Nineteen pregnant women with uncomplicated pregnancies were studied during the first, second, and third trimesters. We measured the following hemostatic parameters: prothrombin time, activated partial thromboplastin time, fibrinogen, antithrombin III, protein C, protein S, platelet number and volume. Platelet function was examined by a cytofluorimetric method, using an anti-GPM-140 antibody which is directed against a platelet alpha granule membrane protein. Activated platelets were expressed as a percentage of the GMP-140-positive platelets over total platelets. Fibrinogen levels showed a steady increase during pregnancy; conversely prothrombin time, activated partial thromboplastin time, protein C, and antithrombin III showed no significant modifications and remained within the reference range. There was a decrease of protein S activity throughout pregnancy, although protein S antigen did not follow this trend. The decrease occurred early in pregnancy and persisted during the second and third trimesters, reaching a stable plateau. We observed no platelet volume change or activation: the percentage of activated platelets was within the normal reference range, even in late pregnancy.