The longer women breastfeed after their first pregnancy, the less likely they are to get an endometriosis diagnosis, according to a study published in the journal The BMJ. The finding suggests that breastfeeding might impact endometriosis disease processes.

However, researchers from Brigham And Women’s Hospital in Boston said they cannot exclude the possibility that breastfeeding is changing the development of severe pain associated with the abnormal tissue growth, and not the risk for endometriosis itself. Either way, the results suggest that hormones are somehow involved.

The report, “History of breast feeding and risk of incident endometriosis: prospective cohort study,” also did not look at women diagnosed before their first pregnancy. Researchers said it would be interesting to study whether breastfeeding could lessen endometriosis symptoms in this patient group.

The research team, led by Leslie Farland, ScD, used data from the Nurses’ Health Study II — a huge prospective study that gathers information about disease risk factors in women.

Their analysis included data on 72,394 women who had been followed for 20 years. Among them, 3,296 were surgically diagnosed with endometriosis after their first pregnancy.

Researchers found that for every three additional months that a woman breastfed after each pregnancy, her risk of ending up with an endometriosis diagnosis dropped by 8 percent.

For women who exclusively breastfed — meaning they did not offer their children solid food or formula — the impact was even greater. This group had a 14 percent drop in risk for each additional three-month period.

The effects also appeared to accumulate. Women who breastfed exclusively for at least 18 months over their reproductive years had a 30 percent lower risk of getting a diagnosis.

“We found that women who breastfed for a greater duration were less likely to be diagnosed with endometriosis,” Farland, a research scientist at the Center for Infertility and Reproductive Surgery at Brigham And Women’s, said in a press release. “Given the chronic nature of endometriosis and that very few modifiable risk factors are currently known, breastfeeding may be an important modifiable behavior to reduce the risk of endometriosis among women after pregnancy.”

An intuitive explanation for this risk would be the absence of periods after pregnancy, since the menstrual cycle is delayed by breastfeeding. Although the team found that this factor could explain some of the reduced risk, it did not account for all of it. Other mechanisms are likely contributing, they said. For instance, breastfeeding triggers an array of specific hormonal changes, involving hormones such as oxytocin, estrogen, or gonadotropin-releasing hormone which are already explored in endometriosis development.

“Our findings lend support to the body of public health and policy literature that advocates for the promotion of breastfeeding. Our work has important implications for advising women who are looking to lower their risk of endometriosis,” Farland said .

But researchers underscored that the association between breastfeeding and endometriosis does not necessarily mean that breastfeeding prevents endometriosis development. Breastfeeding might possibly prevent severe pain caused by an endometriosis lesion, and so, reduce the likelihood that a woman becomes diagnosed.

This potential association opens up for the possibility that breastfeeding might reduce endometriosis-related pain in women who are diagnosed before their first pregnancy — an issue that merits further study.

“We hope that future research will illuminate whether breastfeeding could help lessen the symptoms of endometriosis among women who have already been diagnosed,” Farland said.