If you are trying to conceive and thinking about increasing your exercise intensity, you may want to think again. New research shows that moderation is the key.

A study featured in the March 15, 2012 issue of Fertility and Sterility found that normal-weight women who exercised vigorously five or more hours a week were 42 percent less likely to get pregnant than women who didn’t work out. However, the researchers found that women who were overweight (a body mass index over 25) and who exercised vigorously did not experience a delay in getting pregnant. 

This study included 3,628 women age 18 to 40 who were not receiving infertility treatments. Vigorous activity included running, aerobics, gymnastics, swimming and intense bicycling, while moderate activity included brisk walking, leisurely cycling, golfing and gardening. In women of all body types, moderate exercise was associated with small decreases in the time it took to get pregnant.

The researchers concluded that women at either end of the spectrum—those who are obese and not exercising and those who are normal weight and exercise vigorously—have the hardest time getting pregnant. Obesity has been shown to disrupt menstrual cycles. Competitive athletes who exercise vigorously have also been shown to stop menstruating and have issues with ovulation. 

Lauren Wise, who was one of the study’s authors and is also an associate professor at the Boston University School of Public Health, explained that high-intensity exercise may also disrupt implantation of the fertilized egg, which further explains why vigorous exercisers may have more difficulty getting pregnant.

For more information about enhancing your chances of getting pregnant, click here.