ANN ARBOR—Healthy women who exercise during pregnancy could discard the $99 heart monitor and just listen to their bodies, according to a study from the University of Michigan School of Nursing.

SeonAe Yeo, assistant professor of nursing, conducted a study of the effect of moderate exercise on 18 healthy, non- smoking pregnant women, and on the heart rates of their fetuses.

The women “worked out” at moderate rates in a laboratory every four weeks, from week 16 to week 38 of gestation. The exercise consisted of pedaling a wheel (about 20 inches in diameter) with their hands for 15-20 minutes while Yeo and her assistants measured heart rate, body temperature and oxygen intake, as well as the heart rate of the fetus.

While exercising, the women also were asked to report how much they felt they were exerting themselves, based on a “Rate of Perceived Exertion Scale” (RPE) recommended by the American College of Sports Medicine.

The ACSM scale defines perceived exertion as “the total amount of exertion and fatigue, combining all sensations and feelings of physical stress, effort and fatigue.”

The scale runs from 6-7 (very, very light) to 19-20 (very, very hard). The women in the U-M study exercised at a perceived rate of 13-14 (somewhat hard) for as long as 30-40 minutes.

“The principal concern about pregnant women exercising is that they may overheat,” Yeo said. “If a pregnant woman’s temperature climbs higher than 39-40 degrees Centigrade (102.2- 104 Fahrenheit) in early pregnancy, it may damage the fetus’ central nervous system.”

The temperatures of the women in the U-M study, who exercised at a “somewhat hard” level, never rose above 38 degrees Centigrade—one degree below the danger point. “Although we found that body temperatures varied slightly from woman to woman, the temperature for each individual woman was in the ‘safe’ range throughout each moderate exercise period and from month to month,” Yeo said.

“More significantly, we found that a woman’s perception of her degree of exertion—her RPE, which was verified by the measure of oxygen intake—was quite accurate. All the women knew when their RPE was light and when it was somewhat hard,” Yeo said. “Their bodies and brains were very good monitors.”

The mothers’ heart rates varied during exercise periods, depending on stage of pregnancy, general physical condition, body size, diet, and ethnic group, “so the heart monitors were not terribly good indicators of exertion,” Yeo said. Fetal heart rates were virtually unaffected by the mothers’ exercise periods.

Yeo cautions that the women in her study were healthy non- smokers and exercised in a cool, climate-controlled environment. “Until we have more information about exercise in warmer conditions, we recommend that pregnant women avoid exercise in hot, humid environments. They also should not exercise if they are feeling unwell.”

Yeo presented her findings recently (Sept. 19) at the National Institute of Nursing Research Scientific Symposium. Her study was funded by the National Institute of Nursing Research.

U-M News and Information ServicesUniversity of Michigan