A new study, which followed more than 72 thousand women for more than 20 years, has found that breast feeding is linked to a lower risk of being diagnosed with endometriosis.

Researchers found that among the 72,394 women who had one or more pregnancies (that lasted at least 6 months) there were 3,296 laparoscopically confirmed cases of endometriosis. Compared with women who nursed for less than a month per pregnancy, those who nursed for a year or more had a 32% reduced risk of endometriosis. For each additional three months of breast feeding, the risk was reduced by 8%.

The association between the risk reduction and breast feeding may be mediated at least in part by amenorrhea, according to the researchers who analyzed data from Nurses’ Health Study II (NHS II).

The study, led by Brigham and Women’s Hospital (BWH) in Boston, MA, and published in the British Medical Journal. It is the largest study to date on the link between breast feeding and the risk of endometriosis.

A 40% reduced risk of endometriosis was seen in women who breastfed for ≥ 36 months 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 less than a 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. A 14% lower risk of endometriosis was seen for every additional 3 months of exclusive breastfeeding.

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

Endometriosis is a chronic, incurable gynecologic disorder that affects girls and women. The disorder arises when tissue that is similar to the lining of the uterus (endometrium) is found attached to other areas in the body, generally in the pelvic cavity. As a result, most women with endometriosis experience “chronic pelvic pain, painful periods, and pain during intercourse.