This is an excerpt from Physical Activity and Health Guidelines by Riva Rahl.
The goals of physical activity during pregnancy include maintaining a sense of maternal well-being, avoiding fetal harm, and establishing a pattern of regular activity that will ultimately prevent the onset of chronic disease associated with a sedentary lifestyle. Most women can participate in a wide range of recreational activities and maintain an active lifestyle during pregnancy, although those with medical or obstetrical complications may require modifications. Modifications may also be necessary for women who regularly participated in strenuous recreational or competitive physical activities before their pregnancies. In addition, women who are active before pregnancy may choose to modify their exercise programs somewhat; for example, weight gain or joint laxity may prompt women to walk instead of jog or to exercise indoors instead of outdoors because of improved environmental control. Because of all the physiological changes that occur during pregnancy, activity recommendations for pregnant women are subject to modifications. Finally, concerns for fetal health dictate other changes in order to optimize fetal outcomes.
Nevertheless, the benefits of regular physical activity throughout pregnancy generally outweigh the risks. Women who exercise routinely in the prenatal period have a lower incidence of babies with low birth weight. Starting with the first trimester, regular activity may help ease nausea and lessen fatigue. Body image and mood—both during pregnancy and in the postpartum period—are positively influenced by regular physical activity. Although there are not yet disorder-specific guidelines, it is felt that regular physical activity during pregnancy benefits preeclampsia, hypertension, and gestational diabetes, as mentioned earlier. It is not yet clear whether a regular physical activity program affects gestational length, quality or duration of labor, or incidence of chronic disease in the offspring.
Although the benefits of physical activity for pregnant women are significant, there are still precautions to take and health risks to keep in mind. Because of the increased metabolic rate during pregnancy, thermoregulation during exercise and the potential effect on fetal temperature are a concern. It is important for pregnant exercisers to stay well hydrated in order to improve cooling and maintain blood volume. While the benefits of chronic exercise belong to the mother, the risk of overexercise predominantly affects the fetus (Whaley 2005).
Starting with the second trimester, supine exercises are not recommended, as these can cause the uterus to compress the inferior vena cava and result in decreased cardiac output. Also, prolonged isometric exercise during weightlifting may decrease uterine perfusion. Because of the shift in the expecting mother's center of gravity, exercises that require balance should be avoided later in pregnancy; other anatomical changes of pregnancy are presented in the sidebar. Contact sports and activities that have an increased risk for falls should also be avoided. The new PAGA lists several exercises to avoid; table 6.1 includes these as well as others with risk for maternal or fetal harm. Motionless standing results in venous pooling and a significant decrease in cardiac output and thus should be avoided. In fact, one large study of 7,722 pregnancies found that mothers who had jobs requiring predominantly standing had babies with lower birth weights (Naeye and Peters 1982).