Breastfeeding a baby has a significant effect on a mother's fertility. In the early months, exclusive, or nearly exclusive, breastfeeding (defined as giving nutrition to an infant solely from breastfeeding without supplements or food, water, formula or any other form of nutrition) generally suppresses the return of ovulation and menstruation. Timing, frequency and overall volume of breastfeeding, as well as the age of the baby all affect fertility in a nursing mother.
The act of the infant suckling at the breast stimulates the mother's body to produce milk. This process also influences the production of hormones that are responsible for the development and release (ovulation) of eggs. Effectively, well-established and frequent, exclusive breastfeeding suppresses ovulation and/or the proper development of eggs, both affecting fertility.
While it has always been known that breastfeeding naturally affects child spacing, in the 1980's scientists began studying this phenomenon in a systematic way measuring closely several variables in breastfeeding against specific effects on the reproductive system for women. These studies eventually led to the development of the "Lactational Ammenorrhea Method" (LAM) of child spacing at the Georgetown University Institute for Reproductive Health.
Essentially, LAM indicates that nursing mothers, breastfeeding exclusively day and night (no longer than 4 hours between daytime feedings and 6 hours at night), who have not had their period return, are highly unlikely to conceive in the first six months after the baby's birth. Like any birth control, there is no guarantee, but guidelines indicate less than 2% chance which is as high or higher than several other forms of birth control. When babies begin sleeping through the night, or when regularly pumping instead of breastfeeding directly (even if still providing exclusive breast milk) chances increase of earlier return to fertility.
Length of breastfeeding also impacts the manner in which fertility returns. The Breastfeeding Answer Book, 3rd Edition, indicates that in the first 6 months it is more likely that any return of menses will no actually be preceded by ovulation. The longer breastfeeding continues without menstruation, the more likely it will be that ovulation will occur before menstruation and therefore without any previous warning.
For some women, pregnancy can happen as soon as ovulation returns. For others, the subtle hormones of breastfeeding may continue to disrupt the ability to actually conceive even if ovulating and having regular periods. This can differ greatly from woman to woman, meaning some can continue to nurse and get pregnant, some may need to night wean only and/or reduce frequency of feedings supplementing with foods or formula, and some may need to wean completely.
Understanding the effect of breastfeeding on fertility is important both for women who are trying to avoid pregnancy or delay the return of menstruation, or those who might want to speed along the return of fertility in order to become pregnant again. For details on using breastfeeding as birth control or getting pregnant while breastfeeding, find these articles in related links below (coming soon). Also find a link to an 18-page on breastfeeding, family planning and LAM by the authors of the Institute for Reproductive Health study.
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