The study, led by Brigham and Women's Hospital (BWH) in Boston, MA, and published in the BMJ, uses data from the Nurses' Health Study II (NHSII), which began in 1989.
Corresponding author Dr. Leslie Farland, a research scientist at BWH's Center for Infertility and Reproductive Surgery, says, "We found that women who breast-fed for a greater duration were less likely to be diagnosed with endometriosis."
Endometriosis is a chronic, incurable, and often debilitating gynecologic disorder that affects girls and women. It is diagnosed in around 10 percent of women in the United States, note the authors.
The disorder arises when tissue that is similar to the lining of the uterus, or the 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."
Effects of breast-feeding
Dr. Farland and colleagues say that there is growing evidence that breast-feeding brings lasting health benefits to women, such as helping them to lose excess weight and reduce their risk of breast cancer, ovarian cancer, and other chronic diseases.
It has been suggested that breast-feeding has this effect because it prolongs the temporary absence of menstrual periods (postpartum amenorrhea) and because it alters levels of some hormones that are thought to play a role in endometriosis, such as oxytocin, estrogen, and gonadotropin-releasing hormone.
For their investigation, the researchers used data on NHSII participants who reported at least one pregnancy during the period of the study. After taking out women who had been diagnosed with endometriosis or cancer before data collection began in 1989, the number left for analysis came to 72,394.
This number included 3,296 women who were diagnosed with endometriosis after their first pregnancy during the follow-up and whose diagnosis was confirmed by laparoscopy, which is a surgical procedure that examines abdominal organs.
Because of the detailed answers to the NHSII questionnaires, the researchers were able to determine, for each participant, how long she breast-fed, exclusively breast-fed (that is, without introducing formula or solid food), and how much time elapsed before her periods resumed (the duration of postpartum amenorrhea).
Reduced risk of endometriosis
The researchers found an 8 percent reduced risk of endometriosis for every 3 additional months of breast-feeding per pregnancy. This figure was even higher (14 percent) for women who exclusively breast-fed for those months.
They also found that women who had breast-fed for a total of 36 months or more during their reproductive lifetime had a 40 percent reduced risk of endometriosis compared with women who had never breast-fed.
Also, women who had breast-fed exclusively for 18 months or more over their reproductive lifetime had a nearly 30 percent reduced risk of being diagnosed with endometriosis.
Further analysis revealed that postpartum amenorrhea accounted for some, but not all, of the reduction in risk. This supports the idea that breast-feeding reduces the risk of endometriosis through other means - perhaps through its effect on hormones.
It is also important to note that the study only included women who had been diagnosed with endometriosis. Therefore, it is not possible to draw conclusions from these results about whether breast-feeding affects the development of the disease or just its symptoms.
One explanation could be that women who breast-feed are less likely to seek a diagnosis for endometriosis, perhaps because they experience less pain.
Many women and girls with endometriosis are undiagnosed, and many patients are misdiagnosed.
Also, because the study did not include women who had been diagnosed with endometriosis before their first pregnancy, the researchers were not able to find out if breast-feeding can ease its symptoms. However, they are keen to investigate this further.
"Given the chronic nature of endometriosis and that very few modifiable risk factors are currently known, breast-feeding may be an important modifiable behavior to reduce the risk of endometriosis among women after pregnancy."
Dr. Leslie Farland