When to Start Low-Dose Aspirin for Preeclampsia Prevention
Low-dose aspirin should be initiated between 12 and 16 weeks of gestation in women at high risk for preeclampsia, with optimal timing being before 16 weeks' gestation. 1, 2, 3
Timing of Initiation
Start between 12-16 weeks of gestation for women with high-risk factors, as recommended by the American College of Obstetricians and Gynecologists (ACOG) and endorsed by the Society for Maternal-Fetal Medicine 2, 3
Optimal window is before 16 weeks of gestation, as evidence suggests earlier initiation (before 16 weeks) may be more effective than later initiation, though the U.S. Preventive Services Task Force found no statistically significant difference in their analysis 4
The acceptable range extends up to 28 weeks of gestation, but this represents the outer limit rather than the preferred timing 2, 3
High-Risk Factors Requiring Aspirin Prophylaxis
Women with any one of the following high-risk factors should receive aspirin:
- History of preeclampsia, particularly early-onset preeclampsia with delivery before 34 weeks 1, 2, 3
- Multifetal gestation 1, 2, 3
- Chronic hypertension 1, 2, 3
- Type 1 or type 2 diabetes mellitus 1, 2, 3
- Renal disease 1, 2, 3
- Autoimmune disease (such as systemic lupus erythematosus or antiphospholipid syndrome) 1, 2, 3
Moderate-Risk Factors
Women with more than one of the following moderate-risk factors should be considered for aspirin prophylaxis:
- First pregnancy (nulliparity) 2, 3
- Maternal age ≥35 years 2, 3
- Body mass index >30 kg/m² 2, 3
- Family history of preeclampsia 2, 3
- Adverse sociodemographic characteristics 2, 3
Dosing Recommendations
Standard dose: 81 mg daily as recommended by ACOG for most high-risk women in the United States 1, 2, 3
Higher doses (100-150 mg daily) are recommended by the American Diabetes Association for women with diabetes 1
Evidence suggests doses >100 mg initiated before 16 weeks may be more effective, with some international guidelines recommending 150 mg daily 4, 5
Important caveat: The standard 81 mg dose may be less effective in certain populations, particularly women with chronic hypertension, where studies have shown no significant reduction in superimposed preeclampsia 4
Evidence Supporting Early Initiation
The rationale for starting before 16 weeks relates to the pathophysiology of preeclampsia:
- Defective placentation and inadequate spiral artery remodeling occur in the first trimester 4
- Aspirin initiated early can improve uteroplacental blood flow during the critical period of placentation 6
- Systematic reviews suggest aspirin is more effective when started at 16 weeks compared to later initiation 4
Safety Profile
- Low-dose aspirin does not increase risks of placental abruption, postpartum hemorrhage, fetal intracranial bleeding, or congenital anomalies 4, 7
- Daily use throughout pregnancy until delivery is considered safe 2, 3
Clinical Pitfalls to Avoid
- Do not delay initiation beyond 16 weeks in high-risk women, as effectiveness may be reduced 4, 5
- Do not use 81 mg as a universal dose for all populations; women with diabetes should receive 100-150 mg daily 1
- Do not assume 81 mg is adequate for women with chronic hypertension, as this population may require higher doses or alternative strategies 4
- Do not forget to continue until delivery; stopping early may reduce effectiveness 2, 3