When to Start Aspirin for Preeclampsia Prevention
Start low-dose aspirin between 12 and 16 weeks of gestation (optimally before 16 weeks) in high-risk pregnant individuals to prevent preeclampsia. 1, 2, 3
Optimal Timing Window
- Initiate aspirin at 12-16 weeks of gestation for maximum effectiveness in preventing preeclampsia 1, 4, 5
- Starting before 16 weeks appears more effective than later initiation, as defective placentation and inadequate spiral artery remodeling occur in the first trimester 5, 6
- The latest acceptable initiation is 28 weeks, though this is suboptimal 2, 3
- Continue aspirin daily until delivery 1, 2, 3
The evidence consistently shows that early initiation (before 16 weeks) is critical because aspirin improves uteroplacental blood flow during the critical period of placentation in the first trimester. 5 While guidelines allow initiation up to 28 weeks, the biological rationale and evidence strongly favor starting as early as possible within the 12-16 week window.
Dosing Recommendations
Standard Populations
- 100-150 mg daily is the evidence-based dose for preeclampsia prevention 1, 6
- The commonly available 81 mg dose in the United States may be less effective, particularly in certain high-risk populations 5, 6
- International guidelines (Royal College of Obstetricians and Gynaecologists, National Institute of Health and Care Excellence) recommend 150 mg daily 6
Special Populations Requiring Higher Doses
- Pregnant individuals with type 1 or type 2 diabetes: 100-150 mg daily (162 mg is acceptable; in the U.S., this means two 81 mg tablets) 1, 5
- Individuals with BMI >40 kg/m² may require dose adjustment as higher BMI reduces aspirin effectiveness 5
- Recent evidence suggests 162 mg may be more effective than 81 mg in obese high-risk individuals, with a 78% probability of benefit 7
Critical caveat: The standard 81 mg dose recommended by some U.S. guidelines may be inadequate. Meta-analyses demonstrate that doses <100 mg are not effective in reducing preeclampsia, while doses >100 mg initiated before 16 weeks are most effective. 1, 6
High-Risk Factors Warranting Aspirin Prophylaxis
Single High-Risk Factor (any one warrants aspirin):
- History of preeclampsia, especially early-onset with delivery <34 weeks 5, 2, 3
- Multifetal gestation 2, 3
- Chronic hypertension 2, 3
- Type 1 or type 2 diabetes 1, 2, 3
- Renal disease 2, 3
- Autoimmune disease (e.g., systemic lupus erythematosus, antiphospholipid syndrome) 2, 3
Multiple Moderate-Risk Factors (≥2 warrant aspirin):
- First pregnancy (nulliparity) 1, 2, 3
- Maternal age ≥35 years 1, 2, 3
- BMI >30 kg/m² 1, 2, 3
- Family history of preeclampsia (mother or sister) 1, 2, 3
- Low socioeconomic status 1
- Interpregnancy interval ≥10 years 1
Safety Profile
- Low-dose aspirin does not increase risks of placental abruption, postpartum hemorrhage, fetal intracranial bleeding, perinatal mortality, or congenital anomalies 1, 4, 5
- No significant differences in cesarean delivery rates 4
- Safe for use throughout pregnancy when initiated in the second trimester 1, 2, 3
Additional Benefits Beyond Preeclampsia Prevention
- 14% reduction in preterm birth 4
- 20% reduction in intrauterine growth restriction 4
- Increases mean birthweight by approximately 130 grams 4
- 24% reduction in preeclampsia risk with absolute risk reduction of 2-5% (number needed to treat = 42) 4
Duration of Therapy
- Continue aspirin until delivery in most cases 1, 2, 3
- Some guidelines suggest continuation until 36-37 weeks 1
- Recent evidence suggests aspirin may be safely discontinued at 24-28 weeks in individuals with normal sFlt-1:PlGF ratio, though this requires biomarker testing 8
The weight of evidence strongly supports initiating aspirin at 12-16 weeks (before 16 weeks is optimal) at doses of 100-150 mg daily, continuing until delivery, in all pregnant individuals with high-risk factors for preeclampsia.