Platelets are known to be activated in normal pregnancy, and are further activated in pathological pregnancy states, such as preeclampsia. The factors controlling platelet activation are unknown, but cytokines, such as interleukin 1beta (IL-1beta) and tumor necrosis-alpha (TNF-alpha) have been found to affect platelet function and are believed to be involved in early pregnancy. We assessed the effects of these cytokines on platelets from women at various stages of pregnancy. We compared two methods: platelet in vitro aggregation by aggregometry, and platelet P-selectin expression by flow cytometry. IL-1beta and TNF-alpha had no effect on the in vitro aggregation and P-selectin expression of platelets from women in the first trimester of pregnancy as compared to the inhibitory effects of both in late pregnancy. We conclude that maternal platelet function undergoes a marked change throughout pregnancy.