Pregnant women should avoid intense exercise in view of findings that extremely vigorous activities can slow the heart rate of unborn infants, researchers said Wednesday.

A study, published in the Journal of the American Medical Association, shows some fetal heart rates slowed significantly after pregnant women engaged in exercise boosting their heart rates to 150 beats per minute or more.Although researchers said they do not know what triggers the drop in fetal heart rates, they expressed concern their findings may indicate the unborn babies are being deprived of oxygen.

Studies involving pregnant sheep have shown extreme exertion can result in fetal oxygen deficiencies.