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Objective To determine more precisely the effect of prophylactic low dose aspirin on intrauterine growth retardation and perinatal mortality.

Design Meta-analysis of 13 published randomised clinical trials.

Methods We searched 18 medical databases, including MEDLINE since 1964 and EMBASE since 1974, review articles and the references from each retrieved report to identify all studies evaluating the effect of aspirin in pregnancy and including both intrauterine growth retardation and perinatal mortality as outcome measures.

Results Among 13,234 women from 13 studies between 1985 and 1994, aspirin showed a significant reduction in intrauterine growth retardation (IUGR) (OR 0.82; 95% CI 0.72–0.93; P= 0.003) and a nonsignificant reduction in perinatal mortality (OR 0.84; 95% CI 0.66–1.08; P= 0.18). Subgroup analyses revealed that aspirin was effective at lower doses between 50 and 80 mg/day (IUGR: OR 0.87; 95% CI 0.76–0.99; mortality: OR 0.90, 95% CI 0.70–1.16), but that the preventive effect was greater at higher doses between 100 and 150 mg/day (IUGR: OR 0.36, 95% CI 0.22–0.59; mortality: OR 0.40, 95% CI 0.16–0.97) and among women entered before the 17th week of gestation (IUGR: OR 0.35, 95% CI 0.21–0.58; mortality: OR 0.43, 95% CI 0.17–1.06). We did not identify any specific subgroup of women most likely to benefit from aspirin treatment.

Conclusion The results of this meta-analysis showed that early aspirin treatment reduced the risk of intrauterine growth retardation. Low dose aspirin should not be used routinely in pregnant women until those most likely to benefit from aspirin treatment have been clearly identified.