Fetal platelet counts in thrombocytopenic pregnancy Fetal platelet counts were assessed by percutaneous umbilical blood sampling in 64 pregnancies (62 women) with maternal thrombocytopenia. In 33 pregnancies associated with chronic immune thrombocytopenia, 11 of the fetuses had platelet counts below 150 x 10(9)/l and 4 were severely thrombocytopenic (less than 50 x 10(9)/l). In 31 pregnancies with symptomless maternal thrombocytopenia as an incidental finding, 4 fetuses were thrombocytopenic, 1 of them severely. Maternal indices, including antiplatelet antibodies, did not correlate with risk of fetal thrombocytopenia; and in those with repeat measurements there was no evidence of benefit from treatment with either corticosteroids (4 cases) or intravenous immunoglobulin (3 cases). Percutaneous umbilical blood sampling, a safe procedure in experienced hands, provides accurate platelet counts in thrombocytopenic pregnancy, as an aid to decisions on mode of delivery and to assessment of treatments. 