The use-effectiveness of natural methods of family planning in lactation is evaluated by comparing the incidence of unplanned pregnancies in a group of nursing mothers practising these methods with the incidences reported previously in surveys of breastfeeding women using no contraception. The complexity of the physiological processes involved in the resumption of ovulation after term pregnancy is discussed in relation to NFP and the problems encountered by its users in the context of normal family life. The serum prolactin and gonadotrophin levels are correlated with the postpartum interval and nursing status of the participants and discussed in relation to NFP after childbirth.