Objective: To investigate the effects of aspirin on the distribution of birthweight and its impact on the rates of large-for-gestational age (LGA) neonates. Design: Secondary analysis of the Combined Multimarker Screening and Randomized Patient Treatment with Aspirin for Evidence-based Preeclampsia Prevention (ASPRE) trial. Setting: Thirteen hospitals in England, Spain, Belgium, Greece, Italy, and Israel. Population: Participants of the ASPRE trial at increased risk of preterm pre-eclampsia (PE) who had a live birth. Methods: We compared the birthweight distributions and the rates of LGA neonates between the trial groups. Analyses were stratified according to the presence of pre-existing diabetes mellitus and the development of pre-eclampsia, and logistic regression was used to investigate independent predictors of LGA neonates. Main Outcome Measures: Birthweight distribution and rate of LGA neonates. Results: Among 1,571 singleton, live neonates (777 from the aspirin group and 794 from the placebo group), aspirin was associated with a shift in birthweight from below 2,500 to between 2,500 and 4,000 grams, and birthweight percentile from below the 25 th to between the 25 th and 75 th percentiles, with no significant increase in LGA neonates (5.5% vs. 6.2%, p=0.667). Logistic regression demonstrated a significant interaction between treatment and pre-existing diabetes (p-value 0.034), and a positive association between maternal weight and LGA neonates (adjusted odds ratio 1.040, 95% confidence interval 1.030 – 1.051, p<0.001). Conclusions: Aspirin use is associated with increased birthweight without increasing the rate of LGA neonates. Among women with pre-existing diabetes, however, aspirin may lead to a higher rate of LGA neonates.