BackgroundThe objective of this study was to explore whether the duration of LFEP (late-follicular elevated progesterone) affected pregnancy outcomes in IVF (in vitrofertilization) patients treated with pituitary downregulation protocols.MethodPatients with their first IVF/ICSI cycles between January 2016 and December 2016 were included. LFEP was set either at P > 1.0ng/ml or P > 1.5ng/ml. Clinical pregnancy rate was compared among three different groups (no LFEP; LFEP for 1 day; LFEP for ≥ 2 days). Then multivariate logistic regression analysis was performed to explore the influencing factors of clinical pregnancy rate.ResultsThis retrospective analysis involved 3,521 first IVF/ICSI cycles with fresh embryo transfers. Clinical pregnancy rate was the lowest in patients with a LFEP duration of ≥ 2 days, irrespective of whether LFEP was defined as P > 1.0 ng/ml (68.79% vs. 63.02% vs. 56.20%;P= 0.000) or as P > 1.5 ng/ml (67.24% vs. 55.95% vs. 45.51%;P= 0.000). In addition, LFEP duration was significantly associated with clinical pregnancy outcomes in unadjusted logistic regression analysis. However, in multivariate regression models after adjusting confounders, adjusted OR for LFEP duration (≥ 2 days) in the two models was 0.808 (P= 0.064; LFEP as P > 1.0 ng/ml) and 0.720 (P= 0.098; LFEP as P > 1.5 ng/ml), respectively.ConclusionLFEP adversely affects clinical pregnancy outcomes. However, the duration of LFEP seems to have no influence on the clinical pregnancy rate in pituitary downregulation treatment cycles.