Pregnancy: Physiological Adaptations

A significant benefit of maternal exercise during the first half of pregnancy is that maintaining or increasing exercise time and volume results in increased placental volume and functional capacity (Clapp et al, 2002). A higher functioning placenta helps to improve fetal development while in utero.

The prenatal metabolism increases by 15-20% to support new tissue growth (Clapp et al, 2012). The metabolic system alters the macronutrients utilized for energy, as the maternal system focuses on fats and proteins, prioritizing carbohydrates for the fetus (Clapp et al, 2012). Throughout pregnancy, insulin resistance gradually increases in the maternal system but can be countered by regular exercise (Clapp et al, 2012).

Regular exercise increases the number of mitochondria, enhancing energy production and efficiency in aerobic capacity by 20% (Clapp et al, 2012). Regular exercise helps manage maternal weight gain and reduces insulin resistance, which can support the adaptations to the musculoskeletal system. Overall, regular exercise decreases the stress on the system by improving the efficiency in which the mother can use the minimum required energy to accomplish tasks.

The musculoskeletal system adaptations are in response to the shifts in weight distribution, weight gain, and increased joint laxity. Regular exercise can help to counter the potentially detrimental effects of these adaptations.

Throughout pregnancy, mass increases 15-25% and increases mechanical stress on the lower body (Clapp et al, 2012). This increased mechanical stress can be exaggerated by a change in force distribution through the musculoskeletal system due to a shift in the center of gravity, increased lumbar lordosis, and joint laxity.

As the belly grows, the center of gravity shifts outwards and upwards causing a change in weight distribution. This shift of midline can result in a postural tendency of a posterior pelvic tilt with ribs shifted backwards, or an anterior pelvic tilt with ribs thrusted forward (Wiebe, 2016). More likely, a posterior pelvic tilt will occur to bring more of the weight midline (Wiebe, 2016). This postural tendency could result in a muscular imbalance referred to as lower cross syndrome, where the gluteals and abdominals are inhibited and the erector spinae and iliopsoas are restricted (NASM, 2018). This muscular imbalance can affect the alignment of the pelvis and uterine shape, which could influence fetal positioning during pregnancy and birth. Strengthening the posterior chain could help increase postural endurance to maintain a neutral postural position and minimize the amount of postural compensation patterns, thus improving musculature balance.

The female lumbar spine increases in lordosis to accommodate for the increased mass and shift of center of gravity (Bailey et al, 2016). This increased lumbar lordosis is not necessarily associated with increased back pain (Bailey et al, 2016), but this does change how force is distributed through the lumbar spine and lower extremities. This change in spinal alignment should be considered during exercises, as movements that increase this lordosis would be less comfortable for the woman and may exaggerate an anterior pelvic tilt and rib thrust, thus decreasing stabilization.

As the hormone relaxin increases throughout pregnancy until after 2 weeks postpartum, the joints increase in laxity by 40% (Hemmerich et al, 2019). This increase in joint laxity does not necessarily correlate to increased lower back or pelvic girdle pain (Schauberger et al,1996). If the joint laxity is not supported by increased strength of the surrounding musculature and myofascial slings, then the increased laxity could contribute towards unstable movement. Unstable movement can contribute towards a loss in mechanical integrity and cause injury, which may result in decreased prenatal comfort and function. Regular resistance training exercise, particularly focused on strengthening the connection of the myofascial slings can help to alleviate prenatal discomfort from increased joint laxity.

The local stabilization system is the anticipatory core that activates prior to the initiation of movement, and stabilizes the spine from vertebra to vertebra (NASM, 2018). The local stabilization system is affected by pregnancy: the respiratory diaphragm’s range of motion is restricted as the uterus and fetus grow; the transverse abdominals and internal obliques are overlengthened and stretched, thus inhibited; and the pelvic floor increases in load and force due to the increased mass of the of uterus, fetus, and retained fluids (Wiebe, 2016).

This decreased function of the local stabilization system could result in the diminished stabilization patterns and increased occurrence of movement compensation patterns to overcome lack of stability. Fortunately, the increased pressure within the abdominal cavity from the growing uterus, fetus, and retained fluids helps to stabilize the spine. This instability will be emphasized in the postpartum period after the pressure within the abdominal cavity is suddenly decreased.

All of the musculoskeletal adaptations due to pregnancy accommodate for fetal growth, but can result in diminished function and increased prenatal discomfort. Regular exercise can counter these adaptations by: improving abdominal tone and strengthening the posterior chain to improve postural endurance to counter the shift in the center of gravity; strengthening myofascial slings to support increased laxity of maternal joints; increase ligamentous tensile strength and bone density; and improve stabilization patterns and decrease movement compensation with improved coordination