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.