Being thin in childhood and adolescence increases a woman’s likelihood of developing a painful disorder of the reproductive system called endometriosis.

But this doesn’t mean that it’s a good idea for girls to put on weight so they can prevent the disease, the study’s lead author, Dr. Stacey A. Missmer of Brigham and Women’s Hospital and Harvard Medical School in Boston, told Reuters Health. “Obviously there are a myriad of reasons why being thinner is better. We certainly would never encourage girls to be heavier.”

Instead, according to Missmer, the results offer a window into the biology behind this poorly understood disease, and might also help doctors spot women who should be tested for it.

Endometriosis occurs when tissue that lines the uterus also grows outside it, typically within the ovaries, fallopian tubes, and elsewhere in the pelvic area. This misplaced tissue still behaves as if it were lining the uterus, leading to monthly bleeding. Because the blood has nowhere to go, it can cause scarring and cysts, and the condition doubles a woman’s risk of being infertile.

The main symptoms of endometriosis are pelvic pain and menstrual cramping, Missmer noted, and because some health professionals may see this as normal, “it can be difficult for girls and women to find medical care where that is taken seriously.”

While the cause of endometriosis is still unclear, body weight has been linked to risk, with studies showing a greater likelihood of the condition in slimmer women.

To investigate the role of weight earlier in life, Missmer and her colleagues looked at nearly 90,000 premenopausal women participating in the Nurses Health Study II. Women were 25 to 42 years old when they enrolled in the study, in 1989. At that time, they reported on their body size at ages 5, 10, and 20 years by choosing from nine figures representing body sizes ranging from very thin to obese.

Three years later, about 1,800 had developed endometriosis.

Once the researchers took factors like current heaviness or thinness and childbearing history into account, they found the women who’d been slimmer earlier in life were at 23 percent increased risk of endometriosis. Given the design of the study, the researchers were unable to calculate a woman’s absolute risk of developing endometriosis. However, it’s been shown that endometriosis affects about 10 percent of women in the general population.

Missmer’s study also found that women who’d been heavier earlier in life were at 10 percent lower risk of endometriosis. Women who are heavier have higher levels of androgens, or so-called “male hormones,” which can suppress endometriosis, Missmer noted. The current findings suggest, the researcher said, that even quite early in life weight-related hormonal balance could influence endometriosis risk.

Surgery to visualize the inside of a woman’s pelvic area is necessary for a definitive diagnosis of endometriosis, Missmer said. The current findings could help doctors identify girls and women who are at higher risk of the disease, she added, and for whom this invasive procedure would be warranted.

SOURCE: Human Reproduction, online February 19, 2010.