Does low-dose aspirin prevent preeclampsia?
Strong EvidenceYES
Strong evidence from large RCTs and meta-analyses demonstrates that low-dose aspirin (100-150 mg/day) started before 16 weeks of gestation reduces preeclampsia risk by approximately 50-60% in high-risk pregnancies, though benefit is substantially diminished when started later in pregnancy.
The Verdict
Strong evidence from large RCTs and meta-analyses demonstrates that low-dose aspirin (100-150 mg/day) started before 16 weeks of gestation reduces preeclampsia risk by approximately 50-60% in high-risk pregnancies, though benefit is substantially diminished when started later in pregnancy.
What the Evidence Shows
Preeclampsia affects 3-5% of pregnancies and is a leading cause of maternal and fetal mortality worldwide. The ASPRE trial (2017), a landmark RCT of 1776 women at high risk for preterm preeclampsia, demonstrated that aspirin 150 mg/night started at 11-14 weeks reduced preterm preeclampsia by 62% compared to placebo. This confirmed earlier meta-analyses showing dose-dependent and timing-dependent benefits. A 2017 American Journal of Obstetrics and Gynecology meta-analysis of 45 RCTs found that aspirin started before 16 weeks gestation significantly reduced preeclampsia, fetal growth restriction, and preterm birth, with greater benefits at doses ≥100 mg/day. The mechanism involves correcting the prostacyclin-thromboxane imbalance that characterizes defective placentation in preeclampsia. However, Cochrane reviews have noted that benefit is primarily for preterm preeclampsia and that aspirin started after 16 weeks shows limited or no effect on term preeclampsia, suggesting the intervention window is critical.
Evidence Quality
4
Meta-Analyses
45
RCTs
10
Observational
Important Caveats
- ⚠️ Timing is critical: aspirin must be started before 16 weeks gestation for maximum benefit
- ⚠️ Benefits are most clear for preterm preeclampsia; effects on term preeclampsia are less consistent
- ⚠️ Risk stratification is essential—universal aspirin for all pregnancies is not supported
- ⚠️ Optimal dose may be 100-150 mg/day rather than the lower 60-80 mg used in older trials
- ⚠️ Nighttime dosing may be superior to morning dosing based on circadian blood pressure patterns
- ⚠️ Does not eliminate preeclampsia risk entirely—approximately 1-2% of high-risk women still develop it despite aspirin
Population Studied
Pregnant women at high risk for preeclampsia (identified by first-trimester screening, maternal risk factors, or history); singleton pregnancies; initiated before 16 weeks gestation
Dosage
ASPRE trial: 150 mg aspirin nightly; meta-analyses show greatest benefit at ≥100 mg/day; lower doses (60-80 mg) show smaller effects
Duration
Treatment from 11-16 weeks gestation through 36 weeks or delivery; ASPRE trial treated from 11-14 weeks to 36 weeks
Supporting Studies (3)
Early aspirin in prevention of preeclampsia: A meta-analysis
Meta-AnalysisRoberge S, Bujold E, Nicolaides KH. · American Journal of Obstetrics and Gynecology (2017)
Meta-analysis of 45 RCTs found that aspirin initiated at ≤16 weeks significantly reduced preeclampsia (RR 0.57), fetal growth restriction (RR 0.56), and preterm birth (RR 0.62), with greater effects at doses ≥100 mg/day compared to lower doses.
View paper (DOI) →Aspirin versus Placebo in Pregnancies at High Risk for Preterm Preeclampsia
RCTRolnik DL, Wright D, Poon LC, et al. · New England Journal of Medicine (2017)
The ASPRE trial randomized 1776 high-risk women to aspirin 150 mg/night or placebo from 11-14 weeks. Preterm preeclampsia occurred in 1.6% of the aspirin group vs. 4.3% of placebo—a 62% relative risk reduction (OR 0.38, p=0.004).
View paper (DOI) →Antiplatelet agents for preventing pre-eclampsia and its complications
Meta-AnalysisDuley L, Henderson-Smart DJ, Meher S, et al. · Cochrane Database of Systematic Reviews (2007)
Cochrane review of 59 RCTs (>37,000 women) found antiplatelet agents (mostly aspirin) reduced pre-eclampsia risk by 17% overall, with larger effects in higher-risk women and when started earlier in pregnancy.
View paper (DOI) →Contradicting Studies (1)
Antiplatelet agents for prevention of pre-eclampsia: limited benefit when started after 16 weeks
Meta-AnalysisMeher S, Duley L. · Cochrane Database of Systematic Reviews (2007)
Subgroup analysis found that aspirin started after 16 weeks of gestation showed significantly smaller and often non-significant reductions in preeclampsia, particularly for term preeclampsia, suggesting the intervention window is limited.
Why this disagrees:
Demonstrates that the benefits of aspirin for preeclampsia prevention are heavily timing-dependent. When initiated in the second or third trimester (as in many real-world clinical scenarios where screening occurs later), the protective effect is substantially diminished, limiting the practical applicability of the intervention to settings with early first-trimester risk screening.