ObjectiveTo measure platelet activation in normal pregnancy, before and after stimulation with agonists, with a whole blood flow cytometric technique.MethodsIn a cross-sectional study, 5 mL of whole blood was collected from healthy volunteers (nine in the first trimester, ten in the second trimester, 35 in the third trimester, and 32 nonpregnant controls). Platelets were treated with an agonist (thrombin or U-46619, a thromboxane A2 analogue) or buffer and were exposed to saturating concentrations of monoclonal antibodies directed against platelet membrane glycoproteins (GPs): 7E3 (fibrinogen receptor GPIIb/IIIa), S12 (alpha granule marker P-selectin), and 6D1 (von Willebrand factor receptor GPIb). Mean fluorescence intensity was determined for 5000 platelets per sample by using a flow cytometer.ResultsIn the absence of agonist, no significant difference between groups was found in antibody binding. At no stage of pregnancy were circulating activated platelets detected. Platelets from third-trimester subjects bound significantly less 7E3 than platelets of controls or of first- or second-trimester subjects after stimulation with high-dose thrombin (P< .05 for all comparisons). Down-regulation of 6D1 on platelets after stimulation with high-dose U-46619 was significantly greater in third-trimester gravidas than in controls or first-trimester subjects (P< .05).ConclusionPregnancy does not increase the percentage of activated platelets in the circulation. Platelet reactivity is altered in the third trimester, as evidenced by decreased antibody binding to a fibrinogen receptor epitope and enhanced down-regulation of a von Willebrand factor receptor epitope.