Twelve percent 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: 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 can’t 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.