This prospective cross-sectional (10 women on each occasion) and longitudinal (20 women) study investigated activated protein C (APC) ratio in normal pregnancy. The APC ratio was measured at booking, 20, 30 and 36 weeks of gestation, and compared with a sample of normal nonpregnant women. No significant difference was found between APC ratios for pregnant women at any gestation and those of the nonpregnant population in either the longitudinal or cross-sectional studies. There was a significant decrease in APC ratios throughout pregnancy, but in all but one case values remained within the normal nonpregnant range. The APC ratio can therefore be used as a screening test for the factor V Leiden mutation during pregnancy.