Heparin levels to guide thromboembolism prophylaxis during pregnancy

OBJECTIVE: Our purpose was to determine the dose of heparin required in pregnant women to achieve the same heparin levels as standard doses of 5000 units given subcutaneously every 12 hours in the nonpregnant population. STUDY DESIGN: Fourteen pregnant women placed on heparin prophylaxis for a history of thromboembolism had blood drawn for 64 anti-Xa level determinations in the second and third trimesters. Heparin doses were adjusted in an attempt to achieve a midinterval or peak level of 0.05 to 0.25 U/ml, which corresponds to the range seen in nonpregnant patients given standard doses of 5000 units subcutaneously every 12 hours. RESULTS: A standard heparin dose of 5000 units given subcutaneously every 12 hours was inadequate to achieve the desired range in this pregnant population. In five of nine second-trimester pregnancies 7500 units given subcutaneously every 12 hours was inadequate to attain this range. In six of 13 third-trimester pregnancies, >10,000 units subcutaneously every 12 hours was needed. CONCLUSIONS: Heparin requirements may increase and are highly variable in patients during pregnancy. Until appropriate clinical outcomes trials can determine optimal dosing, measuring anti-Xa activity may be useful to guide therapy.