Doctors prescribe low-dose aspirin during pregnancy primarily to reduce the risk of preeclampsia, a dangerous condition involving high blood pressure that can threaten both the pregnant person and the baby. In large pooled analyses of over 14,000 individuals, low-dose aspirin reduced preeclampsia risk by about 15% and preterm birth risk by about 20% among those at elevated risk. It’s not prescribed to every pregnant person, though. The recommendation targets those with specific risk factors that make preeclampsia more likely.
How Low-Dose Aspirin Protects the Placenta
Preeclampsia is linked to a chemical imbalance in the placenta. In a healthy pregnancy, the placenta produces two substances that work in opposition: one that constricts blood vessels and promotes clotting, and another that relaxes blood vessels and prevents clots. In preeclampsia, the balance tips toward too much constriction and clotting, restricting blood flow to the placenta and driving up blood pressure.
Low-dose aspirin corrects this imbalance in a surprisingly targeted way. Research on human placental arteries found that aspirin significantly reduced the clot-promoting substance (thromboxane) while leaving the vessel-relaxing substance (prostacyclin) essentially unchanged. In lab studies, thromboxane production dropped from about 8.6 to 1.4 units per hour with aspirin treatment. That selective effect is what makes a small daily dose useful rather than harmful: it dials down the dangerous side of the equation without touching the protective side.
Who Gets the Prescription
The American College of Obstetricians and Gynecologists (ACOG) divides the risk factors into two tiers: high risk and moderate risk. If you have even one high-risk factor, aspirin is recommended. If you have a combination of moderate-risk factors, it may also be recommended.
High-Risk Factors
- History of preeclampsia, especially if it led to a serious complication
- Carrying multiples (twins, triplets)
- Chronic hypertension
- Type 1 or type 2 diabetes diagnosed before pregnancy
- Kidney disease
- Autoimmune conditions such as lupus or antiphospholipid syndrome
Moderate-Risk Factors
No single moderate-risk factor triggers a prescription on its own. But when two or more are present together, the combined risk becomes significant enough to warrant aspirin. These factors include:
- First pregnancy
- Obesity (BMI over 30)
- Family history of preeclampsia in a mother or sister
- Age 35 or older
- More than 10 years since a previous pregnancy
- Previous low-birth-weight baby or adverse pregnancy outcome
- IVF conception
ACOG also notes that Black race is included as a moderate-risk factor, serving as a proxy for the documented health impacts of systemic racism rather than reflecting a biological difference. Lower income is similarly listed for the same reason.
How Well It Works
The U.S. Preventive Services Task Force reviewed data from 16 studies covering more than 14,000 people at increased risk. Across those studies, low-dose aspirin reduced preeclampsia risk by 15%. Separately, an analysis of 13 studies with over 13,600 participants found a 20% reduction in preterm birth. These are population-level averages, meaning aspirin doesn’t eliminate risk entirely but meaningfully lowers it. For a condition that can lead to seizures, organ damage, placental abruption, and emergency early delivery, a 15 to 20% reduction from a single daily tablet is considered a strong return.
The benefit appears to be strongest when aspirin is started early in pregnancy, before the placenta has fully developed its blood supply patterns. Once preeclampsia is already underway, aspirin does not reverse it.
When to Start and When to Stop
The standard recommendation is to begin low-dose aspirin (81 mg per day, the same as a “baby aspirin”) between 12 and 28 weeks of pregnancy, with optimal results seen when it’s started before 16 weeks. That early window matters because the placenta is still forming its blood vessel network during the first trimester. Starting aspirin while that process is underway gives it the best chance to influence how blood flows through the placenta for the rest of the pregnancy.
On the other end, there’s no clear benefit to stopping before delivery. Some clinical study protocols stopped aspirin at 36 weeks, while others continued until delivery day. ACOG’s guidance notes that there is no apparent benefit to discontinuing early. Your provider may have a preference based on your individual situation, particularly if you have other bleeding considerations, but continuing through delivery is within the standard of care.
Safety at This Dose
The dose prescribed in pregnancy, 81 mg, is a fraction of what you’d take for a headache (typically 325 to 650 mg). At this low dose, the safety profile is well established. Large reviews have not found increased rates of serious complications like placental abruption or significant maternal bleeding. The dose is low enough to selectively target the clotting imbalance in the placenta without broadly thinning the blood the way a full dose would.
This is specifically about low-dose aspirin taken under medical guidance. Full-strength aspirin and other anti-inflammatory pain relievers carry different risks during pregnancy and are generally avoided, particularly in the third trimester.
What to Expect in Practice
If your provider prescribes aspirin, the daily routine is straightforward: one 81 mg tablet, typically taken at bedtime (some evidence suggests nighttime dosing may be slightly more effective, though this remains a point of ongoing discussion). You’ll likely be told to start at your first or second prenatal visit if you have high-risk factors, or shortly after your risk profile is assessed if moderate-risk factors are identified together.
Aspirin doesn’t replace other monitoring. You’ll still have your blood pressure checked at every prenatal visit, urine tested for protein, and potentially additional bloodwork or ultrasounds if your risk level warrants closer surveillance. Think of aspirin as one layer in a broader monitoring strategy, not a guarantee against preeclampsia. It lowers the odds, and when combined with regular prenatal care, it gives you and your provider more room to catch problems early if they do develop.