In this paper information collected in the World Fertility Survey on durations of breastfeeding and post-partum abstinence and amenorrhoea is used. Rates of conception and of resumption of menstruation are presented for four breastfeeding states: full breastfeeding, partial breastfeeding, weaned, and never breastfed. The various patterns of breastfeeding affect resumption of menstruation in consistent and expected directions. Continued breastfeeding beyond the resumption of menstruation and of sexual relations exerts a considerable contraceptive effect in all twelve countries considered, although interesting differences also emerge. The consequences of these findings are discussed. -Authors