P-766 Re-assessment of the Belgian legislation on embryo transfer in a blastocyst transfer policy

Abstract Study question How does a multiple fresh blastocyst transfer policy impacts live birth rate (LBR) and multiple live birth rates (MLBR) in relation to maternal age ? Summary answer Fresh transferring of ≥ 2 blastocysts only increases LBR in women above the age of 36 and substantially increases risk of multiples in all age categories. What is known already Multiple pregnancies after ART are recognized as a major complication as it significantly contributes to extreme preterm and very low-birthweight births. Even in the presence of a strict Belgian embryo transfer law in some subgroups in which double embryo transfer (DET) is allowed (women aged <36 years in cycle rank 3 up to 6 and in women aged 36-40 years in cycle rank 2), twining rates remain above 10%. A study by Tannus et al., (2017) concluded that double blastocyst transfer (DBT) is associated with both higher LBR and higher twin birth rates compared to elective single blastocyst transfer (eSBT). Study design, size, duration A single center retrospective, observational cohort study was conducted between July 2010 and December 2020. Both fresh IVF and ICSI cycles were included. A total of 8,402 fresh transfers were performed (2,948 on day 3 and 5,454 on day 5) over the course of the study period. SET was performed in 6,108 cases, DET in 2,077 cases, triple embryo transfer (TET) in 202 cases and multiple embryo transfers (MET) in 15 cases. Participants/materials, setting, methods The primary outcomes of this study were LBR per transfer and twin pregnancy rate. Generalized estimating equations (GEE) with the patient as subject was applied to account for clustering of cycles within women. An interaction analysis between the number of embryos transferred and age categories in the day 5 group was performed. A P-value of < 0.05 was considered statistically significant. Main results and the role of chance In women aged <36 years, chances of a live birth were not significantly higher when two or more vs. a single blastocyst were transferred (OR: 1.014; 95%% CI 0.791 - 1.299; p = 0.91). However, transferring more than one blastocyst in women aged 36-40 years and >40 years did result in a higher chance of live birth compared to SBT (OR: 1.631; 95% CI: 1.218 - 2.185; p = 0.0008 and OR: 1.634; CI: 1.219-2.190; p = 0.0008, respectively).` Women aged 36-40 years and > 40 years had an estimated 19.3% and 66.3% lower odds respectively of a live birth after the transfer of a day 5 embryo, vs. women aged <36 years (OR: 0.800, 95% CI: 0.687-0.931; p = 0.0034 and OR: 0.337, CI: 0.268-0.424; p < 0.0001, respectively). Overall, live births were 28.5% higher when 2 or more blastocysts were transferred compared to an SBT (OR: 1.285 95% CI 1.1.077 – 1.533) (p = 0.0046). The estimated odds for a twin pregnancy were 17.8 times higher for DBT/MBT compared to SBT (OR 17.814; 95% CI: 10.74 - 29.55) (p < 0.0001). Limitations, reasons for caution This study is limited by its retrospective design and no interaction analysis between number of twin pregnancies and age categories was possible due to the low number of twins. Wider implications of the findings The data indicates that in women younger than 36 years only one blastocyst should be transferred regardless the embryo quality or the cycle rank. Only for patients older than 40 years, transfer of two blastocyst(s) could be considered. Trial registration number Not applicable