When to Start Low-Dose Aspirin for Preeclampsia Prevention in Pregnancy
Low-dose aspirin (81 mg/day) for preeclampsia prevention should be initiated between 12 and 16 weeks of gestation in high-risk women and continued until delivery. 1, 2, 3
Risk Assessment for Preeclampsia
High-Risk Factors (one or more):
- History of preeclampsia
- Multifetal gestation
- Chronic hypertension
- Type 1 or 2 diabetes
- Renal disease
- Autoimmune disease
Moderate-Risk Factors (more than one):
- First pregnancy
- Maternal age ≥35 years
- BMI >30 kg/m²
- Family history of preeclampsia
- Pregnancy interval >10 years
- Previous adverse pregnancy outcome
Timing of Initiation
The optimal timing for starting low-dose aspirin is between 12 and 16 weeks of gestation 1, 2, 3. This timing is critical because:
- Placentation is actively occurring during this period
- Starting before 16 weeks appears to be more effective in preventing preeclampsia
- Starting too early (before 12 weeks) has limited evidence for safety
- Starting after 16 weeks may be less effective but still beneficial if initiated before 28 weeks
Dosage Considerations
While the standard recommended dose in the United States is 81 mg daily 2, 3, there is emerging evidence suggesting that higher doses may be more effective:
- The Royal College of Obstetricians and Gynaecologists and International Federation of Gynecology and Obstetrics (FIGO) recommend 150 mg daily 4
- For obese women (BMI ≥30 kg/m²), recent evidence suggests 162 mg may provide additional benefit with a 78% probability of reducing severe preeclampsia compared to 81 mg 5
Duration of Therapy
Low-dose aspirin should be continued daily until delivery 1, 2, 3. There is no evidence supporting discontinuation before delivery, and the safety profile supports continued use throughout pregnancy.
Important Clinical Considerations
- Safety profile: Low-dose aspirin in pregnancy has a well-established safety record with minimal risk of maternal or fetal complications 2, 3
- Timing is critical: Starting before 16 weeks appears to be most effective 4
- Obesity impact: Women with BMI >30 kg/m² may benefit from higher doses (162 mg) 5
- Missed opportunity: Failure to identify high-risk women and initiate aspirin therapy before 16 weeks represents a missed opportunity for preeclampsia prevention
Common Pitfalls to Avoid
- Delayed initiation: Starting aspirin after 16 weeks reduces its effectiveness
- Inadequate risk assessment: Failing to identify women with multiple moderate risk factors
- Inconsistent dosing: Intermittent use reduces effectiveness
- Premature discontinuation: Stopping before delivery eliminates protective benefits
- Underdosing in obesity: Standard 81 mg dose may be insufficient for obese women
While some recent research suggests that initiating aspirin even earlier than 12 weeks might be beneficial 6, current guidelines from major medical organizations consistently recommend starting between 12-16 weeks of gestation for optimal benefit with established safety.