The role of breastfeeding in mitigating endometriosis risk

The role of breastfeeding in mitigating endometriosis risk


Breastfeeding has many known benefits for mothers, but is endometriosis protection one of them? To Leslie Farland, ScD, it was a question worth considering, since previous work has shown “a suggestion of a protective association of breastfeeding for other hormonally driven diseases,” such as breast and ovarian cancer.

Plausible contributory mechanisms might include a prolonged period of postpartum amenorrhea, which can extend well beyond a year for some women who exclusively breastfeed their infants. Also, the chain of hormonal events that keep lactation going include ongoing release of oxytocin and prolactin in response to suckling and the breast being drained.

During the period of breastfeeding, there is also an ongoing inhibition of the production of estrogen, gonadotropin-releasing hormone, follicle-stimulating hormone, and luteinizing hormone, said Dr. Farland, assistant director of epidemiologic research at Brigham and Women’s Hospital’s Center for Infertility and Reproductive Surgery in Boston. She presented preliminary results of a large study examining the association between breastfeeding and endometriosis at the 13th World Congress of Endometriosis in Vancouver.


Perspectives on Endometriosis Management

Prior work had found some protective effect against endometriosis from breastfeeding, but the two studies had small sample sizes and didn’t do an effective job of tracking duration and exclusivity of breastfeeding.

Using the large dataset available from the Nurses’ Health Study II, Dr. Farland and her colleagues were able to complete the largest prospective study to date examining the relationship between endometriosis risk and breastfeeding and to examine the effect of multiple variables on the association.

The researchers, working from the theory that endometriosis is at least partially caused by retrograde menstruation, expected to see some effect of prolonged amenorrhea on endometriosis risk. “The frequency, intensity, and duration of menstrual cycles are known to be associated with the risk of endometriosis,” Dr. Farland said.

Robust study methods

Data from the study enabled the researchers to include only women who had a laparoscopically confirmed diagnosis of endometriosis. The study included more than 116,000 women who were registered nurses in 14 states. The women were aged 25-42 years at the time they were recruited into the study.

Beginning in 1993, the Nurses’ Health Study II asked women about the total number of months they had spent breastfeeding. From 1997-2003, the study asked participants to complete a grid that showed breastfeeding duration for each pregnancy. The survey also measured the amount of time spent exclusively breastfeeding and the duration of postpartum amenorrhea after each term delivery. The analysis also included the time since last birth; age; current body mass index (BMI), as well as BMI at age 18; and parity.

The analysis excluded women who had laparoscopically confirmed endometriosis either before the first baseline visit or before their first pregnancy, as well as those who were postmenopausal, had undergone a hysterectomy or received a cancer diagnosis at baseline, or were never pregnant. They also excluded women with no breastfeeding information.

This left about 70,000 women who reported having at least one pregnancy lasting 6 months or more, either at baseline or during the study period. Of these women, more than 3,200 received a laparoscopically confirmed endometriosis diagnosis during the follow-up period.

Dr. Leslie Farland

Protective effect

The researchers found that, overall, breastfeeding was protective for the risk of endometriosis, with a linear trend P-value of less than .0001, meaning that women who had breastfed for a longer total duration across all pregnancies were less likely to receive a diagnosis of endometriosis.

For every 3 months a woman breastfeeds, she experiences a 3% reduction in her risk of endometriosis, according to Dr. Farland’s analysis of the Nurses’ Health Study II data. The analysis also revealed that the protective effect was stronger within the first 5 years after a birth and was stronger with exclusive breastfeeding.

“Among women who have never had endometriosis, the risk of a future diagnosis of endometriosis was lower for women who were breastfeeding,” said Stacey Missmer, ScD, the study’s senior author and a professor of obstetrics and gynecology at Michigan State University in Grand Rapids.

Using a statistical technique called mediation analysis, the researchers looked beyond the association between breastfeeding and amenorrhea and were able to quantify how much of that association could be attributed specifically to postpartum amenorrhea.

They found that longer duration of postpartum amenorrhea had a greater risk-reduction effect. For women who breastfed for 18 months or more, the relative risk for endometriosis diagnosis was 0.58, compared with women who breastfed less than 1 month or not at all. The effect diminished with shorter durations.

Nearly one-third of the protective effect of breastfeeding could be attributed to amenorrhea specifically, according to the mediation analysis. However, among women who breastfed exclusively, amenorrhea contributed a full 50% of the effect.

The independent risk reduction seen with longer amenorrhea duration might be attributable to higher circulating oxytocin levels, as well as inhibition of estrogen and the other hormones associated with ovulation, Dr. Farland suggested.

It is important that the study only included women with laparoscopically confirmed endometriosis, according to Dr. Missmer.

“Because we need a surgical evaluation for diagnosis, no one can speak to what’s actually happening with the underlying presence or absence of disease,” she said. “So, any factors that might influence the disease may also influence symptoms that affect whether or not you’re going to have surgery.”

The lack of a noninvasive biomarker affects who gets diagnosed and how long it takes to get diagnosed because symptoms have to reach a certain threshold before a surgical diagnosis is sought, she said. Since the hormonal changes of lactation are complex and have effects across many aspects of physical mental functioning, “it may be that breastfeeding adequately suppresses pain symptoms such that women are managing well enough that they would never come in for a surgical diagnosis,” Dr. Missmer said.

While amenorrhea accounted for some of the relationship between breastfeeding and endometriosis, it didn’t explain all of it, Dr. Missmer said.

“Beyond lactational amenorrhea, that argued that there are other biologic pathways that could be involved in either suppressing endometriosis itself or suppressing the pathways that get you to a diagnosis.”

The bottom line for clinicians is that attempting breastfeeding is “best for women’s health in general” and may have specific benefits in reducing the symptoms of pelvic pain and future endometriosis risk, Dr. Missmer said.

The analysis was funded by the National Institutes of Health.

Kari Oakes is a writer for Ob.Gyn. News.

Dr. Stacey Missmer