers of single and double embryo transfers were performed between the two groups, which may have affected the results. The cost analysis was performed on a subset of the patients and is therefore exploratory. WIDER IMPLICATIONS OF THE FINDINGS Our study shows no difference in the cumulative pregnancy rate nor costs after fresh and frozen embryo transfers of at most two sequential OR cycles between the Day 3 and Day 5/6 vitrification groups; however, obstetric and perinatal outcomes should be taken into account to determine the best strategy. STUDY FUNDING/COMPETING INTEREST(S) This study was funded as an investigator-sponsored study of S.D. by Merck nv/sa Belgium, an affiliate of Merck KGaA, Darmstadt, Germany, and by Gedeon Richter Benelux (PA18-0162). The authors declare no conflict of interest related to this study. TRIAL REGISTRATION NUMBER NCT04196036. TRIAL REGISTRATION DATE 15 January 2018. DATE OF FIRST PATIENT’S ENROLMENT 15 January 2018.