Antithrombotic therapy is required during pregnancy for the prevention and treatment of venous and arterial thromboembolism and for the prevention of pregnancy loss in women at risk. The choice of anticoagulant for venous thromboembolism during pregnancy is limited to unfractionated heparin or low molecular weight heparin because the use of warfarin is relatively contraindicated. Much of the information surrounding the pharmacokinetics and dosing of unfractionated heparin and low molecular weight heparin obtained from non-pregnant patients has been applied to pregnant women. Whether this is appropriate in the presence of significant physiological changes in pregnancy is unclear. Specific to pregnancy and unfractionated heparin use, activated partial prothrombin time may be unreliable. In addition, the appropriate dosing of low molecular weight heparin is uncertain. Because venous thromboembolism can cause significant maternal morbidity and mortality, these important issues surrounding appropriate drug dosing of anticoagulants should be addressed.