When to Start Low-Dose Aspirin for Preeclampsia Prevention During Pregnancy
Low-dose aspirin (81 mg/day) should be initiated between 12-16 weeks of gestation and continued until delivery in women at high risk for preeclampsia. 1, 2, 3
Risk Assessment for Preeclampsia
High-Risk Factors (any one of these warrants aspirin prophylaxis):
- History of preeclampsia, especially early-onset preeclampsia with delivery <34 weeks 4, 1
- Multifetal gestation 1, 2
- Chronic hypertension 1, 2
- Type 1 or type 2 diabetes 1, 2
- Renal disease 1, 2
- Autoimmune disease (such as systemic lupus erythematosus or antiphospholipid syndrome) 1, 2
Moderate-Risk Factors (consider aspirin if more than one present):
- First pregnancy (nulliparity) 4, 2
- Maternal age ≥35 years 4, 2
- BMI >30 kg/m² 4, 2
- Family history of preeclampsia (mother or sister) 4, 2
- Sociodemographic characteristics (low socioeconomic status) 2, 3
Dosage Recommendations
- Standard dose: 81 mg/day for most high-risk women 4, 2
- For women with diabetes: Consider higher dose (100-150 mg/day) 1, 5
- For women with higher BMI (>40 kg/m²): May require dose adjustment 1
Timing of Initiation
- Optimal timing: Between 12-16 weeks of gestation 4, 2
- Acceptable range: Between 12-28 weeks of gestation 4, 3
- Never before 12 weeks (no trials have evaluated earlier initiation) 4
- Evidence does not show additional benefit of starting before 16 weeks compared to after 16 weeks 4
Duration of Treatment
Special Considerations
Effectiveness
- Number needed to treat to prevent one case of preeclampsia: 42 4
- Number needed to treat to prevent one case of intrauterine growth restriction: 71 4
- Number needed to treat to prevent one case of preterm birth: 65 4
Safety Profile
- Low-dose aspirin has not been shown to increase risk of:
Emerging Evidence and Controversies
- Some recent evidence suggests that doses >100 mg may be more effective than the standard 81 mg dose, particularly for women with diabetes or higher BMI 1, 5
- The International Federation of Gynecology and Obstetrics recommends 150 mg of aspirin (or 2 tablets of 81 mg as an acceptable alternative) 5
- Some researchers have proposed starting aspirin earlier than 12 weeks, but this approach lacks sufficient evidence from clinical trials 6
- Evening or bedtime administration may provide better outcomes than morning dosing 7
Common Pitfalls to Avoid
- Delaying initiation beyond 16 weeks in high-risk women 2, 3
- Failing to identify high-risk women who would benefit from prophylaxis 4, 2
- Discontinuing aspirin before delivery 2, 3
- Using prophylactic aspirin in low-risk women without risk factors 2, 3
- Not adjusting dosage for women with diabetes or high BMI 1, 5