Objective To determine whether antenatal treatment (for ≥= 14 days) with 100 mg aspirin daily, given to mothers with small for gestational age fetuses and abnormal umbilical Doppler, will increase birthweight.

Design Randomised, double-blind placebo controlled trial.

Setting A tertiary referral centre.

Participants Ninety-nine women, of whom 65 were treated for ≥= 14 days (32 with aspirin and 33 with placebo) and comprised the study group. The entry criteria were: singleton pregnancy with ultrasound evidence of a small for gestational age fetus (abdominal circumference < 10%); previous anatomy scan <20 weeks and no evidence of fetal abnormality; gestation between 24 and 36 weeks; umbilical artery Doppler resistance index > 95% for gestation; no previous aspirin treatment in pregnancy; and no contra-indication to aspirin treatment.

Results The mean duration of treatment was 30 days for aspirin treated, and 29 for placebo. No difference was found in birthweight or other measures of fetal growth or newborn morbidity between those treated with aspirin or placebo. Compliance, assessed by thromboxane B2 analysis, showed almost complete suppression of thromboxane B, in aspirin treated women.

Conclusion Low-dose aspirin did not increase birthweight in pregnancies where the fetus has abnormal umbilical Doppler and is thought to be small for gestational age.