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Exercise During Pregnancy: The Risks and Benefits

Regular exercise provides significant and well-documented health and fitness benefits, but concern regarding the potential for adverse maternal and/or fetal outcomes in women who exercise during pregnancy has led to uncertainty and inconsistent advice. The topic has been debated within the medical community, and I will discuss the current mainstream healthcare perspective on the subject, weighing the known and potential risks against the rewards.

The first question to answer is: what are the established and known benefits of exercise during pregnancy? Physically active pregnant women tend to experience improved fitness and sense of well-being, less adverse symptoms of pregnancy, and 50% decrease in gestational diabetes and 40% reduction in preeclampsia.

Second, what are the known or well-established risks of exercise during pregnancy? High intensity third trimester exercise leads to low birth weights, while starting a new exercise program in poorly conditioned women results in higher than normal birth weights.

Next, what concerns of exercise during pregnancy have not been borne out scientifically? Maternal hyperthermia (elevated maternal body temperature) has been associated with fetal neural tube defects, and has thus led to the belief that exercise-related hyperthermia may be teratogenic. However, these fears may be unsupported because of the increased blood volume, earlier initiation of sweating, and larger body habitus generally found in pregnant women may collectively dampen the hyperthermic response to exercise during pregnancy.

Fear of fetal hypoxia during maternal exercise has also led to the discouragement of exercise during pregnancy. However, we need to consider the physiologic adaptations to pregnancy, which may relieve that fear. We know that most functional changes occur by 12 weeks of gestation, including increases in plasma volume, red blood cell mass, heart rate (HR), stroke volume (SV), and thus cardiac output (CO = HR x SV). Thus, exercising women experience 40% greater increase in cardiac output and 20% greater increase in blood volume compared to sedentary women.

Uterine blood flow has been shown to decrease during exercise, but the maternal uterus compensates by shunting blood from the myometrium (the smooth muscle layer of the uterus responsible for contractions) to the placenta, and thus increases uteroplacental tissue oxygen extraction. If fetal hypoxia was significant, we would expect to find a compensatory elevation in fetal erythropoietin levels. However, fetal erythropoietin levels tend not to increase in response to sustained strenuous maternal exercise, which contradicts the belief of fetal hypoxia induced by maternal exercise. Also important is that exercise during pregnancy has not been shown to increase the incidence of miscarriages or pre-term labor.

What are the recommendations for exercise in pregnancy based on a current review of the risks, benefits, and physiologic adaptations? First of all, exercise is not recommended in high-risk pregnancies. Contraindications to exercise during gestation include significant heart disease or anemia, cervical incompetence, uterine bleeding, ruptured membranes, fetal distress, more than 1 prior miscarriage or premature labor, and uncontrolled hypertension or renal disease.

For pregnant women who do not have any of the above contraindications, current recommendations include initiation of a new exercise program with short duration (15 minutes) and low intensity (55-65% of maximum heart rate) at a frequency of 3 times per week. Low impact (walking) and non-impact (swimming) are preferred over high impact (jogging) exercises. The duration, frequency, and intensity of exercise can be gradually increased, but the intensity is not recommended to exceed 85% of the mother’s maximum heart rate (MHR), which is roughly estimated by the equation: 220 minus the pregnant woman’s age. For example, the MHR in a pregnant 20-year-old is 220 minus 20 = 200, and thus the intensity should not surpass a heart rate of 170 (200 x .85).

Supine exercise, such as floor exercises lying on the back, is not recommended after the first trimester due to the elevation in myocardial oxygen demand compared to exercises in the standing position, but exercises to strengthen the pelvic floor are recommended. Strengthening exercises are restricted to only light resistance, and proper breathing technique is emphasized for weight lifting (inhalation during the eccentric phase and exhalation during the concentric phase of muscle contraction). In addition, fluid and caloric intake should be monitored carefully because of the additional nutritional requirements of gestation.

Exercises during pregnancy can be categorized as either low-risk (e.g. walking, swimming, low-impact aerobics, light weight lifting) or high-risk (e.g. mountain biking, heavy weight lifting, contact sports, scuba diving below 10 meters, skiing, ice skating). Immediate discontinuation of exercise is recommended if the mother experiences any of the following symptoms: shortness of breath, dizziness, nausea, weakness, pain, generalized edema, uterine contraction, vaginal bleeding, fluid leakage, or diminished fetal activity.

Tobey Leung, M.D., FAAPMR

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