Antiphospholipid antibody (aPL) is associated with thromboembolism. There is scant evidence of a relationship between the aPL profile and serious adverse pregnancy outcome. The aim of this study was to assess whether aPL measurements during early pregnancy were useful in predicting a serious adverse pregnancy outcome. In this prospective study, we measured aPLs, including lupus anticoagulant (LA), IgG, IgM, IgA anticardiolipin antibody (aCL), IgG, IgM phosphatidylserine-dependent antiprothrombin antibody, and IgG kininogen-dependent antiphosphatidylethanolamine antibody (aPE) during the first trimester in a consecutive series of 1155 women. The 99 th percentile cut-off values in each aPL were determined using samples from 105 women who did not exhibit any pregnancy morbidity. We assessed the predictive risk of a serious adverse pregnancy outcome adjusted for confounding factors. We found that IgG aCL was associated with developing pregnancy-induced hypertension (PIH) (odds ratio 11.4, 95% CI 2.7-48); IgG aPE with PIH (8.3, 2.4-29), severe PIH (20.4, 4.5-91), and premature delivery (PD) (12.7, 3.1-50); and LA with PD (11.0, 2.8-44) and low birth weight (8.0, 2.1-31). The combinations of IgG aPE plus IgG aCL (17.5, 4.7-66.7) or IgG aPE plus LA (22.2, 5.4-909) measurements predicted severe PIH with 30.8% sensitivity and 99.2% specificity. We conclude that aPL measurements during early pregnancy may be useful in predicting adverse pregnancy outcome.