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.