Low-Dose Aspirin for Preeclampsia: Reading ASPRE and the USPSTF Recommendation

A patient hands you two things she found online: a guideline that says take 81 mg of aspirin daily, and a famous trial that used 150 mg. She wants to know which one is right. The honest answer is that both are, because they were built for different jobs. The U.S. Preventive Services Task Force (USPSTF) wrote a rule any clinician can apply in a routine visit, while the ASPRE trial ran a high-precision experiment in women flagged by a first-trimester screening algorithm. The dose gap that looks like a disagreement is mostly a difference in tools, and seeing why is a small lesson in how to read evidence.

Key points#

• The USPSTF gives low-dose aspirin (81 mg daily, started after 12 weeks) a grade B recommendation for people with clinical risk factors for preeclampsia.
• The ASPRE trial tested 150 mg nightly in women selected by a first-trimester screening algorithm and cut preterm preeclampsia sharply in relative terms.
• Both fit inside a wider evidence base supporting benefit across roughly 60 to 150 mg.
• The size of the real-world payoff depends heavily on how carefully the treated population was selected.

The pragmatic rule: what the USPSTF says#

The USPSTF published its current statement in 2021, reaffirming the position it took in 2014. It recommends low-dose aspirin, defined as 81 mg per day and started after 12 weeks of gestation, for pregnant people at high risk of preeclampsia. This is a grade B recommendation, not grade A. Grade B means moderate certainty of a moderate to substantial net benefit. That is a genuine endorsement, worth acting on, but not the Task Force's top tier of confidence.

How does a clinician decide who qualifies? The USPSTF uses a plain risk-factor checklist rather than any imaging or lab test. A single high-risk factor is enough on its own: a prior pregnancy with preeclampsia, chronic hypertension, pregestational diabetes, kidney disease, an autoimmune condition such as lupus or antiphospholipid syndrome, or a pregnancy with more than one fetus. Alternatively, someone with several moderate-risk factors may qualify, including a first pregnancy, obesity, a family history of preeclampsia, age 35 or older, and certain social factors the Task Force ties to inequities in care. The whole appeal of this approach is that it needs nothing beyond a history and a conversation.

The precision experiment: what ASPRE tested#

ASPRE (Aspirin for Evidence-Based Preeclampsia Prevention), reported by Rolnik and colleagues in the New England Journal of Medicine in 2017, took the opposite approach to selecting patients. Instead of a history-based checklist, it used a screening gate at 11 to 13 weeks that combined maternal characteristics, mean arterial pressure, uterine artery pulsatility index (an ultrasound measure of blood flow to the placenta), and the blood marker placental growth factor. Only women the algorithm scored as high risk for preterm preeclampsia entered the randomized comparison.

Those women took either 150 mg of aspirin or a matching placebo each night, from shortly after screening until 36 weeks. The primary outcome was preterm preeclampsia, meaning preeclampsia severe enough to require delivery before 37 weeks. In relative terms the result was dramatic: the odds in the aspirin group were about a third of those in the placebo group, an odds ratio of 0.38 (95% confidence interval 0.20 to 0.74). Aspirin did not meaningfully lower preeclampsia that appeared at term. The benefit clustered at the earlier, more dangerous end of the disease.

Two things temper that headline. The confidence interval is wide because, even after screening thousands of women, the actual count of preterm preeclampsia cases was small, and a rare outcome makes an eye-catching odds ratio less precise than it looks. And a later analysis of the same trial found the effect was strongest among women who took at least 90 percent of their tablets, a reminder that the dose on the box and the dose actually taken are not the same number.

Why the doses differ#

So why does the guideline say 81 mg when the trial used 150 mg? Part of the reason is simply practical. In the United States, 81 mg is the standard low-dose tablet on every pharmacy shelf, and the evidence base the USPSTF weighed was not ASPRE alone. It spanned many trials at doses running from roughly 60 to 150 mg, and benefit showed up across that whole range. ASPRE is one influential study inside that larger body of work, not its foundation.

That leaves a real, unsettled question rather than a contradiction: whether a higher dose or bedtime timing adds anything on top of the standard tablet. Reasonable experts still discuss it. The two documents are not fighting; they are each reasonable readings of an evidence base with a genuinely open edge.

The habit that ties them together: absolute versus relative#

The single most useful skill when comparing sources like these is separating relative effects from absolute ones. An odds ratio of 0.38 describes a roughly two-thirds reduction, but only in relative terms. What a person actually gains depends on how likely the outcome was to begin with. In a group screened to be truly high risk, as in ASPRE, a large relative reduction turns into a meaningful number of preterm preeclampsia cases prevented. Apply the same relative effect to a lower-risk group and it prevents far fewer cases per person treated, simply because there were fewer cases available to prevent.

That is why the method of choosing patients matters as much as the drug itself. ASPRE's algorithm concentrates treatment in the people most likely to benefit, which is efficient but depends on specialized first-trimester screening that many settings do not have. The USPSTF's checklist casts a wider and simpler net, accepting that some people who take aspirin were never going to develop preeclampsia in return for a tool that works in any office. Neither is the wrong answer. They are tuned for different health systems and different resources.

Across these trials, aspirin at these doses has a reassuring safety profile in pregnancy, without signals of increased bleeding or placental abruption large enough to cancel out the benefit, though no preventive medicine is entirely free of tradeoffs.

The bottom line#

ASPRE and the USPSTF are best read as partners, not competitors. ASPRE shows that first-trimester screening plus 150 mg nightly sharply reduces preterm preeclampsia in a carefully selected high-risk group. The USPSTF turns a wider evidence base into a workable 81 mg rule built on a risk-factor checklist that any generalist can use. The dose gap shrinks once you notice both sit inside the same supportive range, and the lasting lesson is to match how impressive an intervention looks to how carefully its population was chosen.