The possibility of using aspirin to prevent pre-eclampsia and its complications stemmed from the results of several small controlled trials. These suggested that aspirin reduced the incidence of proteinuric hypertension and major complications such as intrauterine growth retardation and fetal death.*RF 1-3* The theoretical basis of these benefits is believed to be aspirin's suppression of thromboxane synthesis, which results in less placental aggregation4 and therefore less placental ischaemia (thought to underlie impaired fetal growth and the clinical features of pre-eclampsia).

But concern has been expressed about the potential toxicity of such a potent antiplatelet drug for mothers and fetuses, and questions have been raised about the wisdom of generalising from the results of small trials in highly selected patients. The recent collaborative low dose aspirin study in pregnancy (CLASP) was designed to establish the relative benefits and risks of taking low dose aspirin to prevent or treat preeclampsia and intrauterine growth retardation.5

In this multicentre trial 9354 pregnant women were randomised to either daily aspirin (60 mg) or …