Inclusion Criteria:  Adult male or female patients ≥18 and ≤80 years of age Documented congenital antithrombin deficiency, defined by plasma level of antithrombin ≤60% Personal or family history of TEs or TEEs For the Treatment Phase: either a) non-pregnant surgical patients scheduled for elective surgical procedure(s) known to be associated with a high risk for occurrence of TEs or TEEs, or b) pregnant patients of at least 27 weeks gestational age who are scheduled for caesarean section or delivery For female patients of childbearing potential entering the PK Phase who are not known to be pregnant, and for female surgical patients of childbearing potential entering the Treatment Phase for any procedure other than caesarean section or delivery, a negative urine pregnancy test at screening and at baseline Patient has provided informed consent  Exclusion Criteria:  Requires emergency surgery or emergency caesarean section Has undergone surgery within the last 6 weeks History or suspicion of another hereditary thrombophilic disorder other than antithrombin deficiency (e.g., activated protein C [APC] resistance/Factor V Leiden, Protein S or C deficiency, prothrombin gene mutation [G20210A], or acquired [lupus anticoagulant] thrombophilic disorder) Malignancies, renal failure, or severe liver disease (aspartate aminotransferase [ASAT] >5 times the upper limit of normal) Body mass index >40 kg/m2 Known hypersensitivity or allergic reaction to antithrombin or any of the excipients in Atenativ History of anaphylactic reaction(s) to blood or blood components Refusal to receive transfusion of blood-derived products Administration of any antithrombin concentrate or antithrombin-containing blood product other than the study medication within 14 days of either of the two phases of the study Prior diagnosis of heparin-induced thrombocytopenia TE or TEE within the last 6 months Female patients who are nursing Have participated in another investigational study within the last 30 days