Most pregnant patients with isolated thrombocytopenia will have idiopathic thrombocytopenic purpura. Because the disease is caused by an IgG antiplatelet autoantibody that can cross the placenta, the infant also is at risk for thrombocytopenia. The management of the pregnant patient with idiopathic thrombocytopenic purpura is difficult. The physician must treat the mother, whose platelet count is easily monitored, and the fetus, whose platelet count cannot be assessed.Many recommendations for the management of idiopathic thrombocytopenic purpura in pregnancy have been based on reviews of case reports. This method of analysis, however, may not provide useful information on the true impact of the disease on mothers and infants. Recently, three prospective studies and one study of consecutive pregnant patients have been made. These included unselected patients (90 mothers having 99 conceptions during 95 pregnancies). The present author has analyzed these studies in an effort to provide a more accurate estimate of the implications of the disease.Patients with idiopathic thrombocytopenic purpura have increased numbers of megakaryocytes in the bone marrow and usually have larger than average platelets on the peripheral blood film. Most patients have increased levels of platelet-associated IgG. Although the presence of IgG is a highly sensitive test for idiopathic thrombocytopenic purpura (approximately 90 per cent), its low specificity limits its diagnostic usefulness. Unless an alternative cause of the thrombocytopenia is shown, patients with isolated thrombocytopenia during pregnancy should be presumed to have idiopathic thrombocytopenic purpura and managed accordingly.The prospective studies analyzed here indicate that the risk of significant morbidity is low for both mother and infant. Five of the 99 conceptions in 95 pregnancies died, but the deaths occurred in utero between the 18th and