Anti-Xa directed low-molecular-weight heparin dosing to reduce the risk of venous thromboembolism in pregnant women with inherited antithrombin deficiency.

Background: Inherited antithrombin deficiency (ATD) is associated with a high risk of venous thromboembolism (VTE), particularly during risk periods, including pregnancy. Guidelines vary regarding thromboprophylaxis with low-molecular-weight heparin (LMWH). LMWH use in women with inherited ATD is hampered by the fact that antithrombin is the substrate for its anticoagulant effect, and therefore, larger doses of LMWH are needed.

Objectives: Assess the rate of VTE in pregnant women with inherited ATD after our institution changed practice to using anti-Xa-guided LMWH in 2012.

Methods: This was a retrospective single-center cohort study of pregnant women with inherited ATD, followed from positive pregnancy test to 6 weeks postpartum.

Results: We identified 32 pregnancies in 17 women, 12 with type 1 and 5 with type 2 inherited ATD. Twenty-two (69%) resulted in a live birth. There were 4 cases of VTE (12.5%): 2 occurred in women prior to referral to our unit on subtherapeutic doses of LMWH; 1 in a nonadherent patient with homozygous AT Budapest; and 1 postpartum VTE in the setting of nonadherence. The median starting dose of LMWH in the whole cohort was 6250 units (96 units/kg), increasing to 16 750 units (247 units/kg) by the end of pregnancy. Antithrombin concentrate was given peripartum in 20 pregnancies. There were 4 postpartum hemorrhages (12.5%), 1 minor and 3 moderate.

Conclusion: In this retrospective cohort study of 32 pregnancies, 12.5% of our cohort had a VTE. However, there was no VTE in those who were adherent and received anti-Xa monitored LMWH with risk-assessed use of peripartum antithrombin concentrate.