OBJECTIVE: To investigate whether the consequences of premature P elevation on IVF-ET outcome are modulated by the quality of the ovarian response to controlled ovarian hyperstimulation (COH). DESIGN: Retrospective analysis. SETTING: Assisted Reproduction Unit, Clamart, France. PATIENT(S): One thousand twelve women undergoing 1,189 IVF-ET cycles. INTERVENTION(S): Patients underwent COH with a time-released GnRH agonist and hMG. The ovarian response to COH was classified as strong (< or = 50 hMG ampules, peak E2 levels > 2,500 pg/mL, and > or = 10 mature oocytes; n = 340), weak (> 50 hMG ampules, peak E2 levels < or = 1,500 pg/mL, and < or = 5 mature oocytes; n = 285), or intermediate (remaining cases; n = 564). The IVF-ET outcome in each group was analyzed according to whether or not plasma P levels exceeded 0.9 ng/mL. MAIN OUTCOME MEASURE(S): Pregnancy rates (PRs). RESULT(S): Clinical PRs were similar irrespective of low or high P levels in the strong (30% and 34%, respectively) and intermediate (31% and 30%, respectively) groups. However, in the weak group, P levels > 0.9 ng/mL were associated with lower PRs (3.2% and 23%, respectively). CONCLUSION(S): In the presence of an adequate response to COH, P levels > 0.9 ng/mL were not associated with lower PRs, indicating that good embryo quality may compensate for the adverse endometrial effects of P. Conversely, when the response to COH was weak, premature P elevation led to drastically reduced PRs.