- Endometriosis, the growth of endometrial tissue outside of the uterus, currently affects 10% of U.S. women and has no known cure.
- A new study based on data from over 70,000 women followed for 20 years found the duration of total and exclusive breastfeeding were associated with a decreased risk of endometriosis diagnosis.
- Breastfeeding lowers the risk of endometriosis through postpartum amenorrhea and other hormonal mechanisms, including suppression of steroid hormones.
- These findings support public policies that encourage breastfeeding for infant and maternal health benefits.
The benefits of breastfeeding are nearly always considered from the perspective of the infant, but breastfeeding also has positive effects on the health of the mother. The hormonal changes associated with lactation may lower the risk of developing many chronic diseases, including breast and ovarian cancer. The results of a new study  of over 70,000 women suggest that these same hormonal changes may also lower the risk of developing endometriosis.
Endometriosis is a chronic disorder diagnosed in 10% of U.S. women, but the number affected may be even higher as diagnosis requires confirmation by laparoscopy to rule out other gynecological disorders with similar symptoms, which include painful and heavy periods, and infertility . Despite affecting so many women, endometriosis still has no known cure. Identification of risk factors, particularly those that are behavioral like breastfeeding, is essential in reducing the incidence of this debilitating disorder.
Location, Location, Location
The endometrium is the tissue that lines the inside of the uterus. During the first half of the menstrual cycle, the endometrium thickens in anticipation of a fertilized egg. If no egg attaches, the top layer of the endometrium (called the functional layer) is shed as menstrual blood. Endometriosis occurs if endometrium tissue grows outside of the uterus, usually on the ovaries or fallopian tubes . Having no idea that it is not where it is supposed to be, the endometrium tissue continues along, business as usual, responding to the same hormonal cues in the same manner—thickening in anticipation of a fertilized egg and shedding the top layer if one does not implant. As a result, women with endometriosis generally experience more pain during menstruation and a heavier menstrual cycle.
How does tissue that should only reside in the uterus get to the ovaries and fallopian tubes? The leading hypothesis to explain endometriosis is retrograde menstruation, which is the movement of menstrual blood, containing endometrial tissue, into the fallopian tubes, ovaries, and even the pelvic cavity. It is still not well understood, but a small amount of retrograde menstruation may occur as a normal part of each menstrual cycle . Development of endometriosis depends on how much endometrial tissue ends up in the wrong location—more menstrual cycles mean more retrograde menstruation, and thus a greater risk for developing endometriosis [1-4].
Factors that increase the number of periods a women experiences, such as an earlier age at menarche (≤11 years old) and shorter cycle length (≤27 days), are associated with an increased risk of developing endometriosis . On the other hand, factors that decrease the number of periods, including pregnancy and lactation, have the potential to reduce the risk [1,2].
During pregnancy, high levels of circulating estrogen and progesterone suppress the hormones (i.e., follicle stimulating hormone, or FSH, and luteinizing hormone, LH) that trigger ovulation. Moreover, the implantation of a fertilized egg in the endometrium means that tissue stays right where it belongs. The relationship between breastfeeding and ovulation is less clear-cut, however. “You can’t get pregnant while you are breastfeeding” may be one of the most common pieces of advice given to new mothers, but it is not entirely accurate advice. Breastfeeding can suppress ovulation, resulting in amenorrhea (lack of menstruation) and temporary infertility. But just how long a mother remains infertile while breastfeeding varies with respect to nursing frequency and intensity, as well as maternal condition.
Infant suckling suppresses the production of FSH and LH, thereby inhibiting ovulation and menstruation. However, the stimulus from infant suckling must be consistent in order to suppress these hormones at levels sufficient to prevent follicular development and subsequent ovulation . Thus, lactational amenorrhea requires frequent nursing (approximately every 3–4 hours), like that associated with the earliest months of lactation (< 6 months) before solid foods are introduced .
Lowering the Risks
Based on the relationship between breastfeeding and amenorrhea, a team of researchers from Harvard, UCLA, and Michigan State University hypothesized that, among women who have been pregnant, the duration of breastfeeding would be associated with a lower risk of developing endometriosis . Specifically, they predicted that the strongest protective effects would be associated with the duration of exclusive breastfeeding (i.e., milk is the only source of nutrition) and lactational amenorrhea .
Data to test their hypothesis came from the Harvard-based Nurses’ Health Study II, one of the largest prospective cohort studies on risk factors for disease in U.S. women. Over 100,000 female nurses were originally enrolled to provide information on diet, lifestyle, and health via questionnaires provided every two years. The current study on breastfeeding and endometriosis included 72,394 women who had reported at least one pregnancy lasting at least 6 months between 1989 and 2011. Of these, 3,296 had endometriosis that was confirmed by laparoscopy . This is the largest study to date to address the relationship between breastfeeding and risk of endometriosis.
When looking at the total duration of breastfeeding across their lifetime, women who breastfed for 36 months or more had a 40% lower risk of an endometriosis diagnosis compared with women that never breastfed. For every additional three months of lifetime breastfeeding, the risk decreased an additional 3% .
The results when considering individual pregnancies were similar to those looking at the total duration of breastfeeding; for every additional three months a mother breastfed an individual infant, her risk of endometriosis diagnosis dropped by 8% . The protective effect of exclusive breastfeeding was even more dramatic. For each pregnancy, there was a 14% decreased risk of endometriosis for each 3-month increase in exclusive breastfeeding.
Exclusive breastfeeding has a biologically defined endpoint—as human infants grow, their metabolic needs require provisioning with foods other than breast milk sometime around 6 months of age. Thus, it is extremely important that this study found breastfeeding after solid food introduction still offered a significant protective effect . Moreover, this study identified an inverse linear relationship between breastfeeding duration and risk of endometriosis; rather than plateau out at some point during lactation, risk continued to decrease with each additional three months of nursing.
Taken together, these findings suggest that factors other than postpartum amenorrhea are responsible for the protective effect of breastfeeding on endometriosis. To this end, the study  found the duration of postpartum amenorrhea was inversely associated with risk of endometriosis and explained a significant proportion—but not all—of the risk reduction that was associated with total lifetime duration of breastfeeding and exclusive breastfeeding.
Equally important to considering retrograde menstruation is an understanding of the mechanisms that control this tissue’s ability to adhere, proliferate, and survive outside of the uterus . Supporting this, a study  on rats with surgically induced endometriosis found that although endometrial lesions progressed during pregnancy, no lesions progressed during lactation.
The hormonal profiles of breastfeeding women, even those no longer experiencing amenorrhea, are different from non-nursing (or non-pregnant) women [1,4]. Notably, breastfeeding women have higher circulating levels of oxytocin and prolactin and lower levels of estrogen and progesterone [1,4]. These hormonal changes are believed to offer protective effects on the growth of breast and ovarian cancers and may play a similar role in limiting the growth of endometrial tissue [1,4].
Breastfeeding does not prevent endometriosis. And it is not a cure. But the findings of this prospective cohort study of over 70,000 women suggest it is a significant factor in the development of this disorder and may even play a role in lessening or alleviating symptoms in breastfeeding women with endometriosis .
The takeaway from these findings is not to simply tell all women to breastfeed and to do so for as long as possible. The world we live in is not structured in such a way that this is achievable for all women (or even for most women). Policy must follow the science. When study after study continues to demonstrate the health benefits of breastfeeding (for both mother and child), there should be some response in policy to encourage and support breastfeeding across the population (e.g., longer maternity leave, private and hygienic pumping stations at places of work). This type of research should not make those that are unable to breastfeed feel inadequate, but rather it should encourage action to level the playing field so that all mothers can improve their health outcomes.
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3. Missmer SA, Hankinson SE, Spiegelman D, Barbieri RL, Malspeis S, Willett WC, Hunter DJ. 2004. Reproductive history and endometriosis among premenopausal women. Obstetrics & Gynecology. 104: 965-74.
4. Heilier J-F, Donnez J, Nackers F, Rousseau RJ, Verougstraete V, Rosenkranz K, Donnez O, Grandjean F, Lison D, Tonglet R. 2007. Environmental and host-assocaited risk factors in endometriosis and deep endometriotic nodules: A matched case-control study. Environmental Research. 103: 121-9.
5. Vekemans M. 1997. Postpartum contraception: the lactational amenorrhea method. The European Journal of Contraception & Reproductive Health Care. 2(2): 105-11.
6. Barragán JC, Brotons J, Ruiz JA, Acién P. 1992. Experimentally induced endometriosis in rats: effect on fertility and the effects of pregnancy and lactation on the ectopic endometrial tissue. Fertility and Sterility. 58(6): 1215-9.
Dr. Lauren Milligan Newmark