Moderate exercise is tied to greater success among women trying to get pregnant, but those who work out vigorously take longer to conceive, an international study has found.
"This study is the first to find that the effect of physical activity on fertility varied by body mass index," said lead author Lauren Wise, a reproductive epidemiologist at Boston University. Body mass index (BMI) is a ratio of height to weight.
Researchers in the United States and Denmark followed more than 3,500 Danish women aged 18 to 40 who were trying to conceive over the course of a year for the study, published in the journal Fertility and Sterility.
All reported being in a stable relationship with a male partner and were 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.
While the study was large and well designed, there were some weaknesses, said Bonnie Dattel, an obstetrician at Eastern Virginia Medical School in Norfolk, Virginia, in an email.
Because the amount and intensity of exercise was self-reported, participants could have underestimated or overestimated their activity levels, she said.
The results also don't mean exercise was responsible. Women who took longer to conceive could also have modified their exercise patterns, the researchers noted, making the relationship the opposite of what it appeared.
In general, overweight and obese women have higher rates of fertility issues and a variety of pregnancy complications, said Richard Grazi, a reproductive specialist at Genesis Fertility in Brooklyn, New York, who was not part of the study.
"Fat is metabolically active - it makes estrogens," he said.
That extra estrogen can suppress other hormones responsible for ovulation, which can lead to irregular menstrual cycles or even a lack of menstruation.
On the other hand, it's not clear why lean women who exercise vigorously may take longer to become pregnant.
Having too little body fat may be a factor for some women, and it's known that competitive female athletes and very underweight women sometimes 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 cycling.
"I recommend exercise to all my patients, and a moderate level is always best for conception and pregnancy," said Wise.