Physiological changes in pregnancy

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Under the influence of estrogen, progesterone, and elastin, pregnancy is associated with generalized connective tissue laxity, potentially leading to ligament and joint instability [1]. Additional strain on the musculoskeletal system comes from the change in the body's center of gravity, resulting in progressive lordosis (accentuation of the lumbar curvature of the spine) and kyphosis (curvature of the upper spine) [7]. The change in center of gravity requires greater muscular effort with certain movements, such as rising from a squatting or sitting position or changing directions quickly. The progressive lordosis in pregnancy frequently results in lower back pain, which could be prevented by improving posture and muscular strength preferably prior to pregnancy [8]; such preventative measures are also effective during pregnancy [9]. Providing exercise guidelines to increase core strength prior to pregnancy minimizes these injuries.

Special consideration must be given to changes that occur during each trimester of pregnancy that could result in injury from physical activity in pregnancy. Physical activity in pregnancy can be affected by the following progressive anatomical and physiological changes: change in center of gravity, increased connective tissue laxity resulting in joint instability, lordosis and kyphosis, generalized edema possibly resulting in nerve compression syndrome, increase in blood volume, tachycardia, hyperventilation, and reductions in cardiac reserve and residual lung capacity [10]. Figure 3.1 lists the etiology for potential injury that can occur during exercise in pregnancy and the gestational age during which the injury is most likely to occur. The goal of exercise is to maintain physical fitness within the physiological limitations of pregnancy. Exercise prescriptions should be geared towards muscle strengthening to minimize risk of joint injury and towards correcting postural changes thus diminishing lower back pain.

Physical activity may increase uterine activity (contractions). The effect of exercise on uterine activity has little or no change during the last 8 weeks of pregnancy [11]. While there are no studies reporting that strenuous activity results in preterm labor, until the impact is fully studied, women at risk of preterm labor should be advised to reduce

Physiological Changes During Pregnancy
Fig. 3.1. Potential mechanisms leading to injuries during exercise in pregnancy. (From [10])

activity in the second and third trimesters [12]. There is a link between strenuous physical activity and the development of intrauterine growth restriction in the presence of dietary restrictions. Mothers with physically demanding and repetitive jobs were reported in several studies to deliver early and give birth to small-for-gestational-age infants [13-15]; meanwhile, other studies on vigorous exercise found no difference [16] or an increase [17] in infant birth weight. It appears that infant birth weight is not affected by exercise if energy intake is adequate [18], and that fetal weight can be maintained with adequate nutritional intake.

3.3 hyperthermia

An increase in body temperature during exercise is directly related to the intensity of the activity. During moderate intensity exercise in normal temperature conditions, the body's core temperature tends to rise an average of 1.5°C during the first 30 min of activity, followed by a plateau if the same level of activity continues [19]. Heat is dissipated predominantly through the skin. If heat production exceeds heat dissipation—which can occur if exercise takes place in hot humid conditions, with vigorous exercise, if there is exposure to hot tubs, saunas, or if the woman is running a fever—the core temperature continues to rise. Animal studies have shown that an increase in core temperature greater than 39°C during embryogenesis from 3 to 8 weeks gestation can result in congenital malformations [20-22]. For humans, the association of hyperthermia and congenital malformations is primarily acknowledged in case studies, which suggests a relationship but does not prove causality [23-25]. One prospective study using 165 women exposed to hyperthermia during the first trimester failed to confirm teratogenic effects [26]. As the risk of hypothermia is a concern, pregnant women should be advised to avoid hyperthermic conditions during the entire pregnancy, and particularly the first trimester.

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Dealing With Back Pain

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