A woman with twin pregnancy had a gradual decline in anti-thrombin (AT) activity from 72% at gestational week (GW) 29-3/7 , to 53% at GW31-2/7 , and to 41% at GW32-2/7 , at which time hypertension (148/90 mmHg) and proteinuria (protein-to-creatinine ratio [P/Cr], 0.79 mg/mg) developed in the presence of normal platelet count (159 × 109 /L) and serum aspartate aminotransferase/lactate dehydrogenase (22/164 IU/L). AT product was given three times to maintain AT activity >50% and blood pressure was maintained below 155/95 mmHg with no treatment, but generalized edema with a weekly weight gain of 4.9 kg and increased proteinuria (to P/Cr, 7.6 mg/mg) required cesarean section at GW33-3/7 . This case highlights the occurrence of pregnancy-induced AT deficiency alone in the absence of any other abnormality, including hypertension, proteinuria, or thrombocytopenia. Measurement of AT activity was considered helpful for determination of the appropriate time for delivery in this patient.