Introduction
Aspirin initiated before 16 weeks of gestation at a dose of >100 mg daily reduces the risk of preterm preeclampsia (PE).1 In the Combined Multimarker Screening and Randomized Patient Treatment with Aspirin for Evidence-Based Preeclampsia Prevention (ASPRE) trial,2 women with singleton pregnancies identified as being at high risk of preterm PE based on an algorithm that combines maternal factors and biomarkers at 11 to 13 weeks of gestation were randomized to receive aspirin (150 mg/day) or placebo from 11 to 14 weeks of gestation to 36 weeks of gestation. Aspirin was associated with a significant reduction in the incidence of preterm PE with delivery at <37 weeks of gestation, which was the primary outcome (odds ratio [OR], 0.38; 95% confidence interval [CI], 0.20–0.74; P=.004). There was no significant effect on the incidence of term PE (OR, 0.95 [95% CI, 0.64–1.39]). A subsequent analysis of the ASPRE trial suggested that these differential effects on term and preterm PEs could reflect a mechanism by which the action of aspirin is to delay the onset of and delivery with PE, thereby converting what would be, without treatment, preterm PE to term PE.3…