Vaginal microbiota alterations under supraphysiological estradiol state during in vitro fertilization-embryo transfer (IVF-ET) and the association with reproductive outcomes

Supraphysiological estradiol levels induced by IVF-ET significantly alter vaginal microbiota and shift the CSTs in Chinese infertile women toward patterns of most Asian women. The Lactobacillus dominance under supraphysiological estradiol conditions does not help improve assisted reproductive outcomes. The abundance of Streptococcus, Atopobium, and Bifidobacterium on the day of embryo transfer may serve as predictors for adverse pregnancy outcomes. Among them, Streptococcus correlates positively with peak estradiol levels and may act as a microbial mediator impairing reproductive success under hyperestrogenic conditions. However, further larger-scale researches are needed to identify and elucidate the potential mechanisms.