Abstract:
In most mammalian species lactation suppresses fertility. There is no doubtthat it is the suckling stimulus that provides the controlling signal, and, inhuman reproduction, this is the only truly physiological signal thatsuppresses fertility in normally nourished, healthy women. In breastfeedingwomen, the return of normal fertility follows a relatively well-defined pathprogressing through: an almost complete inhibition of gonadotrophin-releasinghormone/luteinizing hormone (GnRH/LH) pulsatile secretion in the earlystages of lactation; return of erratic pulsatile secretion with some ovarianfollicle development associated with increases in inhibin B and oestradiol; aresumption of apparently normal follicle growth associated with a normalincrease in oestradiol, but often an absence of ovulation, or formation of aninadequate corpus luteum; and a return to normal ovulatory menstrual cycles. Akey element in controlling the rate of this progression is the impact of thesuckling stimulus on the GnRH pulse generator, a common feature of lactationin those species for which there is information. The variability in theduration of lactational amenorrhoea between women is related to the variationin the strength of the suckling stimulus, a unique situation between eachmother and baby. Full breastfeeding can provide a reliable contraceptiveeffect in the first 6 to 9 months, but the precise mechanisms whereby thesuckling stimulus affects GnRH pulsatile secretion remain unknown. Manystudies on the hypothalamic pathways that might be involved in the translationof the neural suckling stimulus to suppression of hypothalamic GnRH secretionhave been undertaken, principally in rats. In women, suckling increases thesensitivity of the hypothalamus to the negative feedback effect of oestradiolon suppressing the GnRH/LH pulse generator, a mechanism that appears to becommon across species. In contrast, the role of prolactin in the control ofGnRH appears to be species-dependent, with the importance varying from none toan important role in late or throughout lactation. In women, there is littleevidence for a role of leptin, opioids or dopamine, although this may merelyreflect the ethical dilemma of being able to give sufficient drug to test thesystem in the mother since these drugs will pass through the breast milk tothe baby. Regardless of mechanism, practical guidelines for usingbreastfeeding as a natural contraceptive have been developed, which allowsmothers to utilize the only natural suppressor of fertility in women as aneffective means of spacing births.