Excessive vigorous exercise can confuse the pituitary gland and shut down ovulation
Too much exercise may be bad for fertility in normal weight women but may help overweight and obese women. In a U.S. News & World Report article, Dr. Jessica Scotchie discusses a study showing that women of a normal weight who do five or more hours a week of vigorous exercise (bicycling, running, aerobics) were 42 percent less likely to get pregnant than women who did not exercise at all.
“Very vigorous exercise can affect ovulation, and thereby disrupt menstrual cycles,” says Dr. Scotchie. “The pituitary gland interprets the strenuous exercise as meaning that this is not an optimal time to further stress the body with reproduction, and thus shuts down the signaling to the ovary to promote ovulation.”