To characterize antithrombin (AT) levels in normal pregnancy.We performed secondary analyses with data from 3 studies. Using a single measurement from each subject in the first analysis (cross-sectional), we correlated AT levels with gestational age from the middle of the second trimester throughout the third trimester of pregnancy. Using serial measurements in a second analysis (cohort), we compared AT levels between the late first and second trimesters of pregnancy and baseline (the level at 6 weeks postpartum). Using serial measurements in a third analysis (cohort), we analyzed the pattern of change in AT levels in the immediate postpartum period. Assays of AT activity were performed using the Dade Behring (Siemens) Berichrom Antithrombin III Chromogenic Assay. AT levels were correlated with gestational age using the Pearson correlation coefficient and compared between the different time points using one-way ANOVA.Overall, AT levels were 20% lower than baseline during pregnancy (p<0.01). There was no significant difference between AT levels obtained between late first trimester and late second trimester. From midtrimester to term, however, AT levels were negatively correlated with gestational age with a 13% drop during this period of time (r=-0.26 [-0.39, -0.11]; p<0.01). Immediately after childbirth, AT levels fell precipitously to 30% below baseline (p<0.05) and reached a nadir 12 hours postpartum before rising and returning to baseline by 72 hours postpartum.It appears that antithrombin (AT) is consumed at the time of delivery. Our findings have implications for AT replacement or even anticoagulation at the time of delivery.