Eleven reports of platelet counts were found in the literature which reported a progressive increase, no change, or a progressive fall during pregnancy. The counts had been made using venous or capillary blood, with either simple dilution or red cell lysis prior to enumeration in a haemocytometer, with or without phase-contrast microscopy. It was therefore decided to examine whole blood platelet counts taken during normal pregnancy. Results from 405 venous blood samples, using an electronic platelet counter, confirmed the finding of a significant progressive fall in platelet counts during normal pregnancy, the lowest counts falling in some women outside the recognized normal range of values.