Even though more women are exercising during pregnancy, and many more pregnant women are involved in strenuous activity because they work outside the home up until delivery, the effect of exercise on birth weight and pregnancy outcome is unclear. A Cochrane Database review was unable to demonstrate risks or benefits to the mother or fetus. Studies have given conflicting results on outcomes such as birth weight, maternal weight gain, rate of operative delivery, and other complications. Interpretation of studies is difficult because of differences in methodology, the populations studied, and confounding variables in the pregnant women studied. Magann and colleagues studied women enlisted in the military to evaluate the effect of exercise on pregnancy outcomes.

They monitored 750 healthy, low-risk pregnant women receiving prenatal care at a military base facility. The women had participated in routine medical screening before pregnancy and worked in similar environments. After gathering extensive data from each participant, the women were divided into four groups based on their reported levels of exercise. Overall, 217 (28.9 percent) of the women were classified as no exercise, 222 (29.6 percent) as light exercise, 73 (9.7 percent) as moderate exercise, and 238 (31.7 percent) as heavy exercise. Pregnancy outcomes were recorded for all participants.

The groups did not differ significantly in demographic characteristics or in significant variables such as smoking, stress, social support, or previous illnesses, but mothers who exercised heavily were older and had higher incomes than women in other groups. The groups were similar in rates of pregnancy loss, neonatal death, pregnancy-induced hypertension or diabetes, anemia, and antenatal hospital admission. Women who exercised reported more acute upper respiratory problems (colds and influenza) but fewer umbilical cord abnormalities than less active women. Exercising women were significantly more likely to require induction of labor and augmentation of labor with oxytocin, and to have longer first stages than nonexercisers. The women who exercised heavily had infants of lower birth weight than sedentary women, and this rate remained significant after controlling for other significant variables. Moderate to heavy exercise was associated with infants averaging 86.5 g less than those of nonexercising women.

The authors conclude that the smaller infants of exercising mothers had restricted growth limited to a restriction of neonatal fat mass but suffered no adverse consequences. They suggest that this restriction may have preventive value in reducing the risk of obesity later in life. Although they observed changes in several aspects of delivery, the authors state that exercise does not affect antenatal, intrapartum, or postpartum complications of pregnancy in healthy low-risk mothers.