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

Risk of Stillbirth After 40 Weeks of Gestation

The risk of stillbirth significantly increases after 40 weeks of gestation, with one additional stillbirth occurring for every 1,449 pregnancies that advance from 40 to 41 weeks. 1

Specific Stillbirth Risks by Gestational Age

The prospective risk of stillbirth increases progressively with advancing gestational age:

• At 40 weeks: 1 in 475 ongoing pregnancies 2
• At 41 weeks: 1 in 1,177 ongoing pregnancies for women under 40 years 3
• At 42 weeks: 3.18 per 1,000 pregnancies (approximately 1 in 314) 1
• At 43 weeks: 1 in 375 ongoing pregnancies 2

The hazard risk doubles from 39 to 40 weeks, from 0.60 per 1,000 ongoing pregnancies to 1.16 per 1,000 ongoing pregnancies 4.

Risk Factors That Further Increase Stillbirth Risk

Several factors compound the risk of stillbirth in post-term pregnancies:

• Advanced maternal age: Women aged 40 or older have a significantly higher risk (RR 5.17,95% CI 3.16-8.46) at 40 weeks compared to younger women 4

  • For women ≥40 years: Risk increases to 1 in 455 ongoing pregnancies beyond 40 weeks 3
  • For nulliparous women ≥40 years: Risk further increases to 1 in 247 ongoing pregnancies 3

• Other significant risk factors 5, 3:

  • Fetal growth restriction
  • Hypertensive disorders
  • Intrahepatic cholestasis of pregnancy (especially with bile acid levels ≥100 μmol/L)
  • Smoking (HR 1.82,95% CI 1.56-2.12)
  • Nulliparity (HR 1.23,95% CI 1.08-1.40)
  • Pre-existing hypertension (HR 2.77,95% CI 1.94-3.97)
  • Pre-existing diabetes (HR 2.65,95% CI 1.63-4.32)

Monitoring and Management Implications

• Antenatal fetal surveillance is recommended for pregnancies that continue beyond 40 weeks 5
• Biophysical profile (BPP) or modified BPP are appropriate monitoring tools with high negative predictive values (>99.9%) 5
• No single antenatal test has been shown to be superior for identifying fetuses at risk for intrauterine demise 5

Important Caveats

1. A normal result on any test of fetal well-being is highly reassuring, but stillbirth can still occur within 1 week of a normal test result 5

2. Antenatal fetal surveillance cannot predict stillbirth related to acute events such as placental abruption or cord accidents 5

3. There is ongoing debate about the "39-week rule" (limiting elective deliveries before 39 weeks) and its potential impact on term stillbirth rates. Some research suggests this policy may be associated with increased rates of term stillbirth by shifting deliveries to later gestational ages 6

4. For women aged 40 or older, especially nulliparous women, induction of labor by 40 weeks gestation may be reasonable given the significantly elevated stillbirth risk 3

5. Unexplained stillbirths remain the most common classification for all women, though stillbirths classified as perinatal infection appear more common in women aged 40 or above 3