The occurrence of thrombocytopenia in 5% of pregnant women at delivery, described as gestational thrombocytopenia, is well documented. A commonly believed concept is that gestational thrombocytopenia is the result of gradually decreasing platelet counts in all women during pregnancy. The goal of our study was to evaluate the data supporting this concept. To learn what is known about platelet counts throughout normal pregnancies, we systematically reviewed all publications describing platelet counts during pregnancy. We identified 3,039 studies; 46 reporting ≥30 women with normal pregnancies were included in our analyses. The combined mean platelet counts from all studies supported the concept that platelet counts decrease during pregnancy and increase postpartum: first trimester, 251,000/µL (95% CI, 238,000-264,000/µL); second trimester, 238,000/µL (95% CI, 222,000-253,000/µL); third trimester, 224,000/µL (95% CI, 213,000-235,000/µL); delivery, 237,000/µL (95% CI, 209,000-264,000/µL); 4-8 weeks postpartum, 247,000/µL (95% CI, 207,000-287,000/µL). However, individual studies were inconsistent. Eleven longitudinal studies compared platelet counts on the same women at different times during gestation: seven reported a decrease; four reported no change. Ten cross-sectional studies compared platelet counts of different women at different times during gestation: five reported a decrease; five reported no change. Five studies compared platelet counts of pregnant to nonpregnant women: three reported that platelet counts were lower in pregnant women; one reported no difference; one reported that platelet counts were higher in pregnant women. These inconsistent data emphasize the need to accurately describe platelet counts throughout normal pregnancies. Accurate data are essential for evaluating the clinical importance of thrombocytopenia during pregnancy.
© 2017 Wiley Periodicals, Inc.