A study found that women who are overweight (a body mass index or BMI rating of 25 to 29.9) have a 26% lower chance of conceiving and women who are obese (a BMI of 30 and greater) have a 43% lower chance when compared to women of normal weight (a BMI of 18.5 to 24.9). In this study, over 3,000 women were followed for one year. These women had no obvious reasons for fertility problems and were all ovulating.
In the past, it was thought that if a woman's obesity was not affecting her ovulatory function, here fertility would be similar to a normal weight woman's, but there does not appear to be true. Studies have shown that losing as little as 5% of body weight can dramatically improve the chances of pregnancy. This does not just apply to women, men can improve their fertility by losing weight too.
Van Der Steeg, J.W. Human Reproduction, December 2007; online edition. Jan Willem van der Steeg, department of Public Health, University Medical Center Rotterdam, Netherlands. William Dodson, MD, Penn State Hershey College of Medicine, Hershey, Pa. Dodson, W., Fertility and Sterility, September 2006; vol86.
Clark AM, Ledger W, Galletly C, Tomlinson L, Blaney F, Wang X, Norman RJ., Weight loss results in significant improvement in pregnancy and ovulation rates in anovulatory obese women. Hum Reprod. 1995 Oct;10(10):2705-12. PubMed PMID:8567797.