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The demographic impact of breastfeeding: a critical review.


 Demography has corrected the old wives tale that nursing mothers cannot become pregnant. Few realize the extent to which knowledge about the effects of breast feeding on birth intervals is based on tenous assumptions, biased observations, and widely divergent methodology. The purpose of this analysis is to provide a word of caution for those who are unaware that the major reasearch on lactation and postpartum ovulation is yet to be done. Data from which the demographic significance of lactation is estimated come primarily from 3 types of studies: 1) analysis of completed inter-birth intervals in populations assumed not to practice birth control; 2) surveys in which information on both breastfeeding and the delay to the resumption of menstruation is collected; and 3) the clinical study where an attempt is made to measure the delay to the resumption of ovulation directly. Each of these 3 types of studies is reviewed, underscoring why most quantitative assertions which derive from these studies about the magnitude of the relationship between nursing and its impact on fertility are conditional at best and in many respects still speculative. 1 promising line of research currently being pursued by Frisch (1975, 1978) is that the amount of reserve energy stored by the nursing mother in the form of fat is a critical intermediate variable in determining the demographic impact of lactation. Another promising line of research is concerned with the hormonal mechanisms involved in letdown. Until the individual data are sampled and aggregated to reflect conditions in the population at large, the kinds of exact inferences about the demographic impact of lactation will continue to be elusive.
