What is the risk of stillbirth after 40 weeks of gestation?

Risk of Stillbirth After 40 Weeks of Gestation

The risk of stillbirth increases significantly after 40 weeks of gestation, with the prospective risk rising from 0.11 per 1,000 pregnancies at 37 weeks to 3.18 per 1,000 at 42 weeks. 1

Specific Stillbirth Risk by Gestational Age

• At 40 weeks: Baseline risk
• At 41 weeks: One additional stillbirth occurs for every 1,449 pregnancies that advance from 40 to 41 weeks 1
• At 42 weeks: The risk increases substantially, with a hazard risk doubling from 39 to 40 weeks (0.60 to 1.16 per 1,000 ongoing pregnancies) 2

Risk Factors That Increase Stillbirth Risk After 40 Weeks

Maternal Age

• Advanced maternal age (≥40 years) significantly increases stillbirth risk at term
• Women ≥40 years have a relative risk of 5.17 (95% CI 3.16-8.46) at 40 weeks compared to younger women 2
• 24.2% of stillbirths in women ≥40 years occur at term pregnancy 2

Specific Medical Conditions

• Intrahepatic cholestasis of pregnancy (ICP): Significantly increased risk of stillbirth, especially with bile acid levels ≥100 μmol/L 3
• Fetal growth restriction: Associated with higher risk of stillbirth in post-term pregnancies 3
• Hypertensive disorders: Increase risk of adverse perinatal outcomes including stillbirth 3

Monitoring and Management to Reduce Stillbirth Risk

Antenatal Testing After 40 Weeks

• Antenatal fetal surveillance is recommended for pregnancies that continue beyond 40 weeks 3
• The biophysical profile (BPP) or modified BPP are appropriate monitoring tools with high negative predictive values (>99.9%) 3
• No single antenatal test has been shown to be superior for identifying fetuses at risk for intrauterine demise 3

Timing of Delivery Recommendations

For uncomplicated pregnancies:

• Consider delivery between 40-41 weeks to balance risks
• Delivery by 41 weeks is associated with reduced stillbirth risk without increasing neonatal mortality 1

For high-risk pregnancies:

• ICP with bile acids ≥100 μmol/L: Delivery at 36 weeks 3
• ICP with bile acids <100 μmol/L: Delivery between 36-39 weeks 3
• Advanced maternal age (≥40 years): Consider delivery by 39 weeks 2
• Fetal growth restriction: Individualized timing based on severity and Doppler findings 3

Pitfalls and Caveats

1. False reassurance: A normal result on any test of fetal well-being is highly reassuring, but a stillbirth can still occur within 1 week of a normal test result 3

2. Limitations of testing: Antenatal fetal surveillance cannot predict stillbirth related to acute events such as placental abruption or cord accidents 3

3. Balance of risks: While earlier delivery reduces stillbirth risk, it must be balanced against potential neonatal complications from early term delivery

4. 39-week rule concerns: There is some evidence suggesting that strict adherence to the "39-week rule" (avoiding elective delivery before 39 weeks) may be associated with increased term stillbirth rates in the US 4

5. Recurrence risk: Women with a previous stillbirth have increased risk in subsequent pregnancies, particularly if the previous stillbirth occurred at very early preterm or post-term gestations 5

In summary, the risk of stillbirth increases progressively with each week beyond 40 weeks of gestation, with specific risk factors like advanced maternal age and certain medical conditions further elevating this risk. Appropriate fetal surveillance and timely delivery decisions are critical to reducing stillbirth risk in post-term pregnancies.