The impact of risk factors on aspirin's efficacy for the prevention of preterm birth

BACKGROUND: The Aspirin Supplementation for Pregnancy Indicated Risk Reduction In Nulliparas trial was a landmark study that demonstrated a reduction in preterm birth and hypertensive disorders of pregnancy in nulliparous women who received low-dose aspirin. All women in the study had at least 1 moderate-risk factor for preeclampsia (nulliparity). Unlike current US Preventative Service Task Force guidelines, which recommend low-dose aspirin for ≥2 moderate-risk factors, women in this study were randomized to receive low-dose aspirin regardless of the presence or absence of an additional risk factor. OBJECTIVE: This study aimed to compare how low-dose aspirin differentially benefits nulliparous women with and without additional preeclampsia risk factors for the prevention of preterm birth and hypertensive disorders of pregnancy. STUDY DESIGN: This was a non–prespecified secondary analysis of the Aspirin Supplementation for Pregnancy Indicated Risk Reduction In Nulliparas trial that randomized nulliparous women with singleton pregnancies from 6 low-middle-income countries to receive low-dose aspirin or placebo. Our primary exposure was having an additional preeclampsia risk factor beyond nulliparity. Our primary outcome was preterm birth before 37 weeks of gestation, and our secondary outcomes included preterm birth before 34 weeks of gestation, preterm birth before 28 weeks of gestation, hypertensive disorders of pregnancy, and perinatal mortality. RESULTS: Among 11,558 nulliparous women who met the inclusion criteria, 66.8% had no additional risk factors. Low-dose aspirin similarly reduced the risk of preterm birth at <37 weeks of gestation in women with and without additional risk factors (relative risk: 0.75 vs 0.85; P=.35). Additionally for our secondary outcomes, low-dose aspirin similarly reduced the risk of preterm birth at <28 weeks of gestation, hypertensive disorders of pregnancy, and perinatal mortality in women with and without additional risk factors. The reduction of preterm birth at <34 weeks of gestation with low-dose aspirin was significantly greater in women without additional risk factors than those with an additional risk factor (relative risk: 0.69 vs 1.04; P=.04). CONCLUSION: Low-dose aspirin's ability to prevent preterm birth, hypertensive disorders of pregnancy, and perinatal mortality was similar in nulliparous women with and without additional risk factors. Professional societies should consider recommending low-dose aspirin to all nulliparous women.