In vivo platelet aggregation was determined in pregnant women at different gestational ages and nonpregnant women by a modification of a method. A higher platelet count ratio (PCR) was found in pregnant women after a gestational age of 13 weeks. At 16–30 and 31–41 weeks of gestation, in vivo platelet aggregation was significantly decreased. The PCR, meaning the ratio of non-aggregated platelets to all circulating platelets, correlated significantly with gestational age. It is suggested that the decrease in in vivo aggregation measured during pregnancy reflects a summarized effect of different factors during pregnancy connected to placenta and uteroplacental vessels.