Background: Pregnancy is a physiological hypercoagulable state with an increased incidence of thromboembolic phenoniena. There is an increase in the concentrations of most clotting factors, a decrease in concentration of some of the natural anticoagulants and reduced fibrinolytic activity. Changes in PS levels have also been reported. Aim: To establish referral range values of functional PS and free PS antigen, during the second (2(nd) T) and third trimester (3(rd) T) of normal gestation. Patients and methods: Forty one normal pregnant women were included in our study 20 during the 2(nd) T (22-24 weeks) and 21 during the 3(rd) T (29-38 weeks). Functional PS was measured by a clot based last and free PS antigen by ELISA. Results. Free PS Antigen was 65.8 +/- 18.3% during the 2(nd) T and 62.3 +/- 65% during the 3(rd) The figure for normal controls were 106 +/- 6.5% Functional PS was 43.8 +/- 13.3 and 25.9 +/- 14.6% during The 2(rd) T and 3(rd) T, respectively. The figures for normal controls were 97 +/- 24% (p < 0.001 compared with pregnant women). Free PS antigen did not change from, the 2(nd) to the 3(rd) T (p=NS), however, functional PS fell significantly from The 2(nd) to tbe 3(rd) T (p < 0.001) and was significantly lower than free PS antigen in both trimesters (P < 0.001). Conclusions. Pregnancy is associated to a decrease in PS. This abnormality is more pronounced for functional PS than free PS antigen and functional PS falls progressively during pregnancy. These assays should not be used to screen for PS deficiency during pregnancy because they could lead to a misdiagnosis.