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ABSTRACT. Objective. The effect of low-dose aspirin (LDA) on pregnancy outcome in antiphospholipid (aPL)-positive women not fulfilling the criteria for antiphospholipid antibody syndrome (APS) was evaluated retrospectively. Methods. We evaluated 139 pregnancies of 114 aPL-positive women not fulfilling the Sydney classification criteria for definite APS (104 treated with LDA, 35 untreated). Inclusion criteria consisted of (1) any titer of aPL and no previous pregnancy or no pregnancy losses (defined as aPL carriers); (2) any titer of aPL and 1 or 2 pregnancy losses before the 10th gestational week. No women had previous thrombosis. The rate of pregnancy loss, gestational age at delivery, and birth weight percentile were compared in the treated and untreated patients. Associations between clinical and laboratory characteristics and pregnancy outcomes were investigated. Results. The rate of pregnancy loss was low in both treated and untreated groups (7.7% vs 2.9%, respectively). There were no statistically significant differences in the rate of pregnancy loss, gestational age at birth, or birth weight percentile in the treated and untreated groups. There were significant associations between gestational age at birth ≤ 34th week and positivity for lupus anticoagulant (p = 0.025) and anti-ß 2 -glycoprotein I IgG antibodies at titers > 99th (p = 0.016). Conclusion. LDA treatment does not appear to improve pregnancy outcome in low-risk women not fulfilling the criteria for APS. Because antibody profile seems to influence pregnancy outcome, further studies of patients stratified according to their antibody profile are warranted. (First Releas