Ambient temperature exposure during oocyte development and subsequent assisted reproductive outcomes: Potentially susceptible period and embryo-related intermediate pathway

Ambient temperature is increasingly recognized as a climate-related environmental stressor with potential implications for human reproductive health; however, potentially susceptible exposure periods and intermediate pathways remain unclear. We aimed to identify potentially susceptible periods during assisted reproduction and explore whether changes in oocyte and embryo yield may contribute to associations between ambient temperature and reproductive outcomes. In this cohort study, 11,359 women undergoing their first embryo transfer cycle between 2018 and 2024 were included. Six predefined temperature exposure windows spanning different reproductive stages were assessed. Associations with laboratory outcomes were evaluated using generalized linear models, and pregnancy outcomes using multivariable logistic regression. Mediation analysis was conducted to explore whether high-quality embryo number may represent an intermediate pathway linking pre-oocyte retrieval temperature exposure with clinical pregnancy. Higher mean temperature during the pre-oocyte retrieval period was associated with lower numbers of retrieved oocytes and high-quality embryos, with less consistent associations for transferable embryos. Temperature exposure during post-transfer periods was not significantly associated with pregnancy outcomes. High-quality embryo number may partially explain the association between pre-oocyte retrieval temperature exposure and clinical pregnancy, although the mediation estimates were uncertain and should be interpreted cautiously. Overall, the pre-oocyte retrieval period may be a potentially susceptible period for temperature-related changes in oocyte and embryo yield, while evidence for downstream pregnancy outcomes was less consistent.