When to Start Aspirin in Pregnancy
Aspirin should be started between 12-16 weeks of gestation (optimally before 16 weeks) in high-risk pregnant women and continued daily until delivery. 1, 2, 3
Optimal Timing Window
- Initiate aspirin between 12-16 weeks of gestation for maximum effectiveness in preventing preeclampsia and related complications 1, 2, 3
- Starting before 16 weeks is critical because defective placentation and inadequate spiral artery remodeling occur in the first trimester, and early aspirin initiation improves uteroplacental blood flow during this critical period 1
- Evidence shows that doses >100 mg initiated before 16 weeks may be more effective than later initiation 1, 4
- The window extends up to 28 weeks of gestation, but efficacy decreases with later initiation 5, 2, 3
High-Risk Indications Requiring Aspirin
One or more of these high-risk factors warrants aspirin prophylaxis: 2, 3, 6
- History of preeclampsia (especially early-onset with delivery <34 weeks) 1, 2, 3
- Multifetal gestation 2, 3
- Chronic hypertension 1, 2, 3
- Type 1 or type 2 diabetes 1, 2, 3
- Renal disease 2, 3
- Autoimmune disease 2, 3
Multiple moderate-risk factors also warrant aspirin: 2, 3
- First pregnancy (nulliparity) 1
- Maternal age ≥35 years 1, 2, 3
- BMI >30 kg/m² 1, 2, 3
- Family history of preeclampsia 1, 2, 3
- Interpregnancy interval ≥10 years 1
Dosing Recommendations
- Standard dose: 81 mg daily for most high-risk women in the United States 5, 2, 3, 6
- Higher dose: 100-150 mg daily for women with diabetes, as diabetes is an independent risk factor requiring more intensive prophylaxis 1
- Consider 150 mg daily for women with chronic hypertension, BMI >40, or when standard 81 mg dosing has failed, as international guidelines suggest higher doses may be more effective 1, 4
- The European guidelines (EASL) recommend 150 mg at night for high-risk women identified through first-trimester screening 5
Important Caveat on Dosing
While ACOG recommends 81 mg daily, emerging evidence suggests that doses >100 mg initiated before 16 weeks are more effective at reducing preeclampsia risk 1, 4. The standard 81 mg dose has shown limited effectiveness in certain populations, particularly women with chronic hypertension, where studies found no significant reduction in superimposed preeclampsia (34.3% without aspirin vs 35.5% with aspirin, p=0.79) 1.
Duration of Therapy
- Continue aspirin daily until delivery in most cases 1, 2, 3
- For planned cesarean section with spinal anesthesia, aspirin should be discontinued 7-10 days before the scheduled procedure 7
- Aspirin can be discontinued 1-2 weeks before expected labor when neuraxial anesthesia is planned 7
Safety Profile
- Low-dose aspirin does not increase risks of placental abruption, postpartum hemorrhage, fetal intracranial bleeding, perinatal mortality, or congenital anomalies 1, 8, 2, 3
- Daily low-dose aspirin use in pregnancy is considered safe with a low likelihood of serious maternal or fetal complications 2, 3
Expected Benefits
When started at the optimal time (12-16 weeks), aspirin provides: 1, 8