Most guidelines recommend stopping baby aspirin at 36 weeks of pregnancy, though some providers advise continuing it until delivery. The exact timing depends on which guideline your provider follows and your individual risk factors, but the goal is the same: keep aspirin working long enough to prevent preeclampsia while stopping early enough to reduce bleeding risk during labor.
The Standard Stop Date: 36 Weeks
The most widely cited stop date across international guidelines is 36 weeks of gestation. Clinical trials studying aspirin for preeclampsia prevention have consistently used this cutoff, instructing participants to take daily low-dose aspirin from early pregnancy through 36 weeks. The logic is straightforward: by 36 weeks, the placental development that aspirin helps protect has largely completed its course, and the benefit of continued use starts to be outweighed by bleeding concerns as delivery approaches.
There is one notable exception. The American College of Obstetricians and Gynecologists (ACOG) and the Society for Maternal-Fetal Medicine recommend continuing low-dose aspirin daily until delivery. This means your provider’s instructions may differ from what you read online or hear from friends with different OBs. Both approaches are considered evidence-based, and the difference reflects genuine clinical debate rather than one side being wrong.
Why Stopping Before Labor Matters
Aspirin works by reducing the blood’s ability to clot. That’s helpful during pregnancy for preventing the blood vessel and placental problems that lead to preeclampsia. But during labor and delivery, when bleeding is at its highest risk, reduced clotting can become a liability. This is why most guidelines recommend discontinuing aspirin before the onset of labor, particularly before 37 weeks (considered full term).
The concern is peripartum bleeding, meaning excessive blood loss around the time of delivery. Stopping at 36 weeks gives your body roughly a week to normalize its clotting function before a typical term delivery. If you’re scheduled for a cesarean section or induction, your provider may time the stop date relative to that planned date rather than using a blanket 36-week rule.
What If You Go Into Labor Early
If labor starts before your planned stop date, the aspirin still in your system does carry some additional bleeding risk, but it’s a manageable one. Your medical team will be aware of your aspirin use and can prepare accordingly. This isn’t a situation where you need to panic, though it’s worth making sure everyone involved in your delivery knows you’ve been taking daily aspirin. Keep this information on your hospital bag checklist and mention it during admission.
When to Start (and Why It Matters for Stopping)
The U.S. Preventive Services Task Force recommends starting low-dose aspirin (81 mg per day) after 12 weeks of gestation for people at high risk of preeclampsia. Most studies that showed benefit initiated treatment before 20 weeks, and ideally before 16 weeks. Some research suggests starting even earlier, before 11 weeks, may influence placental development at its earliest stages, though studies haven’t found major differences in outcomes between starting before 11 weeks versus between 11 and 14 weeks as long as you’re consistent about taking it.
The start date matters for the stop date because aspirin’s protective effect depends on a sustained course through the second and third trimesters. Starting late and stopping early shrinks the window of benefit. If you didn’t begin aspirin until 20 weeks or later, your provider may have a different calculus about when to stop.
Dosage Differences Around the World
In the United States, the standard dose is 81 mg per day. In the UK and much of Europe, providers prescribe 150 mg daily. This isn’t just a regional quirk: a meta-analysis comparing the two dose ranges found that 150 to 162 mg daily reduced preterm preeclampsia by about 66% compared to 75 to 81 mg daily. The higher dose was also associated with a 77% reduction in severe preeclampsia. Both doses are continued through 36 weeks in most trial protocols. Your dose doesn’t typically change the stop date, but it’s worth knowing that aspirin dosing for preeclampsia prevention is not one-size-fits-all globally.
Safety of Aspirin Throughout Pregnancy
One common worry is whether aspirin increases the risk of placental abruption, where the placenta separates from the uterine wall before delivery. A large meta-analysis covering more than 12,500 participants found that aspirin at doses under 100 mg per day had no impact on placental abruption risk regardless of when it was started. At higher doses (100 mg or more), starting before 16 weeks actually trended toward a lower risk of abruption compared to starting later. A 2025 study in Scientific Reports confirmed that 75 mg daily from early pregnancy through the third trimester showed no significant increase in maternal bleeding or other complications.
In short, taking baby aspirin as prescribed throughout the recommended window is safe for both you and the baby. The risks of stopping too early, and losing preeclampsia protection, generally outweigh the risks of the aspirin itself.
What to Do With Conflicting Advice
If your provider says to stop at 36 weeks, they’re following the most common guideline used in clinical trials. If they say to continue until delivery, they’re following ACOG’s recommendation. Both are reasonable. The key things that matter most are starting before 16 weeks, taking it every day without gaps, and not stopping on your own without discussing it first. Abruptly discontinuing aspirin mid-pregnancy because you forgot to refill a prescription or felt uncertain is more risky than the difference between stopping at 36 weeks versus continuing to delivery.
If you’re approaching 36 weeks and haven’t received clear instructions, bring it up at your next prenatal visit. Your provider can give you a specific date based on your due date, your delivery plan, and your individual risk profile.