Accurate diagnosis of deep venous thrombosis (DVT) and pulmonary embolism (PE) is required because treatment can be life-saving, while inappropriate anticoagulation exposes the mother and fetus to haemorrhage and other hazards. Clinicians must be aware of which patients are at risk because DVT is frequently asymptomatic. Clinical diagnosis is unreliable for DVT and PE so objective tests are required. Venography is the gold standard test for DVT but is invasive and has been superseded by less invasive tests such as duplex ultrasound which is now the first-line investigation in pregnancy. However, where doubt remains, venography, CT and MRI have a role. Ventilation-perfusion scanning is the pivotal test for PE in pregnancy, and need not expose the fetus to excess radiation. If the result is unclear deep venous ultrasound can guide management of suspected PE, thus avoiding pulmonary angiography, the invasive gold standard test.