- Anticoagulant therapy during pregnancy is problematic. Patients are frequently treated with long-term low-molecular weight heparin despite a lack of evidence for its effectiveness, and in the absence of validated dosing recommendations. The objectives of this investigation were to characterize the safety and pharmacokinetic behavior of a low-molecular weight heparin (reviparin) administered throughout pregnancy. Forty-two patients followed in a tertiary-care rheumatology clinic who received prophylactic doses of reviparin (4900 anti-Xa units subcutaneously once daily) were enrolled in this investigation. Anti-Xa heparin levels, weights, and gestational ages of the patients were obtained on up to four occasions distributed throughout their pregnancy. The achieved anti-Xa heparin levels were highly correlated with the patient's weight, irrespective of the gestational age. No toxicity other than injection site hematomas was observed. The achieved intensity of anticoagulation with reviparin varies during pregnancy in direct proportion to the patient's weight. This variability may mandate dose adjustment in response to changes in a patient's weight during pregnancy, particularly if low-molecular weight heparin is administered at therapeutic doses.