The safety of antithrombotic therapy during pregnancy.

A number of clinical conditions can require the use of antithrombotic drugs during pregnancy. These mainly include prevention of venous thromboembolism (VTE) and fetal complications in high-risk patients, treatment of VTE and prevention of arterial emboli in patients with mechanical heart valve prostheses. However, there are several problems when using antithrombotic drugs during pregnancy. Warfarin, as well as the other coumarin compounds, crosses the placenta and has the potential to cause both bleeding in the fetus and teratogenicity, therefore its use is not recommended during the first trimester and during the perinatal period. Unfractionated heparin (UFH) and low molecular weight heparin (LMWH) do not cross the placenta and are safe for the fetus, but long-term treatment with UFH is problematic because of its inconvenient administration, the need to monitor anticoagulant activity and because of its potential side effects, such as heparin-induced thrombocytopenia and osteoporosis. LMWH is the drug of choice in the prevention and treatment of VTE during pregnancy because of its practical advantages over UFH and because of a lower risk of side effects. Patients with mechanical heart valve prostheses represent a major clinical challenge. Warfarin, the drug of choice in non-pregnant women, can be administered between the 12th and 36th week. Full-dose UFH is recommended in the first trimester and after week 36. The use of LMWH as an alternative to UFH is still a matter of debate, because inadequate data are available.