Follicle recruitment determines IVF productivity rate via the number of embryos frozen and subsequent transfers

Abstract

IVF productivity rate is an index defined as the sum of all live births from either fresh or frozen embryo transfers arising from a single oocyte collection. This retrospective analysis over 9 continuous years used this index to understand the potential impact on pregnancy rates of milder stimulation regimens with associated reduced egg numbers. The productivity rate per collection increased in a linear and significant rate as more oocytes were recovered, more embryos frozen and more frozen embryo transfers contributed to pregnancy. This observation was true for women aged <35years and less so for women aged 35–39years but not for women aged 40years and older. The contribution of frozen embryo transfer to the productivity rate rose in a linear manner, reaching over 40% of all live births with nine oocytes. The number of live births per oocyte, pronuclear embryos and thawed embryos decreased significantly but the number of live births per embryo transferred (fresh or frozen) rose with rising oocyte numbers, reflecting increasing opportunity for embryo selection. This study suggests that optimal benefits with minimal risks are gained from a model that includes both fresh and frozen transfers under stimulation generating between 8 and 12 eggs.Most of the costs and risks associated with an IVF cycle occur during the stimulation, egg collection and subsequent transfer of fresh embryos and ignore or treat separately the outcome from any subsequent frozen embryo transfers. We have used the term ‘productivity rate’ (or cumulative pregnancy rate per collection cycle) that includes outcomes from both fresh and frozen transfers as a tool to explore the impact of oocyte numbers on the ultimate chance of pregnancy. While the pregnancy rate for individual fresh and frozen transfers were largely unrelated to oocyte numbers, the cumulative chance of pregnancy rose significantly with the number of oocytes recovered. This was due to the increasing contribution of frozen embryo transfers to the overall chance of pregnancy producing more than half of all pregnancies above five oocytes. While the fertilization rate and embryo quality was independent of egg numbers, increasingly more embryos needed to be used to generate sufficient good-quality embryos for transfers as the egg numbers rose. The study found that 8–12 oocytes provided the highest cumulative chance of pregnancy with the least risk of either failed fertilization or ovarian hyperstimulation syndrome and this range could be used as a target number for individualized stimulation regimens.