There are many anatomic and physiologic changes in pregnancy that impact the evaluation and management of a pregnant trauma patient. The enlarging uterus remains protected within the bony pelvis until about 12 weeks and is only vulnerable in cases of severe trauma or if the mother is hemodynamically unstable. At 20 weeks, when the fetus is potentially viable, the uterus becomes an abdominal organ and reaches the level of the umbilicus. At 34-36 weeks, the uterus reaches the level of the costal margins. The distance measured in centimeters from the pubic bone to the top of the uterine fundus can help approximate the gestational age of the fetus (Figure 1). As the uterus enlarges, it can compress the inferior vena cava whenever the patient is lying on her back. This compression can reduce cardiac output by up to one-third, resulting in hypotension.



In general, the physiologic adaptations of pregnancy are the result of either the direct effect of the increased circulating progesterone levels or the hemodynamic changes that occur to meet the increased perfusion requirements and metabolic needs of a developing pregnancy (Table 2).



Cardiovascular Changes

By the end of pregnancy, the uterus and placenta receive approximately 20% of the maternal cardiac output. To meet this increased demand, the resting heart rate is increased by 10-15 beats per minute (BPM). Cardiac output is increased by 1.0-1.5 liters per minute. Blood pressure is decreased 5-15 mm Hg in the second trimester because of decreased vascular resistance and vasodilatation secondary to the effects of progesterone on smooth muscle, and it returns to baseline close as gestation approached term. Maternal blood volume expands significantly during pregnancy. Since red cell production does not increase proportionately, dilutional anemia is common. Because of expanded blood volume, a pregnant patient may lose from 1000-1500 mL of blood or experience 30%-35% of total blood loss before exhibiting overt signs of shock.

Pulmonary Changes

Pulmonary physiology changes significantly during pregnancy. During pregnancy, oxygen consumption is increased. To meet the increased oxygen requirements, tidal volume and respiratory rate are also increased. Consequently, pCO2 will decrease, usually to about 30 mm Hg; a normal pCO2 may indicate respiratory compromise. Functional residual capacity (FRC) is decreased. The end result is a lower pulmonary reserve. The pregnant trauma patient should be aggressively treated when there are any signs of respiratory compromise since she can decompensate rapidly.

Gastrointestinal Changes

During pregnancy, gastric motility decreases, and gastric emptying time increases in combination with a decrease in lower esophageal tone which leads to an increased risk of aspiration.

Orthopedic Changes

Greater ligament laxity during pregnancy increases the risk of orthopedic injuries.

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