Thrombocytopenia is defined as a platelet count of less than 150 x 109/l. Normal pregnancy is generally thought not to affect the platelet count,' but it has been suggested that the normal range is lower in pregnancy, and that the count falls in the third trimester.2`4 This review concentrates on causes of thrombocytopenia with particular reference to pregnancy: most of these involve excessive platelet consumption (Table I).