ObjectiveTo describe the natural history of serum antiphospholipid antibodies (lupus anticoagulant and anticardiolipin antibodies) in pregnant women treated with heparin, and to identify a possible association between changes in antibody status and outcomes of subsequent pregnancies.MethodsThirty-six women with antiphospholipid antibodies who had three or more repeated miscarriages were enrolled. Intravenous heparin was used for each of the first pregnancies after referral. Changes in antibody status were investigated with relation to outcomes of the index and subsequent pregnancies.ResultsEighteen of 23 pregnancies in 36 antibody-positive women treated with heparin resulted in term or preterm deliveries with live-born infants, and five ended in abortions. Antibodies cleared in ten of 12 term pregnancies, in five of six preterm pregnancies, and in one of five abortions. There was a statistically significant difference between the term pregnancy and abortion groups (P< .05). Eleven second and third pregnancies in nine women in whom antibodies cleared resulted in term or preterm deliveries of live-born infants, without heparin therapy. The second and third pregnancies in one woman whose antibodies persisted ended in miscarriages despite repeated heparin administration.ConclusionAntiphospholipid antibodies cleared spontaneously in some pregnant women treated with heparin. Subsequent pregnancies among women in whom antibodies cleared were managed successfully without medication, whereas pregnancies in women with persistent antibodies required treatment.