OBJECTIVE: To report the incidence of prophylactic, subprophylactic, and supraprophylactic anti-factor Xa activity in pregnant patients receiving low molecular weight heparin for venous thromboembolism prophylaxis, and to evaluate whether maternal weight, body mass index, age, gestational age, or the low molecular weight heparin dose correlated with anti-factor Xa levels. METHODS: We reviewed 321 anti-factor Xa levels in 77 patients from one Maternal-Fetal Medicine faculty practice. All patients were administered low molecular weight heparin that subsequently was adjusted based upon serial assessment of peak plasma (at 4 hours postinjection) anti-factor Xa levels at less than 36 weeks gestation. Targeted prophylactic range of peak plasma anti-factor Xa level was 0.2-0.4 units/mL. RESULTS: Only 59% of anti-Xa concentrations were in the prophylactic range, whereas 26% were subprophylactic, and 15% were supraprophylactic. Anti-Xa values were not significantly more likely to be prophylactic in early compared with late pregnancy, obese compared with nonobese patients, or in patients receiving a weight-based minimal dose compared with patients receiving less than a weight-based minimal dose. Anti-factor Xa levels did not correlate with maternal age, weight, body mass index, or gestational age, but there was a positive correlation with the percent of the minimal weight-based dose. CONCLUSION: Even with enhanced low molecular weight heparin dosing, 26% of patients have subprophylactic anti-factor Xa levels. Serial anti-factor Xa assessment for dose adjustment should be considered for all pregnant women receiving low molecular weight heparin.