Objective To examine ovarian function in relation to parity and time since last birth. Design Cross-sectional study. Setting Health-care program in California. Patient(s) 346 naturally cycling women, aged 18 to 39 years. Intervention(s) None. Main Outcome Measure(s) Mean follicular urinary estradiol metabolite concentration (E1C) (cycle days −8 to −1), mean luteal progesterone metabolite concentration (PdG) (days 0 to +10), and cycle phase lengths in ovulatory cycles. Result(s) After the women had collected daily urine samples for up to eight menstrual cycles, we measured the E1C and PdG using enzyme-linked immunoassay. The cycle phase lengths were calculated from the hormone profiles and daily diaries. Women who had given birth within the previous 3 years had lower E1C than the nulliparous women and women who last given birth >3 years earlier. Among the parous women, E1C was positively associated with the time since last birth. Women who last gave birth >3 years earlier had longer follicular phases than the nulliparous women. There were no associations between parity and PdG or luteal phase length. Conclusion(s) Our cross-sectional data suggest that ovarian function differs in nulliparous and parous women and is positively associated with the time since last birth. Longitudinal research is needed to explore within-woman changes in ovarian function prepartum and postpartum.