A prospective trial that demonstrates that dalteparin requirements increase in pregnancy to maintain therapeutic levels of anticoagulation.

Objective: The purpose of this study was to determine whether standard therapeutic doses of dalteparin maintain peak therapeutic levels of anticoagulation during pregnancy.

Study Design: This was a prospective trial in which 13 pregnancies that required therapeutic anticoagulation were treated with dalteparin 100 U/kg every 12 hours; peak and trough (predose) low molecular weight heparin (anti-Xa activity) levels were monitored every 2 weeks. Dosage adjustments were made to maintain peak anti-Xa activity between 0.5 and 1.0 IU/ml. Bone density and bone turnover markers were measured.

Results: A total of 250 peak and trough low-molecular-weight heparin (LMWH) levels were obtained. Eighty-five percent of pregnancies (11/13) required an upward dosage adjustment. Trough levels were in the therapeutic range only 9% of the time, despite the maintenance of therapeutic peak levels. Bone resorption markers and density were unchanged in singleton pregnancies.

Conclusion: Dalteparin dosing, based on weight alone, every 12 hours is inadequate to maintain most pregnant women in the therapeutic range throughout pregnancy as measured by anti-Xa activity. Trough levels are rarely in the therapeutic range, despite maintenance of therapeutic peak levels. These notable changes in low molecular weight heparin peak may explain reported failures in pregnancy.