Expression of antithrombin in neonate and adult mice. Levels of antithrombin in neonate (day +1, n=13) relative to adult (day +50, n=6) mice: (A) plasma antigen, (B) plasma anti-FXa activity, and (C) serpinc1 mRNA. (D) Levels of plasma antithrombin antigen and plasma anti-factor Xa activity at different stages of development relative to adult stage (n=5 for each age). Differences were analyzed by means of Student's t test taking the adult group as a reference group. Statistical significance was taken as p<0.05. **Both measurements were statistically significant. *Anti-FXa activity was statistically significant.