Underweight women and women who were underweight as teenagers or in their mid-30s are at greater risk of experiencing an early menopause compared to lean or normal weight women, according to a study of nearly 80,000 women.
In addition, underweight women who lost 20 pounds or more at least three times between the ages of 18 and 30 were at significantly increased risk of an early menopause, which is defined as a naturally occurring menopause before the age of 45.
The study, which is published today (Thursday) in Human Reproduction , one of the world's leading reproductive medicine journals, is one of very few studies to follow women prospectively from as young as 25 to menopause, and is the largest ever to do so. Information from their teenage years was also gathered.
"Our findings suggest that women who are underweight in early or mid-adulthood may be at increased risk for early menopause. Up to 10% of women experience early menopause and it is associated with a higher risk of cardiovascular disease, and other health conditions such as cognitive decline, osteoporosis and premature death, so these findings have important implications for women and their doctors," said Dr Kathleen Szegda, who, at the time the study was completed, was a post-doctoral research associate at the University of Massachusetts, USA, and led the research. "Underweight women may want to consider discussing the potential implications of these findings with their doctors." 
The researchers looked at body mass index (BMI), weight distribution, weight change and timing of menopause in 78,759 premenopausal women aged between 25-42 who joined the US Nurses' Health Study II in 1989. The women provided information on their medical history and health-related behaviours such as smoking and exercise at the time of joining the study, and then they updated the information by means of questionnaires every two years thereafter. The researchers followed them until 2011, by which time 2804 women had reported early natural menopause.
They found that women who were underweight at any age (BMI of less than 18.5 kg/m2) had a significant 30% increased risk of early menopause compared with lean or normal weight women (BMI between 18.5-22.4 kg/m2). Overweight women with BMIs between 25-29.9 kg/m2 had a 21-30% lower risk of early menopause compared to normal weight women.
Women who were underweight at 18 with a BMI of less than 17.5 kg/m2 had a 50% higher risk of early menopause compared to lean or normal weight women; women who had a BMI of less than 18.5 kg/m2 at the age of 35 had a 59% increased risk.
Underweight women who reported losing 20 pounds or more, three times or more between the ages of 18 and 30, had a 2.4-fold increased risk of early menopause; however, there were only seven women who reported this and so, although the result is statistically significant, it still needs to be treated with caution.
Dr Szegda concluded: "Causes of early menopause are not clearly understood. Our findings suggest that being underweight may have an impact on the timing of menopause. More research is needed to understand how it increases the risk of early menopause."
 "Adult adiposity and risk of early menopause," by K.L. Szegda et al. Human Reproduction journal. doi:10.1093/humrep/dex304.
 Dr Szegda is now Director of Community Research and Evaluation at Partners for a Healthier Community, and is also an adjunct faculty member at the University of Massachusetts.