Low-Dose Aspirin for Preeclampsia Prevention in Women with Previous Pregnancy Hypertension
Despite guideline recommendations, the evidence shows that standard 81 mg aspirin does NOT effectively prevent superimposed preeclampsia in women with chronic hypertension, though it remains recommended for those with a history of preeclampsia in a previous pregnancy. 1
Critical Distinction: History of Preeclampsia vs. Chronic Hypertension
The benefit of aspirin depends critically on which type of "previous high blood pressure in pregnancy" you're addressing:
For History of Preeclampsia (Previous Pregnancy)
- Aspirin 81-100 mg daily is strongly recommended starting at 12-16 weeks gestation (ideally before 16 weeks) and continuing until delivery 2, 3
- This reduces preeclampsia risk by 24%, preterm birth by 14%, and intrauterine growth restriction by 20% 1, 4
- The U.S. Preventive Services Task Force and ACOG both endorse this as a Grade B recommendation for women with prior preeclampsia 1, 3
- Women with history of early-onset preeclampsia requiring delivery before 34 weeks have particularly strong indication 3
For Chronic Hypertension (Pre-existing Hypertension)
- Standard 81 mg aspirin does NOT reduce superimposed preeclampsia rates despite ACOG's 2016 recommendation 1
- A 2020 study of 457 women with chronic hypertension showed no difference in superimposed preeclampsia rates: 34.3% without aspirin vs. 35.5% with 81 mg aspirin (p=0.79) 1
- Paradoxically, severe features of preeclampsia actually increased in the aspirin group (21.7% vs 31.0%, p=0.03) 1
- No reduction in small-for-gestational-age infants or preterm birth was observed 1, 5
Practical Algorithm for Your Patient
Step 1: Identify the specific risk factor
- History of preeclampsia in prior pregnancy → Prescribe aspirin 81-100 mg daily 2, 3
- Chronic hypertension alone → Consider higher doses (150-162 mg) or acknowledge limited benefit with standard dosing 1
- Both chronic hypertension AND history of preeclampsia → Definitely prescribe aspirin, consider higher doses 6
Step 2: Timing
- Initiate between 12-16 weeks gestation, ideally before 16 weeks 2, 3, 4
- Continue daily until delivery 2
Step 3: Dosing considerations
- Standard dose: 81 mg daily for most high-risk women 1, 3
- Consider higher doses (100-150 mg daily) for: women with diabetes, BMI >40, or chronic hypertension where standard dosing has failed 1, 2
- International guidelines (FIGO) suggest 150-162 mg may be more effective, though this requires further study 1
Evidence Quality and Nuances
The evidence reveals a critical gap: while aspirin clearly benefits women with prior preeclampsia 4, the 2020 retrospective cohort study specifically evaluating the ACOG guideline for chronic hypertension found no benefit from 81 mg aspirin 1. This represents the most recent and directly relevant evidence for chronic hypertension populations.
A 2024 study confirmed these findings, showing no reduction in superimposed preeclampsia among 494 women with chronic hypertension prescribed low-dose aspirin, though interestingly found reduced adverse neonatal outcomes (4.0% vs 13%, p=0.002) 5.
Common Pitfalls to Avoid
- Don't assume all "high blood pressure in pregnancy" is the same: Prior preeclampsia and chronic hypertension have different aspirin responses 1, 6
- Don't start too late: Aspirin initiated after 16 weeks may be less effective, as defective placentation occurs in the first trimester 2
- Don't underdose in chronic hypertension: If prescribing for chronic hypertension, acknowledge that 81 mg may be insufficient and discuss higher doses 1
- Don't forget additional high-risk factors: Multifetal gestation, renal disease, autoimmune disease, and diabetes all warrant aspirin regardless of hypertension history 2, 3
Safety Profile
Low-dose aspirin does not increase risks of placental abruption, postpartum hemorrhage, fetal intracranial bleeding, perinatal mortality, or congenital anomalies 1, 2, 4. This excellent safety profile supports its use even when efficacy data are mixed.