A patient came in for her first prenatal visit. Chronic hypertension, obesity, and this was her third pregnancy after two prior preeclampsia diagnoses. Textbook high-risk.
We prescribed 81 mg aspirin daily, just like ACOG recommends. Before 16 weeks, continued until delivery. I checked all the boxes.
Here’s what I didn’t tell her: the dose I prescribed has no proven effect on preventing the type of preeclampsia most likely to harm her and her baby ScienceDirect.
The Dose-Response Curve We’re Ignoring
ACOG’s 2021 Practice Advisory recommends 81 mg aspirin daily for women at high risk of preeclampsia, starting between 12 and 28 weeks ACOG. The recommendation is clear, widely implemented, and based on decades of research showing aspirin prevents preeclampsia.
Except when you look closely at which type of preeclampsia aspirin actually prevents, the 81 mg recommendation falls apart.
Preeclampsia isn’t one disease. Early-onset preeclampsia (delivery before 37 weeks) is a different beast from term preeclampsia. Preterm preeclampsia carries substantially higher risks of maternal and perinatal death and complications New England Journal of Medicine. It’s the cases that end up in the ICU. The babies in the NICU for months.
And 81 mg aspirin doesn’t prevent it.
The Number: When comparing 81 mg to 150 mg aspirin started in the first trimester, the higher dose reduced preterm preeclampsia by 66% (RR 0.34), while 81 mg showed no significant effect ScienceDirect.
That’s not a small difference. That’s the difference between prevention and placebo.
Clinical Pearl: The dose of aspirin we routinely prescribe works for term preeclampsia. It doesn’t work for preterm preeclampsia, which is exactly what kills mothers and babies.
