Pregnant women represent a unique exercising group because of the possible competition between exercising maternal muscle and the fetus for blood flow, oxygen, delivery, glucose availability, and heat dissipation. Also, metabolic and cardiorespiratoy adaptations to pregnancy may alter the responses to acute exercise and the adaptations that result from exercise training. Benefits that are reasonable to expect from a properly designed prenatal exercise program include: improved aerobic and muscular fitness, facilitation of labor and recovery from labor, enhanced maternal psychological well-being, and establishment of permanent healthy lifestyle habits.
Exercise responses, and therefore the exercise prescription, may change during the course of the pregnancy. Some experts have expressed concern that during the first trimester exercise-induced hyperthermia may affect the closure of the neural tube. Also, the possibility of competing maternal and fetal physiological needs is greatest during the third trimester. Thus, it is probably not advisable for pregnant women to begin a NEW strenuous exercise program during the first of third trimesters. However, existing scientific studies generally support the concept that gradual increases in physical activity are appropriate during the second trimester when the discomforts and possible risks of exercise are low. Intensity and duration of an ongoing exercise program should not be increased during the third trimester and should be decreased if there are signs of exertional intolerance and chronic fatigue. Although some pregnant women have undergone maximal exercise testing, this is not recommended in non-clinical settings. Women who are appropriately screened prior to initiating exercise and who are educated regarding signs and/or symptoms for discontinuing exercise typically do not experience problems. Contraindications for exercise during pregnancy have been established by the American College of Obstetrics and Gynecology (ACOG) and are listed in Table 1.
The best modes of exercise during pregnancy are walking and non-weight bearing activities such as stationary cycling or exercise in water. For already exercising women, athletic training for competition should be discontinued. As pregnancy progresses, the center of gravity shifts and may alter balance for safe sports participation. Performance of flexibility exercises has been discouraged due to the looseness of joints; however, stretching to relieve muscle soreness for postural imbalance may be beneficial. The use of weight training for muscular conditioning during pregnancy is controversial. Any exercise that positions the individual in the supine posture during later gestation should be avoided because of a risk of postural hypertension. The pregnant uterus may compress the descending aorta and inferior vena cave resulting in decreased cardiac output and decreased blood flow to the fetus.
Since a major goal of exercise training during pregnancy is to maintain physical work capacity, the appropriate frequency of exercise is 3 to 5 times per week. Durations of 15 to 30 minutes are usually well tolerated. It is important to avoid exposing the fetus to prolonged hypoxic or thermal stresses, therefore longer duration exercise is not recommended.
Prescription of exercise intensity is complicated during pregnancy since resting heart rate is increased throughout pregnancy and maximal heart rate may be reduced in later gestation. Thus, use of conventional heart rate target zones to prescribe exercise intensity is less reliable than in non-pregnant subjects. Available data suggests that perception of exertion remains stable regardless of pregnancy status and therefore, use of RPE scales highly recommended. A target intensity of approximately 12 to 14 on the 6 to 20 point Borg scale is appropriate for most pregnant women. An additional method for preventing over-exertion is to employ the “talk test”. The talk test indicates that the intensity is excessive if an individual cannot carry on a verbal conversation during exercise.
The following precautions for the exercising pregnant woman should be strictly applied. A transition from weight bearing to non-weight bearing activity is recommended as body weight increases. High ambient temperatures and humidities should be avoided during exercise due to problems in thermoregulation during pregnancy. Supine exercise after the fourth month has been identified as problematic, although some individuals may tolerate supine exercise without experiencing problems. Exercise should be discontinued and a physician consulted when any symptoms listed in Table 2 are present. After delivery, exercise can be resumed 8 weeks postpartum or as advised by the obstetrician.
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