The natural pregnancy state is hypercoagulable and prothrombotic. The coagulation changes in pregnancy (see Section “Thrombophilia” of this chapter) promote clot formation and decrease clot dissolution. In addition to normal coagulation system changes, there are numerous vascular alterations in pregnancy which contribute to thrombosis risk through promotion of venous stasis and increased vascular damage. Causes of venous stasis in pregnancy include the large vein compression by the gravid uterus, the right iliac artery overlying and compressing the left iliac vein, hormonally mediated venous dilation, and immobilization. Vascular damage occurring from vascular compression at delivery and assisted or operative delivery also promotes thrombosis.

VTE in pregnancy causes significant morbidity and mortality [30]. In the UK Confidential Inquiry on Maternal Mortality, the most common error leading to death from PE was failure to make the diagnosis; this occurred in over half of the 28 cases reviewed. Typical errors were to only consider infection as a cause of symptoms and failure to investigate because of mistaken belief that radiological testing is contraindicated in pregnancy. Two thirds of the deaths would potentially have been prevented with proper thromboprophylaxis [31]. The hospital mortality of untreated PE is 30 %, making prompt and accurate diagnosis critical [32]. The diagnosis of VTE in pregnancy affects labor and delivery plans due to the added risk of bleeding with anticoagulation. Complications from DVT which extend beyond the pregnancy include post-thrombotic syndrome (PTS). Up to half of patients with proximal DVT outside of pregnancy experience this condition of limb pain, edema, discoloration, and ulcers [21]. Long-term follow-up of women who experienced DVT in pregnancy finds that 40–80 % develop PTS and 65 % have objectively confirmed deep vein insufficiency [33, 34]. Women with a history of VTE are also more limited by their contraceptive choices as estrogen-containing contraceptives are generally contraindicated.

In pregnancy, the laboratory studies of D-dimer and alveolar-arterial gradient are not useful for PE diagnosis. D-dimer is an estimate of blood coagulation and ongoing fibrinolysis and is elevated throughout pregnancy [36]. No diagnostic cohort studies are currently available using D-dimers in the diagnostic approach of suspected PE in pregnancy. An abnormal arterial blood gas can be a clinical sign of PE. However, a normal alveolar-arterial gradient has been shown in 60 % of documented PEs [37]. Part of the initial work-up when a pregnant patient presents with signs and symptoms of a PE includes an electrocardiogram (EKG) and chest x-ray, although neither of these tests should be used to definitively rule in or rule out a PE [38]. In nonpregnant patients, the incidence of sinus tachycardia and evidence of right heart strain (i.e., RBBB) were found to be slightly increased in patients with PE. Chest x-rays may find possible alternative diagnoses such as pneumothorax, pulmonary edema, or pneumonia. If a pregnant patient presents with symptoms of both a DVT and PE, a reasonable first evaluation would be a compression ultrasonography (US) as the anticoagulation regimens for acute DVT and submassive PE are the same. This approach would limit fetal and maternal radiation exposure. However, a negative compression US for DVT alone should not be used to rule out the presence of a suspected PE due to low sensitivity. In one study, only thirty percent of patients with PE had a radiographically proven DVT at the time of presentation [39]. Therefore, a ventilation-perfusion (V/Q) scan or computed topography (CT) pulmonary angiography with US leg studies is recommended for PE diagnosis in pregnancy. There are advantages and disadvantages to either test, and prompt availability of the study varies by medical facility. The V/Q scan when compared to CT angiography has been studied more extensively in pregnancy, with a high (75 %) diagnostic value [40], and does not require contrast administration. V/Q scan results are interpreted through pretest probability; therefore, its negative predictive value is highest when results are normal or low probability [41]. In comparison to a V/Q scan, a CT angiography study when combined with a chest radiograph can offer an alternative diagnosis if no PE is present but also increases breast radiation exposure [42].

The test of choice for diagnosis of DVT in pregnancy is serial compression US. The test is inexpensive and noninvasive and does not expose subjects to radiation [43, 44]. Pelvic DVTs are more common in pregnancy; therefore, the US protocol should include the iliac veins and inferior vena cava at the level of the liver. Magnetic resonance imaging (MRI) and magnetic resonance venography (MRV) should be considered if a pelvic or iliac vein DVT is suspected and the US is negative. MRI does not require ionizing radiation and is generally considered to be safe in pregnancy, although fetal safety data is limited.