OBJECTIVE: To investigate the association between the probability of pregnancy and hormone exposure between the end of menstruation and the day of triggering final oocyte maturation (menstruation-free interval).DESIGN: Prospective study.SETTING: University.PATIENT(S): One hundred women (aged ≤ 39 years) stimulated with a fixed dose of recombinant follicle-stimulating hormone (200 IU).INTERVENTION(S): Daily gonadotropin-releasing hormone antagonist (GnRH, 0.25 mg) used from day 6 of stimulation onward, final oocyte maturation triggered by administration of 10,000 IU of human chorionic gonadotropin (hCG) as soon as ≥ 3 follicles ≥ 17 mm were present, and hormone assessment performed at initiation of stimulation, on the first day after menstruation had stopped, on the day of antagonist initiation, and on the day of hCG administration.MAIN OUTCOME MEASURE(S): The association between hormone exposure during the menstruation-free interval and the probability of ongoing pregnancy.RESULT(S): The exposure to progesterone during the menstruation-free interval was statistically significantly higher in patients who did not become pregnant compared with those who did (4.20 ± 2.54 vs. 3.13 ± 1.14, respectively). Binary logistic regression confirmed the adverse effect of the increased exposure to progesterone for the achievement of pregnancy.CONCLUSION(S): In recombinant follicle-stimulating hormone/gonadotropin-releasing hormone antagonist in vitro fertilization/intracytoplasmic sperm injection cycles, a lower probability of pregnancy is associated with a higher exposure to progesterone during the menstruation-free interval.