Among women who have had one or more pregnancies that lasted for at least 6 months, breastfeeding is inversely associated with the risk of endometriosis, according to a study published in the BMJ.
Leslie V Farland, MSc, ScD, from the Department of Epidemiology at the Harvard TH Chan School of Public Health, in Boston, and colleagues conducted a prospective cohort study in the Nurses’ Health Study II (NHSII) from 1989 to 2011 among 116,430 registered nurses aged 25 to 42. A questionnaire was mailed asking participants to report all pregnancies lasting at least 6 months and provide their lifetime history of breastfeeding.
To estimate total duration of breastfeeding, women were asked, “If you breastfed, at what month did you stop breastfeeding altogether?” and were given the following categories: 1 to 2 months, 3 to 5 months, 6 to 8 months, 9 to 11 months, 12 to 18 months, and 19 months or more. Exclusive breastfeeding was determined by asking women, “At what month did you start giving formula or purchased milk at least once daily?” and “At what month did you start giving solid food at least once daily (baby food, cereal, table food, etc)? Responses included 0 to 2 months, 3 months, 4 to 5 months, 6 to 7 months, 8 to 11 months, and 12 months or more. To determine postpartum amenorrhea, women were asked, “At what month after delivery did your menstrual periods return?” and were given the following categories: 1 to 2 months, 3 to 5 months, 6 to 9 months, 10 months or more, pregnant again, or never.
The primary outcome was laparoscopically confirmed endometriosis. A total of 72,394 women reported having one or more pregnancies that lasted at least 6 months, and 3296 women had laparoscopically confirmed endometriosis. The researchers calculated person-months at risk from report of first pregnancy lasting at least 6 months until confirmed death, cancer (other than non-melanoma skin cancer), postmenopausal status, hysterectomy, laparoscopically confirmed endometriosis, or loss to follow-up.
At baseline in 1989, women who reported a longer duration of total breastfeeding tended to be older, multiparous, and had never smoked or used oral contraceptives. These women were also less likely to be currently overweight or obese, overweight or obese at age 18, and to report earlier age at menarche.
The results indicate that duration of total and exclusive breastfeeding was significantly associated with a decreased risk of endometriosis. Among women who reported a lifetime total length of breastfeeding for less than 1 month, there were 453 endometriosis cases per 100,000 person years compared with 184 cases per 100,000 person years in women who reported a lifetime total of 36 months or more of breastfeeding.
For every additional 3 months of total breastfeeding per pregnancy, women experienced an 8% lower risk of endometriosis (hazard ratio [HR], 0.92) and a 14% lower risk for every additional 3 months of exclusive breastfeeding per pregnancy (HR, 0.86). Women who breastfed for 36 months or more in total across their reproductive lifetime had a 40% reduced risk of endometriosis compared with women who never breastfed (HR, 0.60). The association with total breastfeeding and exclusive breastfeeding on endometriosis was partially influenced by postpartum amenorrhea (34% for total breastfeeding and 57% for exclusive breastfeeding).
“Breastfeeding overall, as well as exclusive breastfeeding, is associated with a lower risk of endometriosis,” the authors stated. “For each pregnancy, women who did not breastfeed were at a significantly increased risk of endometriosis compared with women who followed the recommendations from the American College of Obstetricians and Gynecologists and American Academy of Pediatrics of breastfeeding for at least one year.”
- Farland LV, Eliassen AH, Tamimi RM, et al. History of breast feeding and risk of incident endometriosis: prospective cohort study. BMJ. 2017 Aug 29;358:j3778. doi: 10.1136/bmj.j3778