© 2015 © 2015 Informa UK Ltd. Objective: Late timing of intervention and maternal obesity are potential explanations for the modest effect of aspirin for preeclampsia prevention. We explored whether low-dose aspirin (LDA) is more effective in women at increased risk when initiated before 16 weeks' gestation or given to non-obese women.Methods: Secondary analysis of a trial to evaluate LDA (60 mg/d) for preeclampsia prevention in high-risk women. Participants were randomized to LDA or placebo between 13 and 26 weeks. We stratified the effect of LDA on preeclampsia by (a) timing of randomization (< 16 or ≥ 16 weeks gestation) and (b) body mass index (BMI) class (non-obese and obese). The Breslow-Day test for homogeneity was used to assess for variations in effect of LDA across gestational age and BMI groups.Results: Of 2503 women, 461 (18.4%) initiated LDA < 16 weeks. LDA effect was not better when initiated < 16 weeks (RR: 0.93, 95% CI: 0.67-1.31) versus ≥ 16 weeks (RR: 0.90, 95% CI: 0.75-1.08), (p value for interaction = 0.87). Similarly, LDA effect was not better in non-obese (RR: 0.91, 95% CI: 0.7-1.13) versus obese women (RR: 0.89, 95% CI: 0.7-1.13), (p value for interaction = 0.85).Conclusion: LDA for preeclampsia prevention was not more effective when initiated < 16 weeks or used in non-obese women at risk for preeclampsia. No particular subgroup of women was more or less likely to benefit from LDA therapy.