OBJECTIVE: To determine if an increase in plasma P occurring before hCG administration might impair the outcome of IVF-ET. DESIGN: Five hundred eighty-five IVF-ET cycles were prospectively studied for the occurrence of plasma P elevation before hCG administration. SETTING: Tertiary institution, IVF-ET program, Hopital A. Beclere. PATIENTS: Participating patients included IVF-ET candidates 23 to 42 years of age only, excluding the couples in whom a male factor was a primary or an accessory cause of infertility. MAIN OUTCOME MEASURES: To clarify the practical consequences on IVF-ET outcome of pre-hCG increases in plasma P, we studied 585 consecutive IVF-ET cycles. These were divided into two groups according to plasma P levels observed on the day of hCG administration; plasma P of 0.9 ng/mL (2.9 nmol/L) was taken as an arbitrary cutoff value. Group A included 485 IVF cycles in which plasma P was < or = 0.9 ng/mL (2.9 nmol/L); group B included the remaining 100 cycles in which plasma P was > 0.9 ng/mL (2.9 nmol/L). RESULTS: The number of mature oocytes retrieved, the oocyte cleavage rate, and the number of embryos obtained were similar in groups A and B. In contrast to this apparent similarity in oocyte quality, a decrease in pregnancy rate (PR) and a trend for a decrease in embryo implantation rate were observed in group B in comparison with group A. CONCLUSIONS: The similar fertilization and cleavage rates obtained in groups A and B suggest that pre-hCG elevation in plasma P does not lead to decreased oocyte quality. Yet the lower PR observed when plasma P rises prematurely suggests that the prolonged but discrete elevation in plasma P occurring in these cases might alter endometrium receptivity to embryo implantation.