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The risk of venous thrombosis and pulmonary embolism in otherwise healthy women is considered highest during pregnancy and the puerperium. Indeed, the risk of pulmonary embolism has been estimated to be as much as four- to sixfold higher during pregnancy (Christiansen and Collins, 2006; Marik and Plante, 2008). The incidence of all thromboembolic events averages about 1 per 1000 pregnancies, and about an equal number are identified antepartum and in the puerperium. In a recent study from Norway of more than 600,000 pregnancies, Jacobsen and colleagues (2008) reported that deep-venous thrombosis alone was more common antepartum whereas pulmonary embolism was more common in the first 6 weeks postpartum. The frequency of venous thromboembolic disease during the puerperium has decreased remarkably as early ambulation has become more widely practiced. Even so, there is evidence that it has increased 50 percent from 1999 to 2005 (Kuklina and associates, 2009). Importantly, pulmonary embolism still remains a leading cause of maternal death in the United States (see Table 1-2). By way of example, pulmonary embolism caused approximately 9 percent of the 623 pregnancy-related deaths in the United States in 2005 (Kung and co-workers, 2008).

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In 1856, Rudolf Virchow postulated the conditions that predispose to the development of venous thrombosis: (1) stasis, (2) local trauma to the vessel wall, and (3) hypercoagulability. The risk for each increases during normal pregnancy. For example, compression of the pelvic veins and inferior vena cava by the enlarging uterus renders the venous system of the lower extremities particularly vulnerable to stasis (see Chap. 5, Hemodynamic Function in Late Pregnancy). From their review, Marik and Plante (2008) cite a 50-percent reduction in venous flow velocity in the legs that lasts from the early third trimester until 6 weeks postpartum. This stasis is the most constant predisposing risk factor for venous thrombosis. Venous stasis and delivery may also contribute to endothelial cell injury. Lastly, marked increases in the synthesis of most clotting factors during pregnancy favor coagulation.

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As shown in Table 47-1, there are a number of factors associated with an increased risk of developing thromboembolism during pregnancy. Using data from the Agency for Healthcare Research and Quality that included 90 percent of all hospital discharges during 2000 and 2001, James and co-workers (2006) identified the diagnosis of venous thromboembolism in 7177 women during pregnancy and 7158 during the postpartum period. They calculated that risks for thromboembolism were approximately doubled in women with multifetal gestation, anemia, hyperemesis, hemorrhage, and cesarean delivery. The risk was even greater in pregnancies complicated by postpartum infection. These data are consistent with those reported by Ros and associates (2002), who studied a population-based cohort of more than 1 million deliveries in Sweden. Compared with uncomplicated delivery, they calculated the relative risk of pulmonary embolism to be 4.8 with severe preeclampsia, 3.8 with cesarean delivery, 2.7 with diabetes, and 2.3 with multifetal gestation. By ...

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