Between April 1981 to March 1984, 419 urban middle class postpartum women entered the Natural Family Planning (NFP) program of the Pontificia Universidad Catolica de Chile. This NFP program teaches the Ovulation Method (Billings). Only 1.9% of the women did not learn how to recognize the mucus pattern of fertility awareness. The sample of 378 women who were practicing the method to avoid a pregnancy completed 4,935 months of use of the OM. The cumulative life table unplanned pregnancy rate at the 12th postpartum month was 11.1 +/- 1.9 and the Pearl Rate was 12.1 per 100 woman-years. The Pearl Rate calculation of method-related failure was only 2.1 pregnancies per 100 woman-years. The breastfeeding group showed a significantly lower rate of unplanned pregnancies than the nonbreastfeeding group and there was no significant increase in unplanned pregnancy at the time of menstruation among previously amenorrheic women as compared to later intervals. The protection against unplanned pregnancy shown in this study should be viewed as the combination of two factors: breastfeeding and the Ovulation Method (OM) of NFP.