The hesitance of obstetricians to recommend exercise to pregnant women is rooted in old-fashioned notions of pregnancy as a time of confinement. In the absence of reassuring data regarding the effects of exercise on the mother and fetus, most obstetricians adhered to the principle of doing no harm—advising women to eat for two and not to move.
With ample evidence to show that regular, moderate exercise in women with healthy pregnancies results in no adverse maternal or fetal effects, it could be argued that, in the spirit of “primum non nocere,” obstetricians should make exercise recommendations a top priority.
Indeed, because it is recognized that habits adopted during pregnancy can result in persistent lifestyle improvements, the promotion of exercise during pregnancy is an important public health issue that could significantly reduce the lifetime risks of obesity, chronic hypertension, and diabetes—not only for our patients, but for their families as well.
Recently, exercise has been recognized as an effective alternative to insulin therapy for treating gestational diabetes and as a means of preventing this disorder, which is frequently the first manifestation of what can become a lifelong condition.
Healthy Pregnancy? Few Restrictions
Despite the profound anatomical and physiologic changes of pregnancy, women with healthy pregnancies and no contraindications can exercise just as their nonpregnant counterparts do, combining both aerobic and resistive elements in their workouts. (See box on opposite page.)
A clinical evaluation of each patient is recommended before prescribing exercise, and physicians must consider the type and intensity of exercise—as well as the duration and frequency of exercise sessions—for each patient, based on her level of fitness and familiarity with various activities.
Contact sports and exercises with a high risk of falling or abdominal trauma should be avoided. Scuba diving should be avoided throughout pregnancy because this activity puts the fetus at increased risk for decompression sickness secondary to the inability of the fetal pulmonary circulation to filter bubbles.
Moderate exercise is defined as a level of intensity that still allows normal conversation—equivalent, for example, to brisk walking at 3–4 miles per hour. For women who have been sedentary and are taking up exercise for the first time, a gradual progression to this intensity for up to 30 minutes per day is recommended. Those who are already fit when they become pregnant should be advised that pregnancy is not a time for greatly improving physical fitness and that, in general, overall activity and fitness levels tend to decline during pregnancy.
Pregnant women should exercise caution in increasing the intensity of their workouts, especially when they are extending exercise sessions beyond 45 minutes, because body core temperatures can rise above safe limits after that time. Strenuous exercise has not been proved to increase overall benefit and could actually be harmful, so this level of exercise intensity should be avoided.
Maternal cardiovascular, respiratory, and thermoregulatory adaptation occurs as a result of pregnancy and is further challenged by the addition of exercise. There is decreased availability of maternal oxygen during exercise because of increased maternal oxygen requirements at rest and the increased difficulty in breathing caused by the pressure of the enlarged uterus on the diaphragm. In addition, pregnancy raises basal metabolic rate and heat production, which are then further raised by exercise.
The hesitance of many obstetricians to prescribe exercise for pregnant women centers on the hypothetical fetal risks of impaired transplacental blood flow of oxygen, carbon dioxide, and nutrients during maternal exercise, as well as the potentially teratogenic effects of raising fetal temperature.
Most studies show a minimal to moderate increase in fetal heart rate during maternal exercise, and there is also evidence of fetal heart rate decelerations and bradycardia; however, no lasting fetal effects have been reported.
Data on the effects of increased maternal core temperatures are limited. Hyperthermia during embryogenesis (the first 45–60 days following the last menstrual period) has been shown to cause major congenital malformations (JAMA 1992;268:882–5).
The temperature threshold for human teratogenesis is 39.2° C (103° F). Moderate exercise performed in conditions allowing adequate heat dissipation has been shown to raise core temperatures no higher than 1.5° C during 30 minutes of exercise in nonpregnant women—and this temperature plateaus during as much as 1 hour of exercise.
Loss of fluid through sweat may compromise heat dissipation, so maintenance of euhydration—and thus blood volume—is essential to controlling core temperature.
Extra Nutritional Requirements
Although the published data on a link between low birth weight and maternal exercise are conflicting, it appears that adequate energy intake can offset any exercise-induced decreases in birth weight.