Underweight women are at increased risk for early menopause, a new study has found.

This study, in Human Reproduction, followed 78,759 premenopausal women ages 25 to 42 beginning in 1989. Over the following 22 years, 2,804 of them reported natural menopause before age 45.

After controlling for smoking, pregnancies, oral contraceptive use and other factors, they found that compared with women who had a body mass index of 18.5 to 22.4 (within the normal range), those with a B.M.I. under 18.5 at any age had a 30 percent increased risk of early menopause. Overweight women (B.M.I. 25 to 34.9) had a slightly lower risk of early menopause, and there was no significant increased risk in very obese women with a B.M.I. higher than 35.

The lead author, Kathleen L. Szegda, who was a postdoctoral researcher at the University of Massachusetts, Amherst, when the work was done, said the reasons for the link between weight and the timing of menopause are unclear and that more research is needed to replicate these results.

Still, she said, “Underweight women may want to discuss the findings with their doctors. Early menopause is associated with several health conditions,” including osteoporosis, dementia, heart disease and early death, “and has implications for family planning.”