Objective To determine any benefits or risks, expressed in early childhood, of low dose aspirin treatment in pregnancies at high risk of complications due to pre-eclampsia or intrauterine growth retardation.

Design A questionnaire-based follow-up at 12 and 18 months of age of cohorts of surviving children whose mothers participated in a large randomised, double-blind placebo-controlled trial of 60 mg aspirin.

Setting United Kingdom and Ottawa, Canada.

Subjects 4168 children assessed at 12 months through information provided by general practitioners, and 4365 assessed at 18 months through a questionnaire to parents.

Main outcome measures Hospital visits in the first 18 months for congenital malformations, motor deficit, developmental delay, respiratory problems or bleeding problems; height or weight below the third centile; and delayed acquisition of certain developmental skills.

Results There were no clear differences in any of the main outcome measures, although some confidence intervals were wide.

Conclusions Although an adverse effect can not be ruled out, these findings are reassuring about the safety of low dose aspirin started after the first trimester, at least in respect of congenital malformations, major motor deficit, and severe neuromotor or developmental delay identifiable in early childhood. They provide no clear evidence of benefit. Taking into account evidence from large randomised controlled trials, the place of low-dose aspirin in pregnancy appears to be limited, although it may be beneficial for women at high risk of early onset pre-eclampsia; for them, evidence suggesting that it is not harmful is important.