BACKGROUND Some women exhibit a gradual decrease in antithrombin activity until the time of delivery, with antithrombin activity reaching <65% of the normal level (pregnancy-induced antithrombin deficiency, PIATD). However, the clinical features of such women are not well understood and are unfamiliar to many obstetricians. FINDINGS PIATD is more likely to occur as the number of fetuses increases (approx. 1.0%, 10%, and 40% for singleton, twin, and triplet pregnancies, respectively) and in women with hypertension, isolated proteinuria and/or edema. However, normotensive women account for 60% of women with PIATD. Antithrombin can escape from the blood into the interstitial space. Women with PIATD suffer from a decreased plasma volume and are more likely to develop liver dysfunction, irrespective of the presence or absence of hypertension. Because antithrombin activity continues to decrease until the time of delivery in women with PIATD, women with unrecognized PIATD may be identified as patients with so-called "acute fatty liver of pregnancy" if delivery is delayed. CONCLUSION Knowledge of AT activity in obstetrical practice may improve management of pregnant women. The determination of AT activity should be considered in women with multifetal pregnancies, hypertension, isolated proteinuria and/or edema.