Oocyte or embryo number needed to optimize live birth and cumulative live birth rates in mild stimulation IVF cycles.

Research Question: How many oocytes or embryos are needed to optimize the live birth rate (LBR) per cycle and cumulative LBR (CLBR) following mild stimulation IVF (MS-IVF) in women with uncompromised ovarian reserve?

Design: Retrospective analysis of a 4-year database of five fertility centres. The study population included women with normal/high ovarian reserve, who underwent autologous MS-IVF (daily ≤150 IU gonadotrophin) with fresh and subsequent frozen embryo transfer(s) (FET) from surplus embryos. Only the first cycle of each patient was included. Cycles with >150 IU daily average of gonadotrophin were excluded. 'Freeze-all embryo' (FAE) cycles were analysed separately.

Results: A total of 862 consecutive cycles fulfilled the inclusion criteria; 592 were eligible for fresh embryo transfer, 239 had non-elective 'freeze-all' cycles. Median age (25-75th percentile) of women who had fresh embryo transfer was 35 (32-37) years, median antral follicle count 19 (14-28) and anti-Müllerian hormone 19.2 (13-28.9) pmol/l. LBR/fresh cycle and CLBR inclusive of FAE cycles in the <35, 35-37, 38-39 and 40-42 year age groups were 37.8% and 45.1%, 36.0% and 41.6%, 18.4% and 29.1%, and 8.9% and 18.1%, respectively. The LBR following fresh embryo transfer plateaued after nine oocytes (40.3%) or four embryos (40.8%). The CLBR optimized when 12 oocytes (42.9%) or nine embryos (53.8%) were obtained. The LBR per oocyte peaked in women under 35 years when <5 oocytes were retrieved (11.4%), then declined with age and with higher oocyte yield. There were no cases of severe ovarian hyperstimulation syndrome (OHSS).

Conclusion: Nine oocytes, or four embryos, can optimize fresh transfer cycle LBR in MS-IVF. The CLBR are optimized with 12 oocytes, or nine embryos in predicted normal responders, while safeguarding against OHSS.