Polycystic ovarian syndrome (PCOS), one of the leading causes of infertility, is worsened by obesity and insulin resistance. "All women with PCOS have ovulatory dysfunction that becomes more severe as weight increases," Legro says. However, not all overweight or obese women have PCOS, and not all women with PCOS are overweight. The classic signs of PCOS are irregular periods and excess body hair; if you have these symptoms, consult your doctor.
Though losing weight won't necessarily "cure" PCOS, numerous studies have found that even a small decrease in weight and body fat can improve ovulation in overweight and obese women. "The brain is very sensitive to 'fatness' signals," explains Rose E. Frisch, Ph.D., author of Female Fertility and the Body Fat Connection (The University of Chicago Press).
It works the other way too. In an Arizona State University study, when underweight female athletes who were not having periods increased their body fat from 8.2 percent to 14.4 percent, they started having normal cycles.
Ronda Kelly quickly discovered the impact that gaining a little weight has on fertility: In 2005, weighing 116 pounds, she got pregnant with her second daughter, Audra. On the first try.
Read more on getting pregnant.