Twenty-nine breast-feeding mothers and 10 non-breast-feeding postpartum comparison mothers from a rural area of Mexico were followed longitudinally until ovulation resumed. A simple set of guidelines is described involving three obvious milestones for the breast-feeding mother to safely use the natural contraceptive benefit of breast-feeding. Those milestones are: the first vaginal bleeding episode, the initiation of supplementation, and the child's monthly birthday. In the absence of bleeding and supplementation, 100% of breast-feeding mothers remained anovular for 3 months postpartum, 96% for 4 months, 96% for 5 months, and 96% for 6 months. This suggests that, if a mother understands these three conditions, she can use breast-feeding alone as effectively as modern family planning methods for the prevention of pregnancy.