What is the baseline risk for stillbirth at 40 weeks?

Baseline Risk of Stillbirth at 40 Weeks Gestation

The baseline risk of stillbirth at 40 weeks gestation is approximately 1 in 1,449 ongoing pregnancies (0.69 per 1,000).

Understanding Stillbirth Risk at Term

The risk of stillbirth increases progressively with advancing gestational age at term. According to high-quality evidence from a systematic review and meta-analysis of 15 million pregnancies:

• At 37 weeks: 0.11 per 1,000 pregnancies (95% CI 0.07 to 0.15)
• At 40 weeks: 0.69 per 1,000 pregnancies (baseline)
• At 41 weeks: One additional stillbirth occurs for every 1,449 pregnancies that continue from 40 to 41 weeks 1
• At 42 weeks: 3.18 per 1,000 pregnancies (95% CI 1.84 to 4.35) 1

Risk Factors That Modify Baseline Risk

Several factors can significantly increase the baseline risk of stillbirth at 40 weeks:

Maternal Age

• Women 35-39 years: 1.32 times higher risk (95% CI 1.22-1.43) compared to women under 35 2
• Women ≥40 years: 1.88 times higher risk (95% CI 1.64-2.16) compared to women under 35 2
• For women ≥40 years, the risk beyond 40 weeks is 1 in 455 ongoing pregnancies 3
• For nulliparous women ≥40 years, the risk increases further to 1 in 247 ongoing pregnancies 3

Other Risk Factors

• Smoking: HR 1.82 (95% CI 1.56-2.12) 3
• Nulliparity: HR 1.23 (95% CI 1.08-1.40) 3
• Pre-existing hypertension: HR 2.77 (95% CI 1.94-3.97) 3
• Pre-existing diabetes: HR 2.65 (95% CI 1.63-4.32) 3
• Extreme birth weight percentiles:
  • <1st percentile: OR 9.63 (95% CI 8.39-11.06) 4
  • ≥99th percentile: OR 2.24 (95% CI 1.76-2.86) 4

Clinical Implications

The increasing risk of stillbirth with advancing gestational age has important clinical implications:

1. Risk Assessment: All pregnant women should have individualized risk assessment for stillbirth at term, with special attention to maternal age and other risk factors.

2. Timing of Delivery: For women with risk factors, particularly those ≥40 years of age and nulliparous, consideration should be given to delivery by 40 weeks gestation 3.

3. Risk Communication: Patients should be counseled about the increasing risk of stillbirth with advancing gestational age beyond 40 weeks to facilitate informed decision-making regarding timing of delivery.

Pitfalls and Caveats

• Risk calculations are based primarily on studies from high-income countries and may not be generalizable to all populations 1.
• The absolute risk of stillbirth at 40 weeks remains relatively low for most women without additional risk factors.
• When counseling patients about stillbirth risk, it's important to balance this against other considerations such as the risks associated with induction of labor.
• Registry-based data may have limitations in capturing all confounding variables that could affect stillbirth risk.

Understanding the baseline risk and how it changes with gestational age and maternal characteristics allows for more informed decision-making regarding the optimal timing of delivery to minimize the risk of stillbirth.