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

Two respected bodies of evidence point the same direction with different tools. The U.S. Preventive Services Task Force (USPSTF) gives low-dose aspirin, 81 mg daily started after 12 weeks of gestation, a grade B recommendation for people with clinical risk factors, while the ASPRE trial tested 150 mg taken nightly in women flagged by a first-trimester screening algorithm. Both approaches reduced preterm preeclampsia. The dose, the screening gate, and the way the benefit is framed are where they diverge, and understanding that gap is the whole point of reading the two together.

What ASPRE actually tested

ASPRE (Aspirin for Evidence-Based Preeclampsia Prevention), published in the New England Journal of Medicine in 2017 by Rolnik and colleagues, was a randomized, placebo-controlled trial with a specific and unusual entry gate. Women were not enrolled on clinical history alone. At 11 to 13 weeks of gestation they were screened with a combined algorithm blending maternal factors, mean arterial pressure, uterine artery pulsatility index (an ultrasound measure of placental blood flow), and the serum marker placental growth factor. Only those the algorithm scored as high risk for preterm preeclampsia were randomized.

Those women received either 150 mg of aspirin or placebo each night, from soon after screening until 36 weeks. The primary outcome was preterm preeclampsia, meaning preeclampsia leading to delivery before 37 weeks. The result was striking in relative terms: the odds of preterm preeclampsia in the aspirin group were roughly 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 significantly reduce preeclampsia at term. The benefit concentrated in the earlier, more dangerous end of the spectrum.

Two caveats travel with that headline. First, the confidence interval is wide, reflecting a trial in which the total number of preterm preeclampsia cases was small even though thousands of women were screened. A rare outcome makes a dramatic-looking odds ratio less precise than it appears. Second, a later analysis of the same trial found the effect was strongest in women who took at least 90 percent of their tablets, which is a reminder that the number on the label and the number actually swallowed are different quantities.

What the USPSTF recommends, and why the numbers differ

The USPSTF issued its current statement in 2021, reaffirming its 2014 position. It recommends low-dose aspirin, defined as 81 mg per day, initiated after 12 weeks of gestation, for pregnant people at high risk of preeclampsia. This is a grade B recommendation, not the grade A some summaries assume. Grade B signals moderate certainty of a moderate to substantial net benefit, which is a meaningful endorsement but not the Task Force's highest tier.

The USPSTF does not use ASPRE's biomarker algorithm. It uses a risk-factor list. Someone with any single high-risk factor, such as a prior preeclampsia, chronic hypertension, pregestational diabetes, kidney disease, an autoimmune condition like lupus or antiphospholipid syndrome, or a multifetal pregnancy, qualifies. Someone with several moderate-risk factors, such as first pregnancy, obesity, family history, age 35 or older, or certain sociodemographic factors the Task Force attributes to inequities in care, may also qualify. It is a checklist a clinician can run in any office without specialized ultrasound or assay.

So why 81 mg when ASPRE used 150 mg? Part of the answer is pragmatic: 81 mg is the standard low-dose tablet widely available in the United States, and the broader evidence base the USPSTF reviewed, spanning many trials at doses from about 60 to 150 mg, supported benefit across that range. ASPRE is one influential trial within that evidence base, not its sole foundation. There is genuine scientific discussion, not settled consensus, about whether higher doses or bedtime dosing add benefit, and the two documents reflect that open question rather than a contradiction.

Reading the two together: absolute versus relative

The most important habit when comparing these sources is to separate relative from absolute framing. An odds ratio of 0.38 sounds like a two-thirds reduction, and in relative terms it is. But the absolute benefit depends on baseline risk. In a population screened to be genuinely high risk, as in ASPRE, a large relative reduction translates into a worthwhile number of cases prevented. Applied to a lower-risk population, the same relative effect prevents far fewer cases per person treated, because there were fewer cases to prevent.

This is why the screening strategy matters as much as the drug. ASPRE's algorithm concentrates treatment in people most likely to benefit, which is efficient but requires specialized first-trimester screening infrastructure that is not universal. The USPSTF's risk-factor approach casts a wider, simpler net, accepting that some people who take aspirin would never have developed preeclampsia in exchange for a tool any clinician can apply. Neither is wrong. They optimize for different health systems and different resources.

Aspirin at these doses has a reassuring safety record in pregnancy across the trials, without signals of increased bleeding or placental abruption of the magnitude that would offset the benefit, though no preventive medication is free of tradeoffs. This article is educational and is not medical advice. Whether aspirin is appropriate, at what dose, and when to start are decisions for an individual and their own clinician, weighing that person's specific risk profile.

The takeaway for careful readers

ASPRE and the USPSTF recommendation are not rivals. ASPRE is a precision demonstration that first-trimester screening plus 150 mg nightly sharply lowers preterm preeclampsia in a selected high-risk group. The USPSTF translates a wider body of evidence into a pragmatic, risk-factor-based rule using 81 mg that a general clinician can implement. The apparent dose disagreement is smaller than it looks once you see that both sit inside the same supportive evidence range, and the real lesson is about matching an intervention's absolute payoff to how carefully the population was selected.