Low-dose aspirin prevents preeclampsia by working on two fronts: it reduces inflammation that can damage the placenta’s blood supply, and it helps restore normal blood flow between the uterus and placenta during the critical early weeks of pregnancy. In the landmark ASPRE trial, aspirin taken daily from the first trimester reduced the rate of preterm preeclampsia by 62%, and the rate of very early disease (requiring delivery before 32 weeks) dropped by nearly 90%.
What Goes Wrong in Preeclampsia
To understand how aspirin helps, it’s worth understanding what fails in preeclampsia. Early in a healthy pregnancy, specialized placental cells called trophoblasts burrow deep into the wall of the uterus and remodel the small spiral arteries that supply blood to the placenta. These arteries widen dramatically, creating a high-volume, low-pressure blood supply that can keep up with the growing baby’s needs.
In preeclampsia, this remodeling is incomplete. The trophoblast cells don’t invade deeply enough, the spiral arteries stay narrow, and the placenta ends up chronically underfed. The stressed placenta then releases signals into the mother’s bloodstream that damage blood vessel linings throughout her body, driving up blood pressure and causing protein to leak into the urine. That cascade is what produces the dangerous symptoms of preeclampsia: organ damage, seizure risk, and in severe cases, the need for very early delivery.
How Aspirin Corrects the Problem
Aspirin targets the inflammation that undermines trophoblast invasion in the first place. In preeclampsia-prone pregnancies, inflammatory signaling ramps up early and activates an enzyme called COX-2, which triggers a chain reaction that weakens the ability of trophoblast cells to penetrate the uterine wall. Aspirin blocks this pathway. In laboratory studies, when trophoblast cells were exposed to inflammatory triggers, their ability to invade tissue dropped to about 30% of normal. Adding aspirin more than doubled that invasion capacity, largely rescuing the cells’ function.
Animal research tells a similar story. In a mouse model of preeclampsia, the number of placental cells that successfully invaded the uterine lining dropped by roughly 50% under inflammatory conditions. When aspirin was given alongside the inflammatory trigger, invasion rebounded by 64% compared to the untreated group.
Aspirin also shifts the balance between two competing signals in blood vessels. Healthy placentas produce a substance called prostacyclin that keeps blood vessels relaxed and prevents clotting, while another substance called thromboxane does the opposite, narrowing vessels and promoting clots. In preeclampsia, the balance tips toward thromboxane. Low-dose aspirin preferentially suppresses thromboxane production without significantly reducing prostacyclin, helping to keep the placental blood vessels open and flowing.
Why Timing Matters So Much
Spiral artery remodeling happens primarily in the first and early second trimester. Once that window closes, the architecture of the placental blood supply is largely set. This is why aspirin needs to be started early to be effective. The U.S. Preventive Services Task Force recommends beginning low-dose aspirin after 12 weeks of gestation, and the American College of Obstetricians and Gynecologists says it should optimally be started before 16 weeks. Studies have used start dates ranging from 11 to 32 weeks, but the strongest benefits consistently come from earlier initiation.
Once started, the standard approach is to continue aspirin daily until delivery, or in some cases until 36 weeks. ACOG notes there is no apparent benefit to stopping early before delivery, though some study protocols have varied on this point.
The Recommended Dose
The USPSTF recommends 81 mg per day, which is the standard “baby aspirin” dose available over the counter in the U.S. The ASPRE trial, which produced the strongest results, used 150 mg per day. Both doses are considered low-dose in the context of aspirin therapy, and the difference partly reflects regional prescribing norms (150 mg is more common in European guidelines, 81 mg in American ones). What both share is that they’re far below the doses used for pain relief, which is why side effects are minimal.
Who Should Take It
Aspirin for preeclampsia prevention isn’t recommended for all pregnant people. It’s targeted at those with elevated risk. The USPSTF identifies high-risk factors that, on their own, warrant aspirin use:
- History of preeclampsia in a previous pregnancy, especially if it was early-onset or severe
- Chronic high blood pressure diagnosed before pregnancy
- Type 1 or type 2 diabetes
- Kidney disease
- Autoimmune conditions such as lupus or antiphospholipid syndrome
- Carrying multiples (twins, triplets)
A second category of moderate risk factors can also trigger the recommendation when two or more are present. These include a first pregnancy, obesity, family history of preeclampsia, age over 35, and certain sociodemographic factors associated with higher preeclampsia rates. Your provider weighs these together when deciding whether aspirin makes sense for your pregnancy.
What Aspirin Does Not Do
Aspirin reduces the risk of preeclampsia, but it doesn’t eliminate it. Even in the ASPRE trial’s high-risk population, some women still developed the condition despite taking aspirin as directed. Aspirin is most effective against preterm preeclampsia (the more dangerous form that develops before 37 weeks) and appears less effective at preventing mild, late-onset preeclampsia that develops near the due date. Late-onset disease likely involves somewhat different mechanisms that are less dependent on early placental development.
Aspirin also doesn’t treat preeclampsia once it has developed. If you’re already showing signs of the condition, the management approach shifts entirely. Aspirin is purely a preventive strategy, and its value lies in getting ahead of the problem during the narrow window when the placental blood supply is being built.