Aspirin Use in Pregnancy
Pregnant women at high risk for preeclampsia should receive low-dose aspirin 81-150 mg daily, initiated between 12-16 weeks of gestation (optimally before 16 weeks) and continued until delivery. 1, 2
High-Risk Indications Requiring Aspirin Prophylaxis
Women with any one of the following high-risk factors should receive low-dose aspirin 1, 2, 3:
- History of preeclampsia, especially early-onset preeclampsia with delivery <34 weeks 1, 4
- Chronic hypertension 1, 2
- Type 1 or type 2 diabetes 1, 2
- Renal disease 1, 2
- Autoimmune disease (including SLE or antiphospholipid syndrome) 1, 2
- Multifetal gestation (twins or higher-order multiples) 1, 2
Women with multiple moderate-risk factors should also be considered for aspirin prophylaxis 2, 3:
- Nulliparity (first pregnancy) 1, 2
- Maternal age ≥35 years 1, 2
- BMI >30 kg/m² 1, 2
- Family history of preeclampsia 1, 2
- Low socioeconomic status 1
- Interpregnancy interval ≥10 years 1
Dosing Recommendations
Standard dosing: 81 mg daily is the most commonly recommended dose in the United States 1, 2, 3
Higher dosing (100-150 mg daily) should be strongly considered for: 1
- Women with diabetes (type 1 or 2) 1
- Women with chronic hypertension (as standard 81 mg has shown no benefit in this population) 1
- Women with BMI >40 kg/m² (higher BMI reduces aspirin effectiveness) 1
- Women with multifetal gestation 1
The evidence increasingly supports that doses >100 mg initiated before 16 weeks may be more effective than the standard 81 mg dose, with some international guidelines recommending 150 mg daily 1, 5. A critical study of 457 women with chronic hypertension showed that 81 mg aspirin provided no benefit, with superimposed preeclampsia rates of 34.3% without aspirin versus 35.5% with aspirin (p=0.79), and severe features actually increased in the aspirin group (21.7% vs 31.0%, p=0.03) 1.
Timing of Initiation and Duration
Initiate aspirin between 12-16 weeks of gestation for maximum effectiveness 1, 2, 3. Evidence suggests that starting before 16 weeks is more effective than later initiation, as defective placentation and inadequate spiral artery remodeling occur in the first trimester 1, 5.
Continue aspirin daily until delivery 1, 2, 3. Do not stop at 36 weeks "just to be safe"—this removes protection during a high-risk period without evidence-based rationale, as preeclampsia risk persists throughout pregnancy and even into the early postpartum period 1.
Exception for planned cesarean section: Consider discontinuing aspirin 7-10 days before a scheduled cesarean with spinal anesthesia to allow adequate platelet recovery, and verify platelet count >75 × 10⁹/L immediately before spinal anesthesia 6.
Benefits of Low-Dose Aspirin
Low-dose aspirin provides substantial benefits in high-risk women 7, 4:
- 24% reduction in preeclampsia risk 7, 4
- 14% reduction in preterm birth 7, 4
- 20% reduction in intrauterine growth restriction (IUGR) 7, 4
- Increased mean birthweight by approximately 130g 4
- Number needed to treat: 42 to prevent one case of preeclampsia 4
Safety Profile
Low-dose aspirin does NOT increase the risk of: 7, 1, 4
- Placental abruption 7, 1, 4
- Postpartum hemorrhage 7, 1, 4
- Fetal intracranial bleeding 7, 1, 4
- Perinatal mortality 7, 1, 4
- Congenital anomalies 1
The FDA warning about aspirin in the third trimester refers to high doses (>100 mg), not prophylactic low doses of 75-150 mg 1, 8.
Common Pitfalls to Avoid
- Do not use aspirin for low-risk women without risk factors—current evidence does not support prophylactic low-dose aspirin for prevention of early pregnancy loss, fetal growth restriction, stillbirth, or preterm birth in the absence of high-risk factors 2, 3
- Do not use standard 81 mg dosing in women with chronic hypertension—consider higher doses (100-150 mg) as standard dosing has proven ineffective in this population 1
- Do not start aspirin after 16 weeks if avoidable—efficacy decreases with later initiation, though the window extends up to 28 weeks 1, 2
- Do not stop aspirin at 36 weeks without specific indication—continue until delivery in most cases 1
- Do not overlook risk factors—use systematic screening checklists to ensure all 15 recognized risk factors are assessed 9