What is the absolute risk reduction of preeclampsia with low-dose Aspirin (acetylsalicylic acid) in a healthy 30-year-old woman in her first pregnancy?

Absolute Risk Reduction of Preeclampsia with Low-Dose Aspirin in Healthy First Pregnancy

The absolute risk reduction of preeclampsia with low-dose aspirin in a healthy 30-year-old woman in her first pregnancy is approximately 1.7%, based on a reduction from about 6.3% to 4.6% risk. 1

Risk Assessment and Benefit

• Low-dose aspirin reduces the risk of preeclampsia by 24% in women at increased risk (relative risk reduction), with an absolute risk reduction ranging from 2% to 5% 2
• The number needed to treat (NNT) to prevent one case of preeclampsia is 42 in women at increased risk 2
• For a healthy nulliparous woman (first pregnancy), the NICHD Network of Maternal-Fetal Medicine Units trial showed a reduction from 6.3% to 4.6% incidence of preeclampsia with 60mg daily aspirin 1
• The benefit appears most significant in women with initial systolic blood pressure between 120-134 mmHg, where the reduction was from 11.9% to 5.6% 1

Dosage and Timing Considerations

• The most commonly studied dosages are 60 mg/day and 100 mg/day, with 81 mg/day being a reasonable dosage in the United States 2
• Low-dose aspirin should be initiated between 12 and 16 weeks of gestation for optimal effect 2, 3
• There is no evidence of additional benefit when starting aspirin earlier than 16 weeks compared to later initiation 2
• Some evidence suggests aspirin should be continued until delivery, though practices vary with about 52% of physicians continuing it until delivery 3
• Recent research indicates that aspirin can be safely discontinued at 24-28 weeks in women with normal biomarkers (sFlt-1:PlGF ratio ≤38) without increasing preeclampsia risk 4

Safety Profile

• Low-dose aspirin does not increase the risk of:
  • Placental abruption (pooled analysis) 2
  • Postpartum hemorrhage 2
  • Fetal intracranial bleeding 2
  • Perinatal mortality 2
• However, one study noted an increased risk of abruptio placentae in the aspirin group (11 women vs. 2 in placebo) 1
• No significant differences were found in cesarean delivery rates between aspirin and placebo groups 2

Considerations for Healthy Nulliparous Women

• While the USPSTF recommendations focus on women at high risk for preeclampsia, the NICHD trial specifically studied healthy nulliparous women 1
• For women with moderate risk factors (including first pregnancy), the evidence for aspirin benefit is less certain but may still be beneficial 2
• Early administration of aspirin (≤16 weeks) appears to be more effective at reducing severe preeclampsia (78% reduction) than mild preeclampsia (no significant reduction) 5

Additional Benefits

• Low-dose aspirin also provides:
  • 14% reduction in risk of preterm birth 2
  • 20% reduction in risk of intrauterine growth restriction (IUGR) 2
  • Increased mean birthweight by approximately 130g 2

Clinical Application

• For a healthy 30-year-old woman in her first pregnancy without other risk factors, the absolute risk reduction is modest but may be clinically significant
• The decision to use aspirin should consider the woman's baseline blood pressure, as those with systolic BP 120-134 mmHg appear to benefit most 1
• The low risk of adverse effects and potential benefits beyond preeclampsia prevention (reduced preterm birth and IUGR) may justify consideration even in women without high-risk factors 2

Common Pitfalls and Caveats

• Failing to start aspirin early enough (before 16 weeks) may reduce effectiveness 5
• Discontinuing aspirin too early (before 36 weeks) without biomarker assessment may reduce protective effects 3
• Not considering moderate risk factors in combination when assessing preeclampsia risk 3
• Overlooking the potential benefit in women with higher-normal blood pressure readings 1