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

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. Assess the rate of VTE in pregnant women with inherited ATD, after our institution changed practice to using anti-Xa-guided LMWH in 2012. Retrospective single-centre cohort study of pregnant women with inherited ATD, followed from positive pregnancy test to 6 weeks postpartum. We identified 32 pregnancies in 17 women, 12 with type 1 and 5 with type 2 inherited ATD. 22 (69%) resulted in a livebirth. There were four cases of VTE (12.5%): two occurred in women prior to referral to our unit, on subtherapeutic doses of LMWH; one in a nonadherent patient with homozygous AT Budapest; and a postpartum VTE in the setting of nonadherence. The median starting dose of LMWH in the whole cohort was 6250 units (96units/kg), increasing to 16750 units (247 units/kg) by the end of pregnancy. Antithrombin concentrate was given peripartum in 20 pregnancies. There were four post-partum haemorrhages (12.5%) - 1 minor and 3 moderate. In this retrospective cohort study of 32 pregnancies, 12.5% of our cohort had a VTE. However, there was no VTE in those that were adherent and received anti-Xa monitored LMWH with risk-assessed use of peripartum antithrombin concentrate.