Study Cautions Pregnant Women to Avoid Punishing Exercise
Moderate exercise by a pregnant woman is not likely to harm the health of her unborn child, but exercising to exhaustion can slow down the heart rate of the fetus and therefore should be avoided, a new study has found.
The study, published today in the Journal of the American Medical Assn., advises pregnant women to limit their vigorous exercise to activities requiring no more than 150 heartbeats per minute and to wind down with a gentle, steady reduction of effort.
“What they’ve said is you can exercise to a point, but don’t go crazy,” said Dr. Robert Resnik, chairman of the department of reproductive medicine at UC San Diego. “Everybody says that and this is the data to prove it.”
But one researcher who has also studied the effects of exercise on blood flow to the uterus cautioned that the effects of exertion may vary widely depending on the type of exercise, the size of the muscles involved and the position of the body.
The researcher, Dr. James Clapp of the University of Vermont, said the effects of riding a stationary bicycle like the ones used in the new study may differ significantly from the effects of swimming, aerobic dancing or other forms of exercise that involve more muscles.
“I think the societal issue is what to tell women to do and what to tell them to watch out about,” said Clapp, a professor of obstetrics and gynecology. “And I just don’t think we’re at a point where we can do that with a great deal of certainty.”
In the new study, researchers affiliated with Brown University in Providence, R.I., monitored the heart rates of the fetuses of 45 pregnant women while they rode stationary bicycles at increasing levels of resistance and later as they cooled down.
On only one occasion did the researchers document a slowing of fetal heart rate during so-called submaximal exercise--that is, the equivalent of exercise that prompts flushing, sweating and shortness of breath but can be done easily for 30 minutes to an hour.
However, the team found that fetal heart rate slowed in 15 cases after the women rode the bikes until exhaustion left them unable to continue. In one case, the researchers observed the same reaction after less-than-maximum exercise.
The significance of the heart-rate slowing, known as bradycardia, is not clear. But it can serve as a sign of physiological problems. Heart-rate slowing might be no more than an innocuous reflex; or it might reflect a reduced flow of blood and oxygen to the uterus.
“Based on our relative ignorance of how fetuses respond to uterine blood flow, we would caution against women exercising to their maximum aerobic capacity,” said Dr. Marshall W. Carpenter, chief researcher on the study. “It’s based on ignorance rather than anything else.”
Carpenter’s study is the latest in a series of recent projects aimed at determining whether maternal exercise might harm a fetus--specifically, by reducing blood flow to the uterus in order to supply oxygen to exercising muscles and to carry internal heat out to the skin.
The results of past studies have varied, researchers say. According to Carpenter, some studies using Doppler monitoring of fetal heart rates during vigorous exercise have suggested that fetal heart-rate slowing occurs frequently during vigorous exercise.
But Doppler monitoring--which uses ultrasound to measure the velocity of blood flow through the veins--can pick up maternal movement and produce unreliable results, Carpenter said. So he used two-dimensional ultrasound, a monitoring system that creates an image of the fetal thorax and heart, which he and other researchers say provides more accurate results.
Resnik of UC San Diego said the paper confirms with a new methodology the growing impression that moderate exercise is not harmful to an unborn child. He said studies in animals suggest the fetus may compensate for any reduction in blood flow by extracting more oxygen from the blood.
Clapp, however, warned against generalizations based on the new results.
“I think you have to look at how it applies to a woman exercising on her own--in a health club or on the road,” he said. “It ought to be reassuring, but it’s no guarantee that that’s what’s happening to her baby’s heart rate if she’s competing in a triathlon--which is not an uncommon event in my part of the world.”