Background: The prevalence of spontaneous abortions is 1000-1500/10000 pregnancies per year meaning that 10-15% of all pregnancies will end in an abortion; 1/10 of these abortions are recurrent (1 % of all pregnancies). In about half of women with habitual abortions (HAB) hereditary (F V Leiden, F II (prothrombin) mutation, Protein C, S deficiency and anti-thrombin) or acquired (antiphospholipid antibodies) thrombophilia are observed. Efficacy of the medical treatment of patients with a history of HAB has yet to be completely demonstrated. We have recently shown that low-molecular-weight heparin (LMWH) is as effective as unfractionated heparin in prevention of thromboembolic complications in pregnant women and causes less bleeding complications (UFH) and has no osteoporotic effect. LMWH could be safer than UF-heparin during long treatment periods (7-8 months).