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The old wives’ tale that nursing mothers can not become pregnant has been replaced by a new demographic wisdom. Appearing with ever increasing frequency in the scientific literature are statements to the effect that trends and differentials in breastfeeding practices have a major impact on fertility. There can be no question that a positive correlation exists between the intensity and duration of lactation and length of temporary postpartum sterility. However, confidence about the existence of such a relationship has become translated, for many, into a certainty about magnitude. In point of fact, what we know about the quantitative relationship between nursing and the delay to the resumption of ovulation is sketchy at best. Few realise the fullextent to which our knowledge about the effects of breastfeeding on birth intervals is based on tenuous assumptions, biased observations, and widely divergent methodologies. With more and more writers making claims for the demographic importance of breastfeeding, it is perhaps timely to take stock of the evidence supporting such claims. The purpose of this analysis is to introduce a note of caution for those who seem unaware that the major research on lactation and postpartum anovulation is yet to be done.