Low molecular weight heparin (LMWH) preserves the antithrombotic action but not the anticoagulant activity of heparin. LMWH is safe, does not cross the placenta and is administered as a single daily injection. We report our experience with 6 pregnant women given LMWH for treatment or prophylaxis of thromboembolism. The drug was successfully given to 5 women for periods of 6 weeks--6 months and no thromboembolic complications occurred during pregnancy or pueperium. There were no hemorrhagic complications and no excessive bleeding was observed during delivery. The sixth patient relapsed after 6 weeks of therapy. This patient also showed resistance to standard heparin administered intravenously at a very high dose. LMWH should be considered an alternative to standard heparin in pregnant women requiring antithrombotic prophylaxis and therapy.