In pregnant women, activated partial thromboplastin time (APTT) does not precisely reflect the anticoagulant effect of a therapeutic dose of heparin. However, the measurement of anti-Xa activity can be used to monitor the anticoagulant effect of heparin, since the plasma concentrations of coagulation factors increase in pregnant women. We evaluated the in vitro effects of increased concentrations of fibrinogen and other coagulation factors (FVII, FVIII, and FIX) on the results of assays of APTT and anti-Xa activity in plasma samples with various therapeutic concentrations of unfractionated heparin (UFH). In the presence of UFH, APTT was shortened by increased concentrations of fibrinogen, FVII, or FVIII, and this effect was much stronger when the FVIII concentration was increased. In the plasma samples containing 0.5 or 0.7 U/mL of UFH, the APTT was shortened by approximately half or one-third, respectively, when 6 U FVIII/mL was added to the sample. The anti-Xa activity was not influenced by increased concentrations of the coagulation factors. In the present study, we also evaluated the sensitivities to UHF of four APTT reagents, and found a 1.65-fold difference in the sensitivity to UFH among APTT reagents. Our results demonstrate that increased FVIII concentration shortens APTT under therapeutic doses of UFH, and that APTT thus underestimates the anticoagulant effect of UFH in pregnant women, mainly due to the increased FVIII concentration.