Pregnancy · Risk

Aspirin for Preeclampsia Prevention

Categorical risk stratifier for low-dose aspirin in preeclampsia prevention, per USPSTF 2021 (Grade B), ACOG PB 222 (2020, reaffirmed 2024), NICE NG133, and FIGO 2019. Any one high-risk factor or two-plus moderate-risk factors indicate prophylaxis.

Last reviewed 25 May 2026

Aspirin for preeclampsia prevention — USPSTF / ACOG

Should I take low-dose aspirin in pregnancy?

High-risk factors — ANY ONE = recommend aspirin

Moderate-risk factors — TWO OR MORE = consider aspirin

USPSTF / ACOG band
Low risk

No criteria met for aspirin prophylaxis. Routine antenatal care.

0 high-risk factors · 0 moderate-risk factors
Educational tool only — not medical advice. Aspirin prophylaxis decisions are made with your obstetric provider, who can review medical history, current medications, and contraindications (active GI bleeding, severe asthma triggered by NSAIDs, allergy). Daily aspirin reduces preeclampsia incidence by ~24 % overall and preterm preeclampsia (the most dangerous form) by ~62 % when started before 16 weeks (Roberge 2017 AJOG meta-analysis; ASPRE NEJM 2017).
What does this mean?
Low-dose aspirin is one of the highest-value preventive interventions in modern obstetrics. The landmark ASPRE trial (NEJM 2017) showed 150 mg aspirin from 11–14 weeks in high-risk women cut preterm pre-eclampsia (< 37 wk) by 62 % — among the largest pregnancy effect sizes in any RCT. Mechanism: low-dose aspirin selectively inhibits maternal platelet thromboxane A2, improving placental spiral-artery remodelling. USPSTF/ACOG: start 81–162 mg daily at 12–16 weeks if any HIGH-risk factor or ≥ 2 MODERATE factors, continue through birth. Side-effect profile in pregnancy is excellent at these doses — no increase in placental abruption, postpartum bleeding, or fetal bleeding in pooled trials. If you screen high-risk on this calculator, raise it with your obstetric team at your booking visit — before 16 weeks is the sweet spot.

Introduction

Low-dose aspirin (75-150 mg daily, starting at 12-16 weeks) is one of the most effective interventions in modern obstetrics — it cuts preterm preeclampsia incidence by ~62 % in high-risk women (ASPRE NEJM 2017). The intervention is cheap, well-tolerated, and recommended by every major obstetric society.

This stratifier matches the USPSTF 2021 Grade B Recommendation and ACOG / NICE / FIGO frameworks for identifying who should be offered aspirin prophylaxis.

Background — why aspirin works

Preeclampsia is driven by abnormal placentation: trophoblast invasion of the spiral arteries is incomplete, leading to underperfusion of the placenta. The maternal endothelium responds with widespread vasoconstriction, platelet activation, and the clinical syndrome of hypertension, proteinuria, and end-organ damage that emerges typically after 20 weeks.

Aspirin inhibits cyclooxygenase-1 and platelet thromboxane production, shifting the prostacyclin-to-thromboxane balance back toward vasodilation. Started before placentation is complete (by 16 weeks at the latest), it preserves placental perfusion. Started late (after 16 weeks), the benefit drops sharply because the placental damage is already done.

How to use this stratifier

  1. Tick any of the six high-risk factors that apply.
  2. Tick any of the eight moderate-risk factors that apply.
  3. The output band is determined by the counts.

Decision rules

  • Any 1 high-risk factor → aspirin prophylaxis recommended (USPSTF Grade B).
  • 2 or more moderate-risk factors → aspirin prophylaxis recommended.
  • No factors / 1 moderate factor → routine antenatal care; aspirin not routinely indicated.

Dose and timing

  • Start at 12-16 weeks — starting later loses most benefit.
  • Dose: USPSTF / ACOG: 81 mg daily; NICE / FIGO: 150 mg nightly. Either is acceptable.
  • Continue until 36 weeks or delivery — protocols vary; default is until delivery.

Evidence base

  • ASPRE (Rolnik NEJM 2017) — 1,776 women with high-risk first-trimester PE screen; 150 mg aspirin from 11-14 weeks → 62 % reduction in preterm preeclampsia.
  • Roberge 2017 AJOG meta-analysis — 16,000+ women across 45 trials; benefit confirmed across all risk-level subgroups.
  • USPSTF 2021 — systematic review supporting Grade B recommendation.

What about FMF Triple Test?

The Fetal Medicine Foundation (Nicolaides/Poon) developed a Bayesian competing-risks model that integrates maternal factors with mean arterial pressure (MAP), uterine artery pulsatility index (UtA-PI), and serum PlGF at 11-13+6 weeks. The full model achieves ~75 % detection of early-onset preeclampsia at 10 % false positive (vs ~40 % for maternal factors alone). FMF uses 1:100 as the threshold for aspirin.

The FMF model is more individual but requires specific assays (PlGF) and trained sonographers (uterine artery Doppler). The USPSTF/ACOG categorical approach this widget implements is simpler and globally applicable. Many centres use both — FMF first-trimester for risk refinement; USPSTF criteria as the safety net.

Safety

Low-dose aspirin has decades of pregnancy safety data:

  • No increased rate of placental abruption.
  • No increased rate of postpartum haemorrhage.
  • No increased neonatal bleeding or ductus arteriosus issues at this dose.
  • Some women experience mild GI discomfort or nasal congestion.
  • Higher-dose aspirin (≥ 325 mg) and other NSAIDs are AVOIDED in pregnancy — a different decision.

Contraindications

  • Active peptic ulcer disease or recent GI bleeding.
  • Severe asthma triggered by NSAIDs (aspirin-exacerbated respiratory disease).
  • Known aspirin allergy.
  • Severe liver disease.
  • Bleeding disorders.

Limitations

  • The risk-factor categorical approach catches ~30-40 % of women who eventually develop preeclampsia. The FMF model catches more.
  • Aspirin doesn’t prevent ALL preeclampsia — it reduces the severe / preterm form most.
  • The list of moderate risk factors includes Black race — this is included by USPSTF as a reflection of systemic care inequities and worse outcomes in Black women, not as a biological factor. The 2021 race-neutral VBAC calculator and 2022 race-neutral AAP bilirubin nomogram exemplify the broader move away from race-based clinical algorithms; this guideline has not yet been similarly revised.

Sources

  • US Preventive Services Task Force. Aspirin Use to Prevent Preeclampsia and Related Morbidity and Mortality: USPSTF Recommendation Statement. JAMA 2021;326:1186-91.
  • ACOG. Practice Bulletin 222: Gestational Hypertension and Preeclampsia. 2020, reaffirmed 2024.
  • NICE. Hypertension in pregnancy (NG133). 2019.
  • Rolnik DL, Wright D, Poon LC, et al. Aspirin versus Placebo in Pregnancies at High Risk for Preterm Preeclampsia (ASPRE). N Engl J Med 2017;377:613-22.
  • Roberge S, Bujold E, Nicolaides KH. Aspirin for the prevention of preterm and term preeclampsia: systematic review and metaanalysis. Am J Obstet Gynecol 2018;218:287-293.e1.
  • Poon LC, et al. The International Federation of Gynecology and Obstetrics (FIGO) initiative on pre-eclampsia. Int J Gynecol Obstet 2019;145(suppl 1):1-33.

Frequently asked questions

Why low-dose aspirin in pregnancy?
Daily low-dose aspirin (75-150 mg) started before 16 weeks gestation reduces preeclampsia incidence by ~24 % overall and preterm preeclampsia (the most dangerous form) by ~62 % in high-risk women. The largest randomised trial — ASPRE (Rolnik NEJM 2017, 1,776 women with high-risk first-trimester screen) — found a 62 % reduction in preterm preeclampsia with 150 mg aspirin from 11-14 weeks. Roberge 2017 meta-analysis (AJOG) of 16,000+ women across 45 trials confirmed the benefit. Aspirin appears to work by inhibiting placental thromboxane production, improving placental perfusion and reducing the vasoconstrictive cascade that drives preeclampsia.
What dose and when?
USPSTF (US): 81 mg daily starting between 12 and 16 weeks, continuing until delivery (some protocols continue to 36 weeks). NICE (UK): 150 mg at night from 12 weeks until 36 weeks (NICE NG133). FIGO and ACOG accept 81-150 mg daily. The 150 mg dose has the strongest evidence (ASPRE trial); the 81 mg US dose was chosen for pragmatic reasons (available as low-dose 'baby aspirin'). Both work; higher dose may work better in some women.
Is it safe to take aspirin in pregnancy?
Yes, at the low dose used for preeclampsia prevention. Low-dose aspirin (75-150 mg) has decades of evidence of safety in pregnancy. Major bleeding events are rare, and no increased risk of placental abruption, postpartum haemorrhage, or neonatal complications was seen in ASPRE or in the meta-analyses. Some women experience mild GI discomfort or nasal congestion. Higher-dose aspirin (≥ 325 mg, NSAID-equivalent doses) is AVOIDED in pregnancy because of bleeding and ductus arteriosus risks; that's a different decision.
What if I have multiple risk factors?
More risk factors = stronger indication for prophylaxis. If you have ANY ONE high-risk factor (prior PE, multifetal pregnancy, chronic hypertension, pregestational diabetes, renal disease, or autoimmune disease like SLE/APS), aspirin is recommended. If you have TWO OR MORE moderate-risk factors (nulliparity, BMI > 30, family history, age ≥ 35, prior SGA, IVF conception, interval > 10 yrs, Black race), aspirin is also recommended. Some clinicians use the FMF Triple Test (maternal factors + MAP + uterine artery Doppler) to refine individual risk further.
What's the FMF Triple Test and how is it different?
The Fetal Medicine Foundation (Nicolaides/Poon) Triple Test is a Bayesian competing-risks algorithm that combines maternal factors with mean arterial pressure (MAP), uterine artery pulsatility index (UtA-PI), and serum PlGF (placental growth factor) at 11-13+6 weeks to predict individual risk of preterm preeclampsia. Performance: ~75 % detection of early-onset PE at 10 % false positive (compared to ~40 % for maternal factors alone). FMF uses 1:100 as the threshold for aspirin. The USPSTF/ACOG categorical risk-factor approach is simpler and globally applicable; the FMF approach is more individual but requires specific assays and uterine artery Doppler that aren't universally available.
Can I stop aspirin if my risk seems lower than expected?
Generally no, once started. Aspirin's benefit comes from continuous platelet inhibition through the placentation period. Stopping early may lose the protective effect. Continue until at least 36 weeks (or until delivery, per protocols). Discuss with your obstetric provider before any change.
When should I NOT take aspirin?
Active peptic ulcer disease or recent GI bleeding, severe asthma triggered by NSAIDs (aspirin-exacerbated respiratory disease), known aspirin allergy, severe liver disease, or bleeding disorders. Discuss with your obstetric provider; the decision is individual. The vast majority of women in the indicated risk categories tolerate it without issue.
What's the absolute benefit?
In the high-risk group with positive FMF Triple Test, the number-needed-to-treat (NNT) to prevent one case of preterm preeclampsia is about 30 (ASPRE 2017). For women with multiple risk factors, NNT to prevent any preeclampsia is around 50-70. These are good numbers for a cheap, well-tolerated intervention. For women without risk factors, the benefit is smaller and the recommendation neutral.

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