Low-impact exercise during pregnancy: A study of safety

TL;DR: Low-impact aerobics at about 70% of one's maximum heart rate appears to be safe in terms of risk of maternal hyperthermia, as compared with pre-exercise values.

Abstract: The major concern about pregnant women exercising vigorously is that the consequent hyperthermia might pose a hazard for the fetus. Nonpregnant women may reach a core temperature of up to 40°C while exercising. Animal studies have related heat stress in early pregnancy to a higher risk of neural tube defects. In this study, body temperature, heart rate, and blood oxygen saturation were monitored in 40 pregnant women and 11 nonpregnant control women attending aerobic exercise programs who performed low-impact aerobic exercise at similar levels. Conditioning, weight-bearing activities, stretching, and relaxation were elements of the program. Jumping, running, and exercising when supine were excluded. The study and control groups were similar in body mass. Pregnant women were evaluated at a mean gestational age of 25 weeks. None of the pregnant or control women reached a potentially dangerous core temperature at maximum exercise or afterward. The pregnant women performed at 69% of their maximum heart rate. Preexercise and maximum-exercise core temperature decreased significantly with advancing gestational age. Oxygen saturation was significantly reduced both during and after maximal exercise in the pregnant women, although none had a value less than 95%. Body temperature averaged 97.4°F before exercise and 98.7°F during maximum exercise. No such change was noted in control women. The pregnant women exercised at a slightly lower level than did the control women. Approximately half the women had exercised less since the onset of pregnancy, whereas 12% increased their level of exercise. More than two thirds of pregnant women reported exercising more intensely before the study than during monitoring, and this was the case for all the control women. These findings endorse recent recommendations that pregnant women exercise regularly. Hyperthermia appears not be a risk for women who perform low-impact aerobic exercise at approximately 70% of their maximum heart rate.