To assess the relationship between hemostatic factors and spontaneous abortion, 134 pregnant women presenting to the emergency department were recruited and followed through 22 weeks' gestation. Cases were women experiencing a spontaneous abortion and controls were women who maintained their pregnancy. Fibrinogen, factor VII antigen, activated protein C-sensitivity ratio (APC-SR), protein S, and plasmin-antiplasmin (PAP) were measured. Cases had lower mean levels of fibrinogen and factor VII antigen compared with controls (3.1 g/L vs. 3.7 g/L and 89% of normal vs. 109% of normal, respectively). Regression analyses found that women with fibrinogen levels below 3.0 g/L had a five-fold increased risk of spontaneous abortion (OR = 5.1, 95% CI: 1.8-14.4) and women with factor VII antigen levels below 94% of normal had a threefold increased risk of spontaneous abortion normal (OR = 3.3, 95% CI: 1.2-8.5). Similar mean levels of APC-SR, protein S, and PAP were found in the two groups.