While exercise has been linked to a lower risk of several diseases, such as diabetes, heart disease and depression, its effects on fertility are less clear.
"This study is the first to find that the effect of physical activity on fertility varied by body mass index," Lauren Wise, a reproductive epidemiologist at Boston University and lead author of the study said. Body mass index (BMI) is a ratio of height to weight.
Wise and colleagues in the U.S. and Denmark followed more than 3,500 Danish women aged 18 to 40 trying to conceive over the course of a year. They all reported being in a stable relationship with a male partner and not receiving any fertility treatments.
Participants estimated the number of hours per week they had spent exercising in the past year, as well as the intensity of their workouts.
Over the course of the study, nearly 70 percent of all women became pregnant.
The researchers found that moderate exercise, such as walking, cycling, or gardening, was associated with getting pregnant more quickly for all women, regardless of weight.
Women who spent more than five hours per week doing moderate exercise were 18 percent more likely to become pregnant during any given menstrual cycle than women who performed moderate exercise for less than an hour each week.
However, normal-weight and very lean women who reported high levels of vigorous exercise, such as running or aerobics, took longer to get pregnant. Those who exercised vigorously for more than five hours each week had a 32 percent lower chance of becoming pregnant during a given cycle than women who did not exercise vigorously at all.
There was no association between vigorous exercise and the time it took overweight or obese women -- those with a BMI of 25 or greater -- to become pregnant. A five-foot nine-inch-tall adult weighing 170 pounds would have a BMI of about 25.
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