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Objective: To examine the pathophysiology of reductions in antithrombin (AT) activity during pregnancy and to better characterize the laboratory features of pregnant women with severely depressed AT activity. Methods: Laboratory variables for blood samples obtained within 5 d prior to delivery were compared among three women groups with severely depressed (<45%, n = 6), modestly depressed (45-69%, n = 10), and normal AT activity levels (>70%, n = 134). Results: Pregnancy-induced hypertension was present in 16.7% (1/6), 30.0% (3/10), and 9.0% (12/134) of the women with the above-mentioned AT activities, respectively. The AT activities were significantly and negatively correlated with the D-dimer, urate, and creatinine levels. The D-dimer level was significantly and negatively correlated with the fibrinogen level. Women with AT activity <45% exhibited markedly elevated levels of hemoglobin and liver enzymes and a significantly lower level of fibrinogen than the other women groups, while the platelet count did not differ among the three groups. Conclusions: Enhanced thrombin generation was involved in the decrease in AT activity. AT activity can decrease in the absence of thrombocytopenia. The liver dysfunction that was seen in cases with severely depressed AT activity may have resulted from impairments in liver perfusion caused by microthromboses generated as a result of the relative lack of AT and/or the shortage of circulating plasma in women with reduced AT activities. [ABSTRACT FROM AUTHOR]