The contraceptive efficacy of breastfeeding was assessed in 236 healthy urban women who
were followed at monthly intervals during the first postpartum year. Proportional hazard
models were used to evaluate the influence of time postpartum, menstrual status and
breastfeeding pattern upon the risk of pregnancy. Time and menstrual status had a highly
significant effect on this risk. Those women who remained in amenorrhea had cumulative
probabilities of pregnancy of 0.9% and 17% at 6 and 12 months postpartum, respectively. In …