When should aspirin (acetylsalicylic acid) be started in pregnant women?

Indications to Start Aspirin During Pregnancy

Low-dose aspirin should be started between 12-16 weeks of gestation (optimally before 16 weeks) and continued until delivery in pregnant women at high risk for preeclampsia. 1, 2

High-Risk Populations Requiring Aspirin Prophylaxis

Women with any of these high-risk factors should receive low-dose aspirin:

• History of preeclampsia, especially early-onset preeclampsia with delivery <34 weeks 1
• History of preeclampsia in more than one previous pregnancy 1
• Multifetal gestation 1
• Chronic hypertension 1
• Type 1 or type 2 diabetes 1
• Renal disease 1
• Autoimmune disease (such as systemic lupus erythematosus or antiphospholipid syndrome) 1

Moderate-Risk Factors

Women with more than one of these moderate-risk factors should also receive low-dose aspirin:

• First pregnancy (nulliparity) 1
• Age ≥40 years 1
• Pregnancy interval >10 years 1
• BMI ≥35 kg/m² 1
• Family history of preeclampsia (mother or sister) 1
• Sociodemographic characteristics (low socioeconomic status) 2

Dosing Recommendations

• For most high-risk women: 81 mg/day (U.S. standard low-dose) 1, 2
• For women with diabetes: 100-150 mg/day 1
• In the U.S., 81 mg tablets are commonly available, but 162 mg/day may be acceptable for women with diabetes 1

Timing of Initiation and Duration

• Start: Between 12-16 weeks of gestation (optimally before 16 weeks) 1
• Continue: Until delivery 1

Evidence Quality and Considerations

The recommendation to start aspirin before 16 weeks is supported by strong evidence. Meta-analyses show that aspirin started at ≤16 weeks significantly reduces:

• Preeclampsia by 53% 3
• Intrauterine growth restriction by 56% 3
• Severe preeclampsia by 91% 3
• Preterm birth by 78% 3

In contrast, aspirin started after 16 weeks shows less significant benefits 3, 4.

Special Populations

Women with Diabetes

Pregnant women with diabetes (type 1 or 2) should receive a higher dose of aspirin (100-150 mg/day) starting at 12-16 weeks of gestation 1. This is particularly important as diabetes is an independent risk factor for preeclampsia.

Women with Chronic Hypertension

While all guidelines recommend aspirin for women with chronic hypertension, some studies suggest that the standard 81 mg dose may be less effective in this population 1. Factors such as obesity and altered pharmacokinetics may reduce aspirin's effectiveness in these women 1.

Important Caveats

• Discontinuing aspirin at 28 weeks does not provide the same protective benefits as continuing until delivery 5
• Preconception or very early (<11 weeks) initiation of aspirin has not shown significant reduction in preeclampsia risk, though it may reduce preterm delivery 6
• The FDA notes that aspirin should generally be avoided in the last trimester unless specifically directed by a doctor, highlighting the importance of medical supervision 7
• Higher BMI (>40 kg/m²) may reduce aspirin's effectiveness, possibly requiring dose adjustment 1

Algorithm for Aspirin Use in Pregnancy

1. Assess for high-risk factors at first prenatal visit
2. If any high-risk factor is present → Start aspirin at 12-16 weeks
3. If no high-risk factors, assess for moderate-risk factors
4. If more than one moderate-risk factor → Start aspirin at 12-16 weeks
5. For women with diabetes → Use higher dose (100-150 mg/day)
6. Continue aspirin until delivery
7. Monitor for any bleeding complications throughout pregnancy