Anatomic/Musculoskeletal Considerations

There are various anatomic changes that occur throughout pregnancy that may affect a woman’s ability to participate in certain activities or sports, of which the most obvious and noticeable is gestational weight gain. For a female with a normal prepregnancy body mass index (BMI) (18.5–24.9 kg/m ² ), the optimal weight gain is between 25 and 35 pounds, with an average of 1 pound per week in the second and third trimesters. Increased mass may transfer significant force across the larger weight-bearing joints such as the knees and hips. Over time, this may cause discomfort and limit the ability to participate in activities or sports requiring full load bearing, such as running.

Throughout pregnancy, both the expanding uterus and enlarging breasts will displace the center of gravity. Increased lumbar lordosis and subsequent anterior pelvic rotation on the femur occur in pregnancy, changing a woman’s center of gravity. This is important for activities that require optimal balance such as cycling on a nonstationary bike or walking on uneven ground.

Finally, ligamentous laxity increases throughout pregnancy, secondary to the effects of increased levels of estrogen and relaxin, which can predispose a female to joint instability and the theoretic risk of increased strains and sprains. ^{, ,}

Hemodynamic Adaptations

Significant hemodynamic changes occur during pregnancy, including increased cardiac output, increased resting heart rate, increased stroke volume, and decreased systemic vascular resistance. ^, Cardiac output may increase by as much as 50% in the third trimester. ^, These changes are to supply sufficient blood to the placenta and the growing fetus. ^, During exercise, there can be up to a 50% decrease in splanchnic blood flow and subsequently blood flow to the uterus, as blood is redirected to the exercising muscles. This raises the hypothetical risk of fetal hypoxemia during exercise, which has been the subject of multiple reports. However, flow velocity profiles in the fetal aorta and umbilical circulation in various studies resulted in contradictory and inconclusive results. ^{, ,} At this time, the general consensus is that an increase in fetal heart rate between 10 and 30 beats per minute (bpm) over baseline during maternal exercise does not have a negative sequela on the fetus and that overall fetal injuries are unlikely during a normal uncomplicated pregnancy.

An important consideration is the effect of supine positioning on the fetus, both at rest and during exercise. Primarily after the first trimester, compression of the inferior vena cava by the enlarged uterus reduces cardiac output. ^, A similar phenomenon has been seen with motionless standing for prolonged periods, which leads to decreased venous return and a subsequent increased risk of hypotension. For this reason, it is best to assume a right or left lateral side lying position and avoid motionless standing as pregnancy progresses, especially if one is experiencing hypotensive episodes. Equally important is avoiding supine exercises such as bench press or sit-ups.

Respiratory Adaptations

Along with the cardiovascular adaptations during pregnancy, significant respiratory changes occur. As the uterus enlarges, the diaphragm displaces superiorly, which ultimately causes a profound increase in tidal volume and subsequently minute ventilation, decreasing arterial carbon dioxide. ^{, ,} All these adaptations protect the fetus from an acidic environment, which may significantly affect the function of various organ systems, such as the cardiovascular and central nervous systems, ultimately lowering APGAR (appearance, pulse, grimace, activity, and respiration) scores.

The pressure of the enlarged uterus on the diaphragm causes a decrease in oxygen availability and thereby increased work of breathing and feelings of respiratory discomfort late in pregnancy. ^, In response to increased oxygen requirements of the fetus, there are mild increases in tidal volume and oxygen consumption in pregnant women. To meet the greater oxygen demand during physical activity, pregnant women will have an increase in respiratory frequency and oxygen consumption with just mild exercise. ^{, , ,} Studies have demonstrated that during pregnancy, the subjective effort to perform aerobic exercise is increased and maximum voluntary exercise performance is decreased. ^{, ,}

Thermoregulatory Adaptations

Metabolic rate increases throughout pregnancy subsequently increasing heat production. ^, In the first trimester, core temperatures above 39°C (103°F) should be avoided because of an increased risk of neural tube defects. ^{, ,} In the second and third trimesters, thermoregulatory control improves, during which fetal temperature is maintained approximately 1°C above maternal core temperature, due to fetoplacental metabolism. ^{, ,} Core temperature does not increase significantly with steady-state moderate exercise, which is approximately 60%–70% V o ₂ max. ^, It is critical that heat dissipation remains greater than heat production to protect the fetus and ensure adequate uterine blood flow. For this reason, exercising in hot, humid environments or engaging in strenuous and high-intensity activities should be avoided.

Aerobic Exercises

Aerobic exercises that are advisable in pregnancy include those that focus on the larger muscle groups and those in which intensity is easily modified based on the goal heart rate or the talk test, as mentioned in the section General Recommendations. Brisk walking, jogging, swimming, cycling, rowing, dancing, and cross-country skiing are all examples of aerobic activities that one can partake in while pregnant. Again, this must be catered to each patient and pregnancy based on individual risk factors, previous experience, and preferences.

Strength Training

Overall, there is much less evidence with regard to strength training, especially heavy lifting, during pregnancy. Based on findings from recent studies, lower weights and higher repetitions did not have negative impacts on pregnancy outcomes. ^{, , ,} Although evidence is lacking regarding heavier weight strength training or Olympic lifts, it is advisable to refrain from any lifts that require a Valsalva maneuver. The Valsalva maneuver increases intra-abdominal pressure and in turn hypothetically reduces blood flow and oxygen supply to the fetus, in addition to the potential of damaging pelvic floor muscles. ^{, ,} Pelvic floor muscle training, if done correctly, has been shown to treat and prevent urinary incontinence both during pregnancy and after delivery. Females should aim to perform these exercises at least three times per day on most days of the week for the greatest benefit. ^, It may be prudent to avoid heavy lifts or physical strain during the 6–9 days after estimated ovulation; limited evidence has shown a slightly increased risk of miscarriage during this time of implantation. If done in the appropriate manner, weight lifting during pregnancy can have beneficial effects on the mother without negatively impacting the health of the baby.

Flexibility

Recent studies have shown that prenatal yoga is a promising treatment for maternal depression, providing beneficial effects on maternal comfort during labor, as well as reduced stress, reduced anxiety, and overall improved quality of life. ^, Increased ligamentous laxity during pregnancy causes pelvic instability and misalignment of the spine, which can result in pain and discomfort. Maintaining flexibility during pregnancy enables a female to adapt effectively, efficiently, and safely to pregnancy-associated changes in alignment, joints, tendons, and ligaments. Stretching during pregnancy should focus on maintaining a normal range of motion required for activity, stretching the muscle belly rather than at the tendon or ligament, and performing stretches in a slow and controlled manner for maximum effectiveness.