Relative contributions of anovulation and luteal phase defect to the reduced pregnancy rate of breastfeeding women

Objective: To evaluate the contribution of anovulation and luteal phase defects to lactational infertility. Design: Prospective longitudinal follow- up. Setting: Outpatient clinic. Subjects: Forty-nine women fully nursing and amenorrheic on day 75 postpartum and 25 cycling, interval non-nursing women. Interventions: Plasma prolactin, luteinizing hormone, estradiol (E₂), and progesterone (P) levels twice a week up to the second postpartum menses. Main Outcome Measures: Ovulation rate and endocrine profile of the menstrual cycles. Results: Ovulation rates were 37% and 97% at 6 and 12 months postpartum; 67% of ovulations occurred in amenorrhea. The luteal phase was shorter, and E₂ and P levels were lower in lactating women than in non- nursing women. These parameters were closer to normal in the second cycle than the first, in spite of active nursing. The risk of ovulation and pregnancy in amenorrhea was 27.7% and 0.9% at month 6 postpartum. After the first menses, these risks were 93% and 7%, respectively. Conclusion: The abnormal endocrine profile of the first luteal phase offers effective protection to women who ovulate during lactational amenorrhea within the first 6 months after delivery. Later luteal phases are improved and women are at risk of pregnancy.