When should a pregnant patient at high risk of preeclampsia be started on aspirin (acetylsalicylic acid) and for how long?

Aspirin for Preeclampsia Prevention in Pregnant Patients

Pregnant patients at high risk for preeclampsia should be started on low-dose aspirin at 12-16 weeks of gestation (optimally before 16 weeks) and continued daily until delivery at 36-37 weeks. 1, 2

Risk Assessment for Preeclampsia

High-Risk Factors (single factor sufficient for aspirin therapy):

• History of preeclampsia
• Multifetal gestation
• Chronic hypertension
• Type 1 or type 2 diabetes
• Renal disease
• Autoimmune disease (including SLE)

Moderate-Risk Factors (more than one needed for aspirin therapy):

• First pregnancy (nulliparity)
• Maternal age ≥35 years
• BMI >30 kg/m²
• Family history of preeclampsia
• Sociodemographic risk factors

Dosing Recommendations

The most recent guidelines from the American Diabetes Association (2025) recommend:

• 100-150 mg/day of aspirin 1
• In the US, where 81 mg tablets are standard, two tablets (162 mg) may be acceptable 1

This represents an important update from previous recommendations, as meta-analyses have shown that aspirin doses <100 mg are not effective in reducing preeclampsia 1, 3.

Different organizations recommend varying doses:

• ACOG and USPSTF: 81 mg/day
• WHO: 75 mg/day
• RCOG and European guidelines: 150 mg/day 2

Timing of Initiation and Discontinuation

• Start: Between 12-16 weeks of gestation, optimally before 16 weeks 1, 2
• Stop: At 36-37 weeks of gestation 2

Starting aspirin before 16 weeks is critical because this is when placentation occurs. Aspirin works by inhibiting thromboxane A2, promoting prostacyclin, and improving placental perfusion during this critical period 2, 4.

Special Considerations

• For patients with diabetes (type 1 or 2): Low-dose aspirin (100-150 mg/day) is specifically recommended starting at 12-16 weeks 1
• For patients with Systemic Lupus Erythematosus (SLE): Low-dose aspirin (81-100 mg daily) is strongly recommended during pregnancy 2
• For patients with Antiphospholipid Syndrome (APS): Low-dose aspirin should be combined with prophylactic-dose heparin/LMWH 2

Safety Profile

Low-dose aspirin in pregnancy has been shown to have no increased risk of:

• Placental abruption
• Postpartum hemorrhage
• Fetal intracranial bleeding
• Congenital anomalies 2

However, the FDA label notes that aspirin should be used with caution during the last 3 months of pregnancy unless definitely directed by a doctor, as it may cause problems in the unborn child or complications during delivery 5.

Clinical Efficacy

Low-dose aspirin reduces:

• Risk of preeclampsia by approximately 24% in high-risk women
• Risk of preterm birth by 14%
• Intrauterine growth restriction risk by 20% 2

A cost-benefit analysis has concluded that this approach reduces morbidity, saves lives, and lowers healthcare costs 1.

Common Pitfalls to Avoid

1. Starting aspirin too late (after 16 weeks) reduces its effectiveness
2. Using insufficient dosage (<100 mg)
3. Discontinuing too early (before 36 weeks)
4. Not identifying all relevant risk factors when assessing patients
5. Poor patient education about medication adherence 2

For optimal effectiveness in preventing preeclampsia and its complications, proper timing, appropriate dosing, and continued use until 36-37 weeks are essential components of this preventive strategy.