Physiological Effects of Aerobic Exercise During Pregnancy
Many women who have been doing aerobic exercises choose to continue exercising during pregnancy to maintain their cardiopulmonary fitness. Maternal and fetal responses have been well studied; therefore this information is used to guide both the therapist and the patient in determining necessary modifications to an existing exercise program.

Maternal Response to Aerobic Exercise

Blood Flow
Aerobic exercise does not reduce blood flow to the brain and heart. It does, however, cause a redistribution of blood flow away from the internal organs (and possibly the uterus) and toward the working muscles. This raises two concerns: that the reduction in blood flow may decrease the oxygen and nutrient availability to the fetus and that uterine contractions and preterm labor may be stimulated. Stroke volume and cardiac output both increase with steady-state exercise. This, coupled with increased blood volume and reduction in systemic vascular resistance during pregnancy, may help offset the effects of the vascular shunting.

Respiratory Rate
The maternal respiration rate appears to adapt to mild exercise but does not increase proportionately with moderate and severe exercise when compared with a nonpregnant state. The pregnant woman reaches a maximum exercise capacity at a lower work level than a nonpregnant woman because of the increased oxygen requirements of exercise.

Hematocrit Level
The maternal hematocrit level during pregnancy is lowered; however, it rises up to 10 percentage points within 15 minutes of beginning vigorous exercise. This condition continues for up to 4 weeks postpartum. As a result, cardiac reserve is decreased during exercise.

Inferior Vena Cava Compression
Compression of the inferior vena cava by the uterus can occur after the fourth month of pregnancy, with relative obstruction of venous return. This leads to decreased cardiac output and orthostatic hypotension. It occurs most often in supine or static standing positions, and therefore prolonged time in these positions should be avoided.

Energy Needs
Hypoglycemia occurs more readily during pregnancy; therefore, adequate carbohydrate intake is important for the pregnant woman who exercises. A caloric intake of an additional 500 calories per day is suggested to support the energy needs of pregnancy and exercise, dependent on the intensity and duration of the exercise. In comparison, a sedentary pregnant woman requires a 300 calorie per day increase.

Core Temperature
Vigorous physical activity and dehydration through perspiration leads to increased core temperature in anyone who exercises. Concern has been expressed over this occurring in the pregnant woman because of the relationship of elevated core temperature to neural tube defects of the fetus. Studies report that during pregnancy the core temperature of physically fit women actually decreases during exercise. These women appeared to be more efficient in regulating their core temperature, and thus the thermal stress on the embryo and fetus is reduced.

Uterine Contractions
Norepinephrine and epinephrine levels increase with exercise. Norepinephrine increases the strength and frequency of uterine contractions. This may pose a problem for the woman at risk of developing premature labor.

Healthy Woman Response
Studies have shown that healthy women who continue to run throughout pregnancy deliver on the average of 5 to 7 days sooner compared with controls. Clapp found that exercise, including weight bearing (even with ballistic motions such as during aerobic dancing), can be performed in mid- and late pregnancy without risk of preterm labor or premature rupture of the membranes. Women who wish to continue strenuous or competitive exercise or participate in specific athletic training require close supervision by a specialist during pregnancy.

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