Antiphospholipid antibodies, i.e. lupus anticoagulants and anticardiolipin antibodies, are associated with obstetric complications. Fetal death and recurrent spontaneous abortions represent the obstetric criteria of the antiphospholipid syndrome. They occur with similar frequences and have an overall prevalence of 15-20%. Lupus anticoagulants carry a risk 3.0 to 4.8 times, and anticardiolipin antibodies 0.86 to 20 times higher than controls. The mechanism(s) by which antiphospholipid antibodies cause these events still has to be defined: thrombosis in the placental vessels, and impairment of embryonic implantation have been proposed. Unfractionated or low-molecular-weight heparin, alone or in combination with low-dose aspirin, represent the current standard treatment of pregnant antiphospholipid-positive women for preventing recurrent obstetric complications. Upon treatment, the live birth rate increases from 0-40% to 70-80%. However, there is still an excessive frequency of maternal and/or fetal complications, indicating the necessity of a better calibration of the dosage, duration and timing of administration of heparin treatment.