We carried out a retrospective cohort study to construct reference ranges for free protein S (FPS) levels during pregnancy and identify any conditions or factors that may affect FPS levels. Patients that were ordered thrombophilia screening tests during gestational period were identified. Patients demonstrated to have hereditary or acquired thrombophilia were excluded. Reference ranges were constructed using regression analysis. Outcome of the index pregnancy and pregnancy complications was used to identify any confounding factors. A total of 455 pregnant women were included. The quadratic equation for FPS according to gestational age (GA) was [75.497 + (-1.516*GA) + 0.018*GA*GA]. FPS level and GA were negatively correlated (Spearmans rho statistic [r(s)] = -0.436, p = 0.001). FPS level and fetal growth restriction (FGR) were negatively correlated ([r(s)] = -0.093, p = 0.049). FPS level and placental abruption were positively correlated ([r(s)] = 0.098, p = 0.039). Stepwise linear regression model constructed to predict FPS level with gestational age, placental abruption and FGR as the predictor variables. Gestational age was the only variable retaining statistically significant relation with FPS level (chi(2) = 0.216, df = 3, p = 0.001). FPS levels decrease significantly throughout gestation in gravidas without hereditary and/or acquired thrombophilias. In patients without thrombophilia FPS levels are not associated with pregnancy complications. The obtained reference intervals may be useful for the clinicians ordering FPS during pregnancy.