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.
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.
"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."
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.
Role of Obesity Is Complex
It is not clear how obesity affects fertility in women who ovulate normally. Van der Steeg suggests that disruptions in the hormone leptin, which regulates appetite and energy expenditure, may prevent successful fertilization.
Reproductive endocrinologist William Dodson, MD, tells WebMD that it is increasingly clear that the role of obesity in reproduction is more complex than was once thought.
"What we once held as dogma is now starting to fall apart," he says. "We thought that if a woman's obesity was not affecting her ovulatory function, her fertility would be similar to a normal-weight woman's. But this does not appear to be true."
Dodson's own recent research at the Penn State Hershey College of Medicine confirmed that obese women undergoing infertility treatments needed higher doses of infertility drugs than normal-weight or overweight women.
Like the newly published study, all the women in the Penn State study had normal ovarian function.
"The issue of obesity and reproduction is complex, and we are only beginning to understand it," he says.