Women who nurse their babies are less likely to develop endometriosis, and the longer they breastfeed, the lower their risk, a study shows.

“For women who are interested in reducing their risk for endometriosis, breastfeeding is one potential modifiable way that women can reduce their risk,” Leslie V. Farland of Harvard Medical School in Boston, one of the study’s authors, told Reuters Health in a telephone interview. “I think the research builds off of what we already know, that breastfeeding can be beneficial for both mother and child.”

Endometriosis causes chronic pain and can lead to fertility problems. There is no cure; treatments include pain medication, hormone therapy, surgery to remove the misplaced uterine tissue and, in some cases, total hysterectomy.

Some researchers have proposed that that retrograde menstruation, in which menstrual blood “backs up” through the uterus into the fallopian tubes and out into the abdomen, could eventually lead to endometriosis. Because many women stop menstruating while they are breastfeeding, being exposed to fewer periods could thus have a protective effect.

In a study published in The BMJ, Farland and her team looked at 72,394 women participating in the Nurses’ Health Study who had been pregnant for at least six months. None had endometriosis at the beginning of the study.

During the follow-up, 3,296 were diagnosed with endometriosis. Women who had breastfed for at least three years during their reproductive years were 40 per cent less likely to develop endometriosis than those who breastfed for less than a month. On a per-pregnancy basis, each additional three months of nursing was associated with an eight per cent lower endometriosis risk.

Women who stopped menstruating for six to 12 months after their baby’s birth were 42 per cent less likely to develop endometriosis than those who never missed a period after pregnancy.

But exposure to fewer menstrual periods accounted for only some of the protective effect of breastfeeding. Farland and her colleagues suggest that the hormonal changes women experience during breastfeeding, including higher oxytocin levels and lower estrogen levels, could also play a role.

“Our results aren’t generalisable to women who have endometriosis before pregnancy,” Farland said. Future studies should investigate whether breastfeeding could reduce symptoms in these women, she added, and clarify the mechanism responsible for breastfeeding’s protective effect.