Physiological Effects of Aerobic Exercise During Pregnancy Maternal
Physiological Effects of Aerobic Exercise During Pregnancy
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 steadystate 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 non-pregnant state. • The pregnant woman reaches a maximum exercise capacity at a lower work level than a non-pregnant 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 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.
Responses to healthy women • 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.
Fetal response to maternal aerobic exercise
• No human research has conclusively proven a detrimental fetal response to mild- or moderate-intensity maternal exercise. • Recent studies suggest that even vigorous exercise does not have the detrimental effects on the fetus that once were feared, • Therefore restrictions on exercise because of concerns for the effects on the embryo and fetus have been lessened. • In fact, fit women who maintained their volume of exercise after 20 weeks’ gestation delivered babies with lower fat mass than those who decreased exercise intensity mid-way through the pregnancy.
Blood flow • A 50% or greater reduction of uterine blood flow is necessary before fetal well-being is affected (based on animal research). • No studies have documented such decreases in pregnant women who exercise, even vigorously. • It is suggested that the cardiovascular adaptations in exercising women offset any redistribution of blood to muscles during exercise.
Fetal heart rate • Brief submaximal maternal exercise (up to 70% maternal aerobic power) does not adversely affect fetal heart rate (FHR). • The FHR usually increases 10 to 30 beats per minute at the onset of maternal exercise. • After mild to moderate maternal exercise, the FHR usually returns to normal levels within 15 minutes, • strenuous maternal exercise the FHR may remain elevated as long as 30 minutes. • Fetal bradycardia (indicating fetal asphyxia) during maternal exercise has been reported in the literature, with the return to pre-exercise FHR levels within 3 minutes after maternal exercise, followed by a brief period of fetal tachycardia. • The healthy fetus appears to be able to tolerate brief episodes of asphyxia with no detrimental results.
Heat dissipation • The fetus has no mechanism such as perspiration or respiration by which to dissipate heat. However, physically fit women are able to dissipate heat and regulate their core temperature more efficiently, thus reducing risk.
Newborn status • Newborn children of women who continue endurance exercises into the third trimester of pregnancy are reported to have an average decrease in birth weight of 310 g. There is no change in head circumference or heel–crown length. Further study of these children (up to 5 years of age) has shown slightly better neurodevelopmental status in addition to higher percentage of lean body mass.
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