Pulmonary thromboembolism is the main cause of maternal death in the UK and current trends show an increase. Deep-vein thrombosis underlies this disorder. Important issues include pathophysiology, diagnosis, and management of thrombosis in pregnancy, especially the use of anticoagulants. Congenital and acquired thrombophilias contribute to the pathophysiological processes that underlie miscarriage, intrauterine growth restriction, and pre-eclampsia, and raises new possibilities for intervention. The high prevalence of thrombophilic defects in the population, the association of defects with maternal and fetal disorders, and special considerations for management make it essential for obstetricians to understand this area.