Seasonal variability does not impact in vitro fertilization success.

It is unknown whether seasonal variation influences the outcome of in vitro fertilization (IVF). Previous studies related to seasonal variation of IVF were all small sample size, and the results were conflicting. We performed a retrospective cohort study evaluating the relationship between seasonal variability and live birth rate in the year of 2014-2017. Patients were grouped into four seasons (Winter (December-February), Spring (March-May), Summer (June-August), and Autumn (September-November)) according to the day of oocyte pick-up (OPU). Multivariate logistic regression analysis was performed to evaluate association between seasonal variation and live birth. Models were adjusted for covariates including temperature, sunshine hour, infertility type, infertility duration, infertility factor and BMI. In total 38,476 women were enrolled, of which 25,097 underwent fresh cycles, 13,379 were frozen embryo transfer. Live birth rates of fresh embryo transfer were 50.36%, 53.14%, 51.94% and 51.33% for spring, summer, autumn and winter, respectively. Clinical pregnancy rate between the calendar months varied between 55.1% and 63.4% in fresh embryo transfer (ET) and between 58.8% and 65.1% in frozen embryo transfer (FET) (P-values 0.073 and 0.220). In the unadjusted model and adjust model, seasonal variation was not associated with live birth. In conclusion, there was no significant difference of seasonal variations in the outcome of IVF with fresh embryo transfer and frozen embryo transfer.