A longitudinal study of twenty-six breast-feeding and twelve non-breast-feeding postpartum women was conducted in Assiut, Egypt in order to determine the time that ovulation resumed after childbirth, and the effect of breast-feeding frequency on the period of lactational anovulation. Breastfeeding women experienced the onset of follicular development, vaginal bleeding, ovulation and pregnancy significantly later than women who did not breast-feed. Ovulatory and non-ovulatory breast-feeders reported similar frequencies of breast-feeding episodes. The introduction of dietary supplements commonly preceded ovulation. An algorithm using three simple variables observable to the breast-feeding mother was found to predict up to 100% of the first ovulations. All breast-feeding women who did not give supplements and did not have a vaginal bleeding episode by 6 months postpartum were anovular by strict criteria for ovulation. Ovulation did not precede bleeding or supplementation in the women who experienced these events before 6 months, yielding a highly effective formula for preventing unplanned pregnancy by the informed use of breast-feeding.