Antithrombin deficiency can be hereditary or acquired. Genetic deficiency is a rare disorder affecting 1 : 10000 individuals in the general population and about 1% of patients diagnosed with Familial venous thrombosis. The estimated thrombotic risk is 12-20 fold in patients with this deficiency. Heterozygous individuals have plasma Antithrombin levels around 50% of normal and are usually symptomatic. Homozygous deficiency is incompatible with life. Type I deficiency shows decreased activity and antigenic levels whereas Type II deficiency shows decreased activity with normal levels of antigen. Most common clinical presentation is Deep vein thrombosis in lower extremities and Pulmonary embolism. Antithrombin is characteristically low in newborns, reaching adult levels by the age of 6 months.