An interdisciplinary international group of researchers in the area of lactational infertility gathered with the purpose of coming to a consensus about the conditions under which breastfeeding can be used as a safe and effective method of family planning. The consensus of the group was that the maximum birth spacing effect of breastfeeding is achieved when a mother "fully" or nearly fully breastfeeds and remains amenorrheic. When these two conditions are fulfilled, breastfeeding provides more than 98% protection from pregnancy in the first six months. Data are reviewed from thirteen prospective studies in both developed and developing countries supporting the consensus. The rationale for the consensus is given in detail. Recommendations are made based on what is currently known of the antifertility effects of breastfeeding. Research should continue to measure a broad spectrum of variables so that these guidelines can be refined as new information becomes available.