Researchers from the University of Iowa Hospitals and Clinics followed 116,430 women from September 1989 to June 2011 and found that higher BMI was inversely associated with endometriosis. In fact, morbidly obese participants (those with a BMI of 40 or higher) were 39 percent less likely than normal-weight women to develop endo. And among women with fertility problems, those with a BMI of 40 or above were 62 percent less likely to develop endometriosis than those on the low end of the BMI range.
Of the women studied, about 5,500 were diagnosed with endometriosis, and researchers found that the women's current weight and their weight at age 18 was tied to their risk—the heavier the woman, the lower her chance of being diagnosed.
In news coverage of the study, researchers cautioned that this does not mean women should try to gain weight to reduce the risk of endo. It's possible that the strong weight association is because women with infertility tend to have higher rates of other hormonal conditions that may lead to higher weights, such as PCOS.
Knowing this association, however, may be useful as researchers look for underlying causes of endo. In addition, the researchers hope, doctors will be more aware of the increased likelihood of endo in leaner women, which might help them reach a quicker diagnosis.