Pregnancy · Pre-eclampsia Prevention
Aspirin in Pregnancy — PE Prevention
Low-dose aspirin (75-150 mg) from before 16 weeks reduces pre-eclampsia by 24% overall, and preterm PE by 62% in high-risk women. Who needs it, dose, side effects, when to start + stop. NICE NG133 / USPSTF / ASPRE NEJM 2017.
Last reviewed 2 June 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.
Why aspirin in pregnancy?
Low-dose aspirin (75-150 mg) reduces pre-eclampsia by ~24% overall, and preterm PE (the most dangerous form) by ~62% in high-risk women.
Works by inhibiting placental thromboxane → improves blood flow + reduces inflammation cascade.
ASPRE trial (NEJM 2017) confirmed major benefit. Roberge 2017 meta-analysis (16,000+ women) confirmed.
Who should take it?
NICE NG133 / USPSTF / ACOG: ONE high-risk factor OR TWO moderate:
High-risk (any one triggers):
- Previous pre-eclampsia.
- Chronic hypertension.
- Type 1 or 2 diabetes.
- Chronic kidney disease.
- Autoimmune disease (lupus, APS).
Moderate (two or more):
- First pregnancy.
- Age ≥35.
- BMI ≥30.
- Pregnancy interval >10 years.
- Family history of PE.
- Multiple pregnancy.
- Black / Hispanic ethnicity (US).
When to start
Before 16 weeks — earlier the better. Ideal: 12-14 weeks after dating scan.
Started after 16 weeks much less effective. Continue until 36 weeks.
Dose
- UK: 150 mg nightly.
- US: 81 mg daily.
- Take with food; nighttime ideal (less GI irritation).
- Don’t use: 300+ mg high-dose (bleeding risk).
Side effects
Generally very well tolerated. Possible:
- GI irritation (~10%) — take with food.
- Bruising slightly increased.
- Gum bleeding when brushing.
- Allergic reaction rare.
Not linked to miscarriage, birth defects, or placental abruption (low-dose aspirin REDUCES placental complications).
Safe for baby?
Yes — low-dose aspirin safe throughout pregnancy. Not teratogenic. No increase in birth defects or growth restriction (improves growth in placental insufficiency).
High-dose (300+ mg, NSAID) AVOID — premature ductus arteriosus closure, bleeding, oligohydramnios.
When to stop
- Standard: 36 weeks (some say 37-38).
- Stop 7-10 days before planned C-section.
- Unexpected labour: doesn’t contraindicate epidural (much weaker effect than LMWH).
- Postpartum: stop unless continued for other indication.
What if I miss a dose?
Take when you remember the same day. If next day, skip; don’t double-dose. Effect is cumulative — single miss doesn’t ruin protection. Set daily routine, phone reminder.
NHS prescriptions cover aspirin in pregnancy free.
FMF / enhanced first-trimester PE screen
Combines history + uterine artery Doppler + PlGF + PAPP-A + MAP at 11-14 weeks. Calculates individual PE risk.
Catches additional ~50% of women who’d benefit from aspirin beyond history-alone screening. NHS in some trusts; private £200-400.
Other painkillers compatible
- Paracetamol: fully compatible; first-line.
- NSAIDs (ibuprofen, diclofenac): avoid in pregnancy.
- Codeine / tramadol: fine.
Different scenarios — aspirin in pregnancy
Scenario 1: First baby, age 38, BMI 31, no PE history
Three moderate factors (or 2 + first pregnancy = qualify). Aspirin 150 mg nightly from 12-14 wk.
Scenario 2: Previous PE at 34 weeks, now planning second pregnancy
High-risk. Aspirin from positive pregnancy test or 12 wk latest. Specialist consultant care.
Scenario 3: T1DM, age 30, no other risk factors
T1DM high-risk alone. Aspirin from 12 wk. Tight glucose control.
Scenario 4: Healthy 28-year-old, first baby, BMI 24
Only one moderate factor (first pregnancy). Aspirin not routinely indicated. FMF screen could refine.
Scenario 5: APS positive, recurrent miscarriage history
Aspirin + LMWH from positive pregnancy test. Specialist rheumatology + obstetric care.
Care guidance — aspirin in pregnancy
- Discuss at booking — risk stratification.
- Start <16 weeks for effect.
- 150 mg UK, 81 mg US.
- Take with food, nighttime ideal.
- Continue to 36 weeks.
- Stop 7-10 days before planned C-section.
- Don’t double-dose if missed.
- Tell any healthcare provider you’re on aspirin.
- Paracetamol for any pain (not NSAIDs).
- FMF screen if available for refined risk.
- Future pregnancies: aspirin recommended if PE history.
Sources
- NICE NG133. Hypertension in pregnancy.
- Rolnik DL, et al. ASPRE: Aspirin versus Placebo in Pregnancies at High Risk for Preterm Preeclampsia. NEJM 2017.
- Roberge S, et al. The role of aspirin dose on the prevention of preeclampsia and fetal growth restriction: systematic review and meta-analysis. AJOG 2017.
- USPSTF. Low-Dose Aspirin Use for the Prevention of Morbidity and Mortality From Preeclampsia. 2021.
- ACOG Committee Opinion 743. Low-Dose Aspirin Use During Pregnancy.
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Frequently asked questions
Why aspirin in pregnancy if it's normally avoided?
AT LOW DOSE (75-150 mg/day), aspirin REDUCES PRE-ECLAMPSIA risk by ~24% overall and PRETERM PRE-ECLAMPSIA (most dangerous form) by ~62% in high-risk women. STARTED before 16 WEEKS. WORKS by inhibiting placental thromboxane production → improves placental blood flow → reduces inflammation cascade that causes PE. LARGEST trial — ASPRE (Rolnik NEJM 2017, 1,776 women) — confirmed major benefit. ROBERGE 2017 meta-analysis (16,000+ women, 45 trials) confirmed. HIGH-DOSE aspirin (300+ mg) AVOIDED in pregnancy due to bleeding + Reye's. 75-150 mg = SAFE, evidence-based for PE prevention.
Who should take aspirin in pregnancy?
NICE NG133 / USPSTF / ACOG recommend if ONE HIGH-RISK factor OR TWO MODERATE: HIGH-RISK (any one triggers aspirin): (1) Previous pre-eclampsia; (2) Chronic hypertension; (3) Type 1 or 2 diabetes; (4) Chronic kidney disease; (5) Autoimmune disease (lupus, antiphospholipid syndrome); (6) Previous severe PE / preterm PE / IUGR-PE. MODERATE (two or more): (1) First pregnancy (nulliparous); (2) Age ≥35; (3) BMI ≥30; (4) Pregnancy interval >10 years; (5) Family history of PE; (6) Multiple pregnancy (twins+); (7) Black / Hispanic ethnicity (US guidance); (8) Lower socioeconomic status. OPTIONS: discuss with midwife / consultant at booking.
When should I start aspirin?
BEFORE 16 WEEKS — earlier the better. IDEAL: 12-14 weeks (after dating scan), OR if known high-risk preconception, can start at positive pregnancy test. EVIDENCE: aspirin started AFTER 16 weeks much less effective; trials show optimal window is 11-16 weeks. CONTINUE: until 36 weeks (some say 37-38). STOP: 7-10 days before planned C-section (some practitioners continue); STOP at labour onset usually. DON'T stop without clinical advice.
What dose of aspirin?
NICE NG133 / UK: 150 mg nightly. USPSTF / US: 81 mg daily (US doesn't have 150 mg formulation). ASPRE used 150 mg = standard most evidence. CHEAPEST + best evidence dose. DON'T USE: paracetamol (different drug); high-dose aspirin (300+ mg, NSAID effect, bleeding risk). LOW-DOSE aspirin tablets: 75 mg in UK ('baby aspirin') = older dose; many trusts now 150 mg. TAKE WITH FOOD; nighttime ideal (less GI irritation; smoother placental blood flow during sleep).
Are there side effects?
GENERALLY VERY WELL TOLERATED. POSSIBLE: (1) GI IRRITATION — heartburn, nausea (~10%); take with food, evening dose helps; (2) BRUISING — slightly increased; usually minor; (3) GUM BLEEDING when brushing — usually mild; (4) ALLERGIC reaction — rare; if asthma + nasal polyps, AERD (aspirin-exacerbated respiratory disease) — avoid; (5) ULCERS — rare at low dose; (6) HEMORRHOIDS bleeding sometimes. SERIOUS: significant bleeding rare at 75-150 mg. NOT linked to: miscarriage, birth defects (no teratogenic effect), placental abruption (low-dose aspirin reduces, doesn't cause).
Will aspirin harm baby?
NO. LOW-DOSE aspirin (75-150 mg) is SAFE for baby throughout pregnancy. NOT teratogenic. NO increase in: birth defects; growth restriction (actually IMPROVES growth in placental insufficiency); miscarriage; stillbirth. HIGH-DOSE aspirin (300+ mg, NSAID use): AVOID — premature closure of ductus arteriosus (baby's heart vessel), bleeding, oligohydramnios. STICK to 75-150 mg/day max. DELIVERY: stop 7-10 days before planned C-section (sometimes continued); stop at labour for safety.
What if I miss a dose?
TAKE IT WHEN you remember THAT day if you remember within several hours. If next day already: skip the missed dose; take regular next dose. DON'T double-dose. EFFECT cumulative — single missed dose doesn't ruin protection. MULTIPLE MISSES weekly: less protective. SET DAILY ROUTINE — same time each day (evening with dinner often easiest). PHONE REMINDER. PARTNER reminder. PILL ORGANISER. NHS prescriptions cover aspirin during pregnancy free.
Should everyone take it 'just in case'?
NO. UNIVERSAL aspirin NOT recommended. Reserved for: high-risk OR ≥2 moderate risk factors. WHY NOT EVERYONE: (1) Most women won't get PE — exposure to drug without need; (2) Small but real bleeding risk; (3) Compliance issues lower with universal screening. RISK-STRATIFICATION via FMF first-trimester screen (history + biomarkers) identifies high-risk women — better than history alone. FMF / NICE PE screen at 11-14 weeks (private + some NHS) — IDENTIFIES additional women who'd benefit.
Can I take aspirin if I've never had PE?
DEPENDS on OTHER risk factors. If you have ZERO PE risk factors: aspirin NOT indicated; healthy pregnancy doesn't need it. If you have ≥1 HIGH-RISK or ≥2 MODERATE: yes (even without prior PE — first pregnancy is itself a risk factor; combined with age 35+ + BMI ≥30 = 3 moderate). NICE risk stratification at booking determines. ALWAYS discuss — sometimes individual circumstances warrant. SOME women take aspirin in IVF / fertility journey (separate indication — recurrent miscarriage workup, implantation support — controversial).
What about aspirin for recurrent miscarriage / APS?
DIFFERENT INDICATION same drug. ANTIPHOSPHOLIPID SYNDROME (APS): aspirin + LMWH from positive pregnancy test (raise live birth from ~10% to ~70-80%). UNEXPLAINED recurrent miscarriage: aspirin alone NOT effective (ALIFE 2010 trial). IVF: aspirin sometimes used; mixed evidence. DOSE same (75-150 mg). DOUBLE INDICATION sometimes: APS + PE risk — aspirin covers both. SPECIALIST guidance.
Can I take other painkillers with aspirin?
(1) PARACETAMOL — yes, fully compatible; first-line for headaches, fever, pain in pregnancy. (2) NSAIDs (ibuprofen, diclofenac, naproxen) — AVOID in pregnancy generally (heart vessel issue, oligohydramnios after 28 weeks); also additive bleeding risk with aspirin. (3) CODEINE / TRAMADOL — fine alongside aspirin (but constipation worse). (4) PRESCRIPTION painkillers — discuss with doctor; most fine. ALWAYS: tell any healthcare professional you're on aspirin (dental work, surgery, post-delivery).
When do I stop aspirin?
STANDARD: 36 weeks (some say 37-38 wk). REASONS: (1) Placental development complete; main protective period past; (2) AVOID bleeding at delivery; (3) Stop 7-10 days before planned C-section (some continue, evidence supports; depends on anaesthetist comfort). UNEXPECTED labour: aspirin in last 7 days doesn't contraindicate epidural (much weaker effect than LMWH on bleeding). POSTPARTUM: stop UNLESS continued for another indication (T2DM, cardiac disease, APS).
Does aspirin help with growth restriction?
PARTIALLY YES. ASPRE + meta-analyses: aspirin reduces IUGR (intrauterine growth restriction) by ~20% in high-risk women. MECHANISM: improves placental blood flow. NOT a treatment for established IUGR (delivery often only option); a preventive measure. SOME WOMEN start aspirin specifically for IUGR risk (previous SGA baby, hypertension etc). DISCUSS at booking review of risk factors.
Do I need a private FMF first-trimester PE screen?
OPTIONAL. ENHANCED first-trimester screen (Fetal Medicine Foundation): combines maternal history + uterine artery Doppler + PlGF + PAPP-A + MAP at 11-14 weeks. CALCULATES individual PE risk — identifies high-risk women not flagged by history alone. AVAILABLE: some UK trusts FREE; private ~£200-400. BENEFIT: catches additional ~50% of women who'd benefit from aspirin. COSTS: extra appointment, drug exposure for woman who turned out low-risk. INDIVIDUAL choice.
What about preeclampsia in next pregnancy?
PREVIOUS PE: recurrence ~15-50% depending on severity, gestation. STRONG indication for aspirin from <16 weeks NEXT pregnancy. EVEN STRONGER if severe / early-onset PE / HELLP previously. PRECONCEPTION: BP, BMI, HbA1c optimisation; check antiphospholipid antibodies if recurrent / early-onset / severe. PLAN with consultant. WITH ASPIRIN: recurrence rate roughly halved. NEXT PE often milder than previous.
How does this relate to other calculators on BumpBites?
Companion: /calculators/preeclampsia-risk for risk stratification; /calculators/preeclampsia-diagnosis if PE develops; /calculators/fmf-pe-screen for first-trimester enhanced screen; /calculators/hellp-classifier for severe variant; /calculators/recurrent-miscarriage for APS pathway; /calculators/vte-prophylaxis-pregnancy (often co-prescribed); /calculators/pcos-pregnancy (moderate risk factor).