Does breastfeeding help lower risk of endometriosis?

A new analysis of data from Nurses’ Health Study II (NHS II) shows that following the American College of Obstetricians and Gynecologists’ recommendations on breastfeeding may lower a woman’s risk of endometriosis. The association between the risk reduction and breastfeeding, the authors say, may be mediated at least in part by amenorrhea.

Published in The British Medical Journal, the findings are from the largest study to date to prospectively estimate this particular association and the first to look at the components of breastfeeding that may play a role in risk of endometriosis. The study was well powered because NHS II has more than 20 years of follow-up and validated measurement of endometriosis.

For the cohort research, the authors looked at data from 72,394 women who reported having 1 or more pregnancies that lasted at least 6 months, 3,296 of whom had laparoscopically confirmed endometriosis. For each pregnancy, the women reported on NHS II surveys their duration of total breastfeeding, exclusive breastfeeding, and postpartum amenorrhea.

The main outcome measure was incident self-reported laparoscopically confirmed endometriosis in parous women and the researchers calculated hazard ratios (HRs) and 95% confidence intervals (CIs) for endometriosis diagnosis using multivariable Cox proportional hazard models.

A 40% reduced risk of endometriosis was seen in women who breastfed for ≥ 36 months total during their reproductive lives versus women who never breastfed. Incidence of endometriosis was 453 per 100,000 person-years in the women who breastfed for < 1 month versus 184 per 100,000 person-years for those who reported a lifetime total ≥ 36 months. Every additional 3 months of total breastfeeding per pregnancy was associated with an 8% lower risk of endometriosis (HR 0.92, 95% CI 0.90-0.94; P <0.001 for trend). A 14% lower risk of endometriosis was seen for every additional 3 months of exclusive breastfeeding (HR 0.86, 95% CI 0.81-0.90; P <0.001 for trend.)

The protective association with breastfeeding was strongest among women who had given birth within the last 5 years P = 0.04 for interaction). Postpartum amenorrhea partially influenced the association with breastfeeding (34% total breastfeeding and endometriosis and 57% for exclusive breastfeeding and endometriosis).

Commenting on the results, the authors said that “given the chronic and incurable nature of endometriosis, breastfeeding should be further investigated as an important modifiable behavior to mitigate risk for pregnant women.”