An abnormally low platelet count occurring during pregnancy (usually less than 70,000 platelets/mm3). Serious illnesses that cause low platelet counts (e.g., disseminated intravascular coagulation, HELLP syndrome, idiopathic thrombocytopenic purpura, pre-eclampsia, systemic lupus erythematosus, or leukemia) should be ruled out. If no illness is present, the condition is usually benign.