Thrombosis prophylaxis in pregnancy with use of subcutaneous heparin adjusted by monitoring heparin concentration in plasma.

Twenty-six pregnant women were given prophylactic heparin treatment because of previous thromboembolic complications. The mean duration of treatment was 25 weeks (range, 6 to 32 weeks). The amount of heparin was adjusted to a plasma concentration of 0.08 to 0.15 IU/ml, measured as anti-factor Xa activity. This effect was compared with that on activated partial thromboplastin time. The average dose was 16,400 IU/24 hours or 225 IU/kg of body weight per 24 hours. When the plasma concentration was within the stipulated range, the dose was 234 IU/kg of body weight/24 hours. No significant prolongation of activated partial thromboplastin time was found in two thirds of the samples. Blood coagulation was either not activated or only slightly activated, as verified by a low level of fibrinopeptide A. Platelet counts and antithrombin III levels were generally not depressed. No thromboembolic complications occurred during the pregnancies or puerperium. Bleeding during delivery was not increased. No fractures of the spine caused by osteoporosis were found on radiologic examination post partum.