['To evaluate the association between serum P levels on the day of hCG administration and pregnancy outcome in women undergoing controlled ovarian hyperstimulation, prevention of premature ovulation by GnRH analogues, and fresh ET after 5 days of embryo culture.', 'Retrospective, observational, cohort study.', 'Private IVF center.', 'A total of 2,555 women undergoing fresh ET on day 5 in 2,062 GnRH agonist and 493 GnRH antagonist cycles.', 'None.', 'Live birth rate.', 'Live birth rate in cycles with GnRH agonists was significantly lower in women with P levels ≥ 2.0 ng/mL (17.4%) on the day of hCG administration as compared with women with P levels <1.5 ng/mL (24.6%) and 1.5-1.99 ng/mL (26.7%). No such significant differences in live birth rates in cycles with GnRH antagonist could be observed.'] Is subtle progesterone rise on the day of human chorionic gonadotropin administration associated with lower live birth rates in women undergoing assisted reproductive technology : a retrospective study with 2,555 fresh embryo transfers?