Why is aspirin (acetylsalicylic acid) recommended during pregnancy?

Low-Dose Aspirin in Pregnancy: Prevention of Preeclampsia

Low-dose aspirin (81 mg/day) is recommended during pregnancy specifically for the prevention of preeclampsia in high-risk women, and should be initiated between 12-16 weeks of gestation and continued until delivery. 1

Mechanism and Benefits

Aspirin works through several mechanisms to prevent preeclampsia:

• Inhibits thromboxane A2 while promoting prostacyclin
• Improves placental perfusion during critical placentation
• Provides anti-inflammatory, antiangiogenesis, and antiplatelet properties 2, 1

These effects help counter the placental ischemia, inflammation, and oxidative stress that contribute to preeclampsia development.

Low-dose aspirin has been shown to:

• Reduce preeclampsia risk by approximately 24% in high-risk women
• Decrease preterm birth risk by 14%
• Lower intrauterine growth restriction risk by 20% 1

Who Should Receive Aspirin Prophylaxis

High-Risk Factors (any one factor qualifies):

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

Moderate-Risk Factors (more than one factor qualifies):

• First pregnancy (nulliparity)
• Maternal age ≥35 years
• BMI >30 kg/m²
• Family history of preeclampsia 1, 3

Dosing and Timing

The optimal regimen for aspirin prophylaxis:

• Dose: 81 mg/day (USA recommendation) 1, 3
• Initiation: Between 12-16 weeks of gestation (optimal efficacy when started before 16 weeks) 1
• Duration: Continue daily until delivery 1
• Discontinuation: Should be discontinued at 36 weeks of gestation according to some guidelines, while others recommend continuation until delivery 1

Important Considerations and Cautions

1. Timing is critical: Starting aspirin after 16 weeks significantly reduces its effectiveness 1

2. FDA warning: The FDA label cautions about using aspirin during the last 3 months of pregnancy unless specifically directed by a doctor, as it may cause problems in the unborn child or complications during delivery 4

3. Surgical considerations: Aspirin should be discontinued approximately one week before planned surgical procedures, including cesarean section 1

4. 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 1

Special Populations

• Systemic Lupus Erythematosus (SLE): Low-dose aspirin (81-100 mg daily) is strongly recommended for all SLE patients during pregnancy 1

• Antiphospholipid Syndrome (APS): Low-dose aspirin should be combined with prophylactic-dose heparin/LMWH 1

Emerging Evidence

Recent research suggests that higher doses (150-162 mg) may be more effective, particularly in obese women. A 2025 study showed a 78% probability of benefit with 162 mg versus 81 mg in reducing preeclampsia with severe features in high-risk obese individuals 5.

Common Pitfalls to Avoid

1. Starting aspirin too late (after 16 weeks), which significantly reduces effectiveness
2. Discontinuing too early (before delivery in some guidelines)
3. Not identifying all relevant risk factors when assessing patients
4. Using aspirin for prevention in low-risk women without indication 1

In summary, low-dose aspirin is a critical preventive intervention for preeclampsia in high-risk pregnant women, with clear benefits for maternal and fetal outcomes when properly prescribed and administered.