It's possible that the tests used in this study can't pick up small but important changes that occur after exercise, Heo said.
All the women in the study gave birth to healthy babies, although the study was too small to make a conclusion about the effect of exercise on health after delivery.
The authors plan to study exercise in pregnant women who develop complications, such as high blood pressure, as well as competitive elite athletes. "There's a lot of questions about what happens at very high levels of exercise," Szymanski said.
Find out more about healthy pregnancy at womenshealth.gov.
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