THURSDAY, March 15 (HealthDay News) -- Exercise is a plus for
women trying to become pregnant, but overdoing workouts might make
it harder to conceive -- unless you're overweight, researchers
Usually a risk factor for most health problems, being overweight
or obese didn't hinder fertility in heavy women who logged vigorous
workouts -- running, fast cycling and aerobics. However,
healthy-weight women who performed more intense workouts were more
likely to experience delays becoming pregnant.
The study was led by U.S. and Danish researchers who tracked
physical activity and fertility in thousands of Danish women.
While moderate physical activity was associated with a small
increase in fertility rates among all women, study author Lauren
Wise, an associate professor of epidemiology at the Boston
University School of Public Health, said, "Our study found that
higher levels of vigorous exercise were associated with lower
fertility rates in normal-weight women, but not overweight and
Wise said the findings indicate that physical activity of any
type might improve fertility among heavier women, but their
normal-weight counterparts who want to improve their pregnancy odds
should sub in low-key workouts such as brisk walking and gardening.
In other words, marathon runners who want to conceive might want to
scale back on those pavement-pounding workouts, she said.
Other studies of competitive female athletes suggest that
intense workouts disturb women's monthly menstrual cycles and lead
to a lack of ovulation and even the absence of menstrual periods,
among other problems, Wise said. But, she added, high-intensity
exercise might also impair implantation when a fertilized egg
attaches to the wall of the uterus.
The researchers of the observational study recruited and
administered questionnaires over the Internet to 3,628 women who
ranged in age from 18 to 40. They had to be in stable relationships
with male partners and planning to become pregnant, but not
involved in any fertility treatments.
The researchers collected information on height, weight,
reproductive and medical history, plus lifestyle and behavioral
details, and then sent out follow-up questionnaires by email every
two months for 12 months, or until a woman became pregnant.
At the study's start, the women were asked about the average
number of hours per week they exercised and about what types of
moderate or vigorous physical activity they performed. Running,
fast cycling, aerobics, gymnastics and swimming were considered
vigorous. Brisk walking, leisurely cycling, golfing and gardening
were defined as moderate.
The participants were categorized by their exercise levels and
the results were evaluated according to body mass index (BMI, a
ratio of weight to height). A BMI over 25 is considered overweight
While moderate physical activity was linked to becoming pregnant
faster across all BMI ranges, the researchers found that there was
an "inverse association" between vigorous physical activity and how
long it took to become pregnant for normal-weight women (a BMI
under 25). In overweight or obese women, there was no link between
vigorous exercise and a longer time to pregnancy.
The study findings, which did not prove a cause-and-effect
relationship, were published March 15 in the journal
Fertility and Sterility.
Wise noted that in other research, being overweight or obese has
been associated with lower fertility rates compared to
normal-weight women. Obesity has also been associated with
menstrual cycle disturbances. She said increased physical activity
in this study might reverse the harmful effects of obesity and
improve the overweight women's overall fertility rates.
The exact mechanisms for why physical activity might enhance
fertility in overweight and obese women remain unclear, though, and
the researchers didn't delve into them, Wise said.
Dr. Dimitrios Mastrogiannis, director of the division of
maternal-fetal medicine and an associate professor of obstetrics,
gynecology and reproductive sciences at Temple University School of
Medicine in Philadelphia, said, "Obesity is a different ballgame.
Different hormones are playing around in obesity -- fatty tissue
produces more female hormones, more estrogens. Other hormones are
transformed into female hormones in the adipose tissue."
The research does not suggest packing on pounds to get pregnant,
or backing off physical activity completely, he said.
"Exercise is a good thing. It's linked to less cardiovascular disease, less cancers, less diabetes," said Mastrogiannis, noting that physical activity is also linked to better pregnancies, easier labors, less pain and fewer induced births.
The bottom line of the study is that normal, non-obese women who
want to become pregnant should stick to moderate aerobic exercise,
"We recommend our patients get moderate aerobic exercise, akin to 30 minutes a day is usually what we say. Walking is very important," he said.
"If they engage in very vigorous exercise -- running, fast cycling, gymnastics or swimming -- more than five hours a week, it makes them less fertile," Mastrogiannis said.
For more on pre-conception health, visit the
American Pregnancy Association.