Although high-titer IgG antiphospholipid antibody (aPL) is a predictor of mid-pregnancy fetal death in women with systemic lupus erythematosus (SLE), some SLE patients with high-titer aPL carry pregnancies normally, and to term. To determine potential antibody differences between IgG aPL-positive women with and without fetal death, we studied aPL isotype, subclass, anticoagulant activity, phospholipid specificity, and antibody avidity in selected sera from pregnant SLE patients with high-titer IgG aPL. For controls, we selected sera from pregnant SLE patients who had negative results on tests for IgG aPL (with and without fetal loss). None of the specified antibody characteristics distinguished between the aPL-positive patient groups, nor were other specificities defined in IgG aPL-negative sera from women with fetal death. Although high-titer aPL is a good predictor of fetal death, currently known characteristics, other than a high titer of aPL, do not identify which women will experience this complication.