The present study was developed to verify whether a reduction in phospholipid concentration could increase the activated partial thromboplastin time (APTT) sensitivity to detect lupus anticoagulant (LA) during pregnancy. The authors studied 38 pregnant women (10 normal subjects and 28 patients with associated clinical complications) and 40 nonpregnant control subjects. Tests to detect LA, including APTT, platelet neutralization procedure (standard APTT), the kaolin clotting time, the diluted Russell viper venom test neutralized by lysed platelets, and factor assays, were performed. Positive results were found in 5 of 28 pregnant women with associated clinical complications. The APTT, using three different phospholipid concentrations (standard and more diluted cephalin), was performed on plasma samples and on its 1:1 mixture with normal plasma. The behavior of standard and diluted APTT was similar in negative LA pregnant women and nonpregnant control subjects. The mean values showed nonsignificant differences. Four of five pregnant women with positive LA findings had a prolonged APTT, which was not corrected by the addition of normal plasma using standard conditions. When diluted phospholipids were used, only one of them had a prolonged APTT that was corrected by the addition of normal plasma. Therefore, the highest sensitivity (80%) and specificity (100%) of the APTT to detect LA in pregnant women were obtained using the standard conditions.