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 told Reuters Health. 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.

While the study was large and well designed, there were some weaknesses, wrote Dr. Bonnie Dattel, an obstetrician at Eastern Virginia Medical School in Norfolk, told Reuters Health by email.

Because amount and intensity of exercise was self-reported, participants could have underestimated or overestimated their activity levels, which could have had an impact on the results, she said.

The results also don't mean that exercise was responsible. Women who took longer to conceive could also have modified their exercise patterns, researchers noted in the journal Fertility and Sterility, making the relationship the opposite of what it appeared.

DOES FAT PLAY A ROLE?

In general overweight and obese women have higher rates of infertility issues and a variety of pregnancy complications, according to Dr. Richard Grazi, a reproductive specialist at Genesis Fertility in Brooklyn, New York who was not involved in the current research.

"Fat is metabolically active -- it makes estrogens," Grazi told Reuters Health.

That extra estrogen can suppress other hormones responsible for ovulation, he said. This can lead to cycle irregularity and even amenorrhea, or lack of menstruation.

On the other hand, it's not clear why lean women who exercise vigorously may take longer to become pregnant, said Wise.

Having too little body fat may be a factor for some women. Competitive female athletes and very underweight women, for example, are known to experience menstrual irregularities.

Exercise may also affect the fertilized egg's ability to implant in the uterus. One previous study of women undergoing in vitro fertilization found a higher risk of implantation failure among women who did a lot of running or bicycling.

Very lean women who do vigorous exercise, such as marathon training, several hours a week, and are trying to become pregnant, may want to cut back to more moderate activities, Wise said.

"I recommend exercise to all my patients, and a moderate level is always best for conception and pregnancy," said Dattel.

SOURCE: bit.ly/GGNTMY Fertility and Sterility, online March 16, 2012.

(This story corrects headline from the previous, incorrect: "Among overweight, exercisers conceive more quickly." It corrects first paragraph from previous: "For heavy women trying to conceive, exercise may be linked to getting pregnant faster.")

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