"We found that obesity is an additional risk factor for infertility in women who have regular [menstrual] cycles," he tells WebMD. "This is important given the increase in obesity worldwide."

The study is among the first to examine the relationship between body weight and infertility in women who ovulate, says researcher Jan Willem van der Steeg, MD, of Amsterdam's Academic Medical Center.

Women in the study who were severely obese were 43% less likely to achieve pregnancy than normal-weight women or women who were considered overweight but not obese during the yearlong study.

Dec. 11, 2007 -- Obesity is a known risk factor for ovulation problems, but it also contributes to infertility in women who ovulate normally, new research shows.

Obesity and Infertility

Van der Steeg and colleagues followed 3,029 couples who were having trouble conceiving on their own.

All the couples had spent a year or more trying to conceive, and none had obvious reasons for fertility problems -- the women were ovulating and had at least one functioning fallopian tube, and the men had normal semen analyses.

The couples were followed until pregnancy was achieved or until they started fertility treatments. In addition to a fertility history, the women's weight, height, and smoking status were measured at study entry.

The women were classified as underweight, normal weight, overweight, or obese based on their body mass index (BMI).

As measured by the BMI, a 5-foot 6-inch woman who weighs 115 to 154 pounds is considered normal weight (BMI of 18.5 to 24.9). If she weighs between 155 to 185 pounds she is considered overweight (BMI of 25 to 29.9), and she would be considered obese at a weight of 186 or more (BMI of 30+).

The vast majority of the study participants (86%) were either normal weight or overweight.

An additional 10% were obese, with BMIs of 30 or more. These women had the most trouble conceiving during the yearlong observation.

For example, a woman with a BMI of 35 was found to be 26% less likely to achieve a spontaneous pregnancy than women who were normal weight or overweight but not obese.

A woman with a BMI of 40 or more was 43% less likely to get pregnant.

The study appears in the December issue of the journal Human Reproduction.