P-241 Cumulative live birth rates after embryo transfers at Day-2/3 versus Day-5/6: a comparative nationwide cohort study
Authors: Fauque P , Zebina A , Guérif F 
Published: June 2023

Abstract

Study question
Are there significant differences between cumulative live birth rates (CLBR) after a short and an extended embryo culture when comparisons are performed per cycle?

Summary answer
The CLBR per cycle was significantly lower in the Day-5/6 group and higher per embryo transfer as compared with the Day-2/3 group.

What is known already
Previous evidence has shown the probability of live birth following a fresh embryo transfer in In Vitro Fertilization (IVF) is higher after extended embryo culture (E-EC: Day-5/6) than after a shorter embryo culture (S-EC: Day-2/3). A further controversy literature has arisen on cumulative pregnancy rates, related to the higher risks of embryo transfer cancellation after E-EC compared with S-EC. Moreover, all of these studies used slow freezing/thawing embryo protocols with a higher survival for Day-2/3 embryos than Day-5/6 embryos. Finally, a recent study combining vitrification highlighted CLBR improvements after E-EC but CLBR were expressed per embryo transfer, not per cycle.

Study design, size, duration
A French national register study including all IVF cycles with at least one cleaved embryo at day-2 performed in France from 01/2016 to 12/2018 was carried out (n = 165,808 cycles), after exclusion of gamete donation cycles, attempts with preimplantation genetic diagnosis, surgical spermatozoa, viral context, and for which the embryo freezing was performed by slow-freezing. We chose to include only cycles from IVF centers with standardized CLBR similar or above to the national mean (99%CI).

Participants/materials, setting, methods
From the biomedicine Agency’s registry, two groups were identified: cycles with embryo(s) at Day-2/3 with/without embryo transfer (S-Group) and cycles with all embryos cultured to Day-5/6 with/without embryo transfer (E-Group). The CLBR defined as delivery of at least one live-born infant in fresh or in subsequent frozen-thawed cycles were compared. Only the first delivery was considered. Regression model adjusted for women age, parity, Day-2 embryos, IVF-method and attempt rank was used.

Main results and the role of chance
In total, 48,293 attempts fulfilled the inclusion criteria. Among them, 31,769 cycles were included in the S-Group and 16,524 in the E-Group. Overall, the CLBR per cycle was significantly lower in the E-Group compared with the S-G group (32.9% versus 33.5%; aOR: 0.89, 95%CI [0.86-0.93], p&lt;.0001). A significant higher rate of fresh embryo transfer cancellation was observed in in the E-Group compared with the S-Group (19.8% versus 3.1%, p &lt; .0001). However, when we performed further comparisons restricted to data from IVF centers with LBR strictly significantly above to the national mean LBR, the CLBR were similar between both groups (36.4% versus 38.5% in S-Group and E-Group, respectively; aOR: 0.97 [0.90-1.04], p = 0.402).
Finally, the CLBR in E-group was significantly higher than in S-group when expressed per embryo transfer (aOR: 1.15 [1.10-1.21], p&lt;.0001).

Limitations, reasons for caution
Main limitations are: the retrospective design and the 1 year follow-up that could not include all frozen embryo cycles performed.

Wider implications of the findings
Overall, the nationwide results per cycle suggest that extended embyo culture until blastocyst stage, even used in combination with vitrification, could not improve live birth rates.

Trial registration number
not applicable