OBJECTIVE: To determine the incidence of maternal antiplatelet antibodies in cases of thrombocytopenia during pregnancy, using the monoclonal antibody-specific immobilization of platelet antigens assay; and to assess the usefulness of this assay for predicting risk of neonatal thrombocytopenia. METHODS: A total of 6770 pregnant women were included in the study, and the monoclonal antibody-specific immobilization of platelet antigens assay was done when platelet counts were less than 150 x 10(9)/L. Platelet counts were determined in 6103 newborns. RESULTS: The incidence of maternal thrombocytopenia was 11.6% (95% confidence interval [CI] 10.8, 12.4). Among newborns, 1.3% (95% CI 0.5, 2.7) born to thrombocytopenic mothers were thrombocytopenic, compared with 0.4% (95% CI 0.2, 0.6) born to nonthrombocytopenic women. Antiplatelet antibodies were detected in 37 (8.6%) of 430 thrombocytopenic women; autoantibodies were detected in 28 cases (circulating or bound to platelets), alloantibodies in eight cases, and an association of alloantibodies and autoantibodies in one case. The positive and negative likelihood ratios for predicting neonatal thrombocytopenia were 4.6 and 0.7, respectively. CONCLUSION: The monoclonal antibody-specific immobilization of platelet antigens assay did not predict the risk of neonatal thrombocytopenia in an unselected population of thrombocytopenic pregnant women.