A study by investigators at Brigham and Women’s Hospital finds that women who breastfed for longer periods of time had significantly lower risk of being diagnosed with endometriosis, offering new insights into a condition that, up until now, has had very few known, modifiable risk factors.
Endometriosis is a chronic and incurable gynecologic disorder that affects approximately 10% of women in the US. Its symptoms can be debilitating and include chronic pelvic pain, painful periods and pain during intercourse.
“We found that women who breastfed for a greater duration were less likely to be diagnosed with endometriosis,” said corresponding author Dr Leslie Farland, a research scientist at the Centre for Infertility and Reproductive Surgery at BWH. “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.”
The team used data from the Nurses’ Health Study II (NHSII), a prospective cohort study that began in 1989. In the current analysis, researchers followed thousands of women for more than 20 years. During that time period, 3,296 women in the study were surgically diagnosed with endometriosis after their first pregnancy. The research team examined how long each woman breastfed, exclusively breastfed (breastfed without the introduction of solid food or formula), and how much time passed before their first postpartum period.
The team found that for every three additional months that mothers breastfed per pregnancy, women experienced an 8 percent drop in risk of endometriosis. This drop was even higher for mothers who exclusively breastfed: risk of endometriosis dropped 14% for every three additional months of exclusive breastfeeding per pregnancy. Researchers also looked at the effect of breastfeeding across reproductive lifetime – that is, breastfeeding more than one child. Women who breastfed exclusively for 18 months or more across their reproductive lifetime had a nearly 30% lower risk of being diagnosed with endometriosis.
The team investigated whether the decreased risk was due to postpartum amenorrhea – the temporary absence of menstrual periods that occurs when a woman is breastfeeding. They found that this accounted for some – but not all – of the effect, suggesting that breastfeeding may influence endometriosis risk through other mechanisms as well. Breastfeeding changes many of the hormones in a woman’s body, including oxytocin, estrogen, gonadotropin-releasing hormone and others for which there is evidence of a role in endometriosis pathophysiology.
The authors note that although they find a robust association between breastfeeding and lower risk of endometriosis, they cannot disentangle whether women who breastfeed are less likely to develop the disease itself, or whether women who breastfeed are less likely to experience pain symptoms severe enough to indicate a surgical evaluation.
The study did not include women who had been diagnosed with endometriosis prior to their first pregnancy, but the researchers are interested in investigating whether breastfeeding could help ease the symptoms of endometriosis for women who already have been diagnosed with the disease.
“Our findings lend support to the body of public health and policy literature that advocates for the promotion of breastfeeding,” said Farland. “Our work has important implications for advising women who are looking to lower their risk of endometriosis. We hope that future research will illuminate whether breastfeeding could help lessen the symptoms of endometriosis among women who have already been diagnosed.”
Objective: To investigate the association between lifetime breast feeding, exclusive breast feeding, postpartum amenorrhea, and incidence of endometriosis among parous women.
Design: Prospective cohort study.
Setting: Nurses’ Health Study II, 1989-2011.
Participants: 72 394women who reported having one or more pregnancies that lasted at least six months, 3296 of whom had laparoscopically confirmed endometriosis. For each pregnancy, women reported duration of total breast feeding, exclusive breast feeding, and postpartum amenorrhea.
Main outcome measures: Incident self reported laparoscopically confirmed endometriosis (96% concordance with medical record) in parous women. Multivariable Cox proportional hazard models were used to calculate hazard ratios and 95% confidence intervals for diagnosis of endometriosis.
Results: Duration of total and exclusive breast feeding was significantly associated with decreased risk of endometriosis. Among women who reported a lifetime total length of breast feeding of less than one month, there were 453 endometriosis cases/100 000 person years compared with 184 cases/100 000 person years in women who reported a lifetime total of ≥36 months of breast feeding. For every additional three months of total breast feeding per pregnancy, women experienced an 8% lower risk of endometriosis (hazard ratio 0.92, 95% confidence interval 0.90 to 0.94; P<0.001 for trend) and a 14% lower risk for every additional three months of exclusive breast feeding per pregnancy (0.86, 0.81 to 0.90; P<0.001 for trend). Women who breastfed for ≥36 months in total across their reproductive lifetime had a 40% reduced risk of endometriosis compared with women who never breast fed (0.60, 0.50 to 0.72). The protective association with breast feeding was strongest among women who gave birth within the past five years (P=0.04 for interaction). The association with total breast feeding and exclusive breast feeding on endometriosis was partially influenced by postpartum amenorrhea (% mediated was 34% (95% confidence interval 15% to 59%) for total breast feeding and 57% (27% to 82%) for exclusive breast feeding).
Conclusion: Among women who experienced at least one pregnancy that lasted at least six months, breast feeding was inversely associated with risk of incident endometriosis. This association was partially, but not fully, influenced by postpartum amenorrhea, suggesting that breast feeding could influence the risk of endometriosis both through amenorrhea and other mechanisms. Given the chronic and incurable nature of endometriosis, breast feeding should be further investigated as an important modifiable behavior to mitigate risk for pregnant women.
Leslie V Farland, A Heather Eliassen, Rulla M Tamimi, Donna Spiegelman, Karin B Michels, Stacey A Missmer