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Fady I. Sharara, MD, FACOG
Effect of Weight On Fertility
Virginia Center for Reproductive Medicine

Effect of Weight On Fertility

Currently 50% of Americans are overweight, and more than 30% of Americans are obese. Along with a sedentary life style, fast foods, cooking and eating habits are the purported causes of the increase in obesity rates. Obesity brings to mind associations with hypertension, diabetes and heart disease. Yet, most people are surprised to learn that there is a significant association between obesity and infertility in both women and men. Couples trying to conceive face an extra challenge when both the man and the woman are overweight (body mass index, or BMI > 25) or obese (BMI > 30). According to a recent study, obese couples were almost three times as likely to take more than a year to achieve a pregnancy compared with normal-weight couples.
Weight can affect fertility in women, but a recent Danish study was the first to examine the impact of overweight or obesity in couples. Obese women had a 78% greater risk of being sub-fertile than normal-weight women, and obese men had a 49% increased risk for sub-fertility than normal-weight men. In men, a 3-unit increase in BMI increased the risk of infertility by about 10%, mainly through a decline in sperm quality.
According to the American Society for Reproductive Medicine, 12% of all infertility cases are a result of a woman either weighing too little or too much. The main ingredient in the body weight and fertility mix is estrogen a sex hormone produced in fat cells. If a woman has too much body fat, the body produces too much estrogen and begins to react as if it is on birth control, limiting her odds of getting pregnant. A woman with too little body fat cannot produce enough estrogen and her reproductive cycle begins to shut down. Both under and overweight women have irregular cycles in which ovulation does not occur or is inadequate.
The good news is the fact that more than 70% of women who are infertile as the result of body weight disorders will conceive spontaneously if their weight is corrected through a weight-gaining or weight-reduction diet, as appropriate. Yet, body weight is often considered last in an infertility evaluation. Awareness of the importance of body weight on reproduction allows couples to maintain appropriate body weight or to correct a body weight disorder before subjecting themselves to expensive, time consuming infertility evaluation and treatment.
The decline in fertility in both overweight and underweight women (BMI < 19) is also noted when assisted reproductive technologies, such as IVF, are used. In a 2001 study, we were the first to report that overweight/obese women have up to 50% lower chance at having a live born compared to women with normal BMI. This was later confirmed in larger studies.
For overweight or obese women (and men), the traditional methods of weight loss diet and exercise are essential elements of the treatment program. Weight loss does not occur immediately, but must be measured over time. Overweight and obese individuals should set their goal to reach a body weight that is 110% of predicted ideal body weight, however many women will conceive spontaneously before they even attain their target weight.
The key to success with slender women is convincing them that low body weight is associated with infertility. Slender women should be encouraged to increase their body weight to 100% of predicted ideal body weight for their height. Weight gain should be slow and one expects that up to six months may be required to restore normal reproductive function and pregnancy.
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