In this section we consider the indirect effects of breast-feeding on infant health through its effect on birth intervals. First, we examine the evidence that breast-feeding is associated with longer intervals between births. Then we discuss studies that have attempted to show that short birth intervals are related to poorer infant health.
EFFECT OF BREAST-FEEDING ON INTERVALS BETWEEN BIRTHS
Breast-feeding is associated with a delay in the return of ovulation after a birth, with longer intervals between births, and with lower fertility rates occurring in populations where this practice is prolonged. In a recent issue of Population Reports,13 numerous studies that evaluate the contraceptive effect of breast-feeding were reviewed. Studies that link breast-feeding and fertility include clinical reports based on small numbers of women, larger prospective studies, and single-round demographic surveys which may be representative of national populations.
Because ovulation is difficult to measure, studies linking breast-feeding with ovulation have been limited to small numbers of clinic-based subjects.4,14,17 These studies have shown that women who breast-feed—and those who breast-feed for longer periods—tend to ovulate later following a birth than other women. This effect is believed to be related to the hormone prolactin, which is released through the stimulus caused by the infant's sucking. Prolactin promotes the production of breast milk and is believed to be related to the inhibition of ovulation. Levels of prolactin and the anovulatory effect are related to the frequency and intensity of nursing. Thus, women who breast-feed partially on infrequently, while giving supplementary food, are more likely to ovulate than those who breast-feed fully.