Eligibility, timing, and outcomes of low-dose aspirin prophylaxis in pregnancy at risk of preeclampsia: a retrospective cohort study

Aims: To evaluate, in a real-world cohort, guideline-based eligibility for low-dose aspirin (LDA) prophylaxis, the rate and timing of its initiation, the factors associated with non-initiation and late initiation, and the association between the timing of prophylaxis and clinical outcomes in pregnancies at risk of preeclampsia. Methods: Single-center retrospective cohort of singleton pregnancies delivered at 20 weeks or later between 2022 and 2024. Guideline-based eligibility (≥1 high-risk or ≥2 moderate-risk factors), prescription, timing of initiation (≤16 weeks, >16 weeks, not started), and dose were abstracted from medical records. The initiation rate among eligible women was tested against an 80% benchmark. Multivariable logistic regression, propensity-score weighting, and multiple imputation were used to assess determinants and outcomes (preeclampsia, fetal growth restriction [FGR], small-for-gestational-age [SGA] birth, iatrogenic preterm birth). Results: Of 480 pregnancies, 270 (56.3%) were eligible for LDA. Among eligible women, prophylaxis was initiated in 202 (74.8%; 95% confidence interval [CI] 69.3-79.6), below the 80% benchmark (p=0.025); only 154 (57.0%) began at or before 16 weeks (p<0.001). A daily dose of at least 100 mg was used in 89.3% of prescriptions. Higher body-mass index independently predicted initiation (adjusted odds ratio [aOR] 1.09 per kg/m²; p=0.002). After adjustment, the timing of aspirin showed no statistically significant association with preeclampsia (≤16-week aOR 0.83; 95% CI 0.39-1.73), SGA, FGR, or iatrogenic preterm birth; estimates were unchanged after weighting and imputation. Conclusion: Guideline-recommended LDA prophylaxis was under-applied, with delayed initiation being common despite largely adequate dosing. System-level quality-improvement measures are warranted.