Effect of breast-feeding patterns on human birth intervals.

This discussion describes the methods which have been used to investigate lactational infertility and examines how different breastfeeding patterns influence human birth intervals. Lactational infertility can be measured in 3 different ways: by the duration of the interbirth interval; by the duration of lactational amenorrhea; and by the return of ovulation. The ultimate test of fertility is pregnancy. Several reports have demonstrated that breastfeeding increases the interval between pregnancies. For example 2 studies compared the time to next conception in nursing and nonnursing mothers from Alaskan Eskimo and rural Indian populations. Despite the wide differences in climate and culture the conception rates were similar in the 2 populations and conception occurred sooner in the nonlactating than lactating mothers. Despite clear evidence that breastfeeding is associated with prolonged interbirth intervals it cannot be assumed that breastfeeding per se is directly responsible for this effect. In many cultures and particularly in Africa sexual taboos are imposed on nursing mothers and reduced frequency of intercourse could explain at least partially the fertility inhibiting effect of breastfeeding. The return of menstruation postpartum has been used in many studies as an indirect index of resumed ovulation. This is a convenient method because it is easy to measure and can be applied to large populations. Many studies have shown that duration of postpartum amenorrhea is longer in breastfeeding than nonnursing mothers. In nonnursing mothers the duration of postpartum amenorrhea averages about 3 months. Among nursing mothers it may last for more than 2 years. Most studies which have attempted to define the timing of ovulation after childbirth have used endometrial biopsy. As an alternative the use of plasma or urinary steroid concentrations provides objective evidence of ovulation and enables a quantitative estimate of menstrual cycle adequacy. The interbirth consists of 3 phases: lactational amenorrhea; the menstrual interval (the interval between the return of postpartum menstruation and next conception); and the length of gestation itself. Only the period of gestation is relatively fixed. Thus it is important to consider the timing and frequency of ovulations during lactational amenorrhea and during the menstruating interval. The combined evidence suggests that the menstruating interval is associated with a reduction of fecundity which is less complete than it is during the phase of lactational amenorrhea. Suckling is a major variable in the control of postpartum ovulation yet relatively few studies have attempted to measure the suckling stimulus. A study of Konner and Worthman (1980) of ]Kung hunter gatherers suggested that very frequent suckling exerts a profound inhibitory effect upon reproduction. The early and regular use of supplementary food will have a detrimental effect on the contraceptive effect of breastfeeding. Malnutrition and age are additional factors that have been suggested as an influence on lactational infertility. For individual mothers breastfeeding cannot be relied upon as a guarantee against pregnancy. Its main importance as a contraceptive method is in developing countries.