What is the age of viability in pregnancy?

Age of Viability in Pregnancy

The age of viability in pregnancy is generally considered to be around 24 weeks of gestation, with survival rates increasing significantly after this point. 1

Definition and Classification

Viability represents a physiological continuum impacted by multiple factors, with gestational age being the primary determinant. According to current guidelines:

• Previable period: Before 23 weeks of gestation, when a fetus would not survive outside the uterus (survival rates 5-6%) 1
• Periviable period: 20 0/7 to 25 6/7 weeks of gestation, representing the transition zone where survival becomes possible but with high morbidity risk 1
• Viable period: Generally considered to begin around 24 weeks, when survival becomes more likely 1

Survival Rates by Gestational Age

Survival rates to hospital discharge increase dramatically with each additional week of gestation during the periviable period:

• Before 23 weeks: 5-6% survival with nearly universal serious morbidity (98-100%) 1
• 23 weeks: 23-27% survival 1
• 24 weeks: 42-59% survival 1
• 25 weeks: 67-76% survival 1

Factors Affecting Viability

While gestational age is the primary determinant, several other factors influence viability:

• Estimated fetal weight: Lower weights correlate with poorer outcomes
• Multiple gestations: Generally associated with lower survival rates
• Fetal anomalies or genetic diseases: May significantly impact viability
• Antenatal steroid administration: Improves outcomes
• Level of neonatal care available: Tertiary care centers have better outcomes 1, 2

Long-Term Outcomes

Neurodevelopmental outcomes also improve with increasing gestational age:

• 22-23 weeks: 45% with moderate to severe impairment at 30 months 1
• 24 weeks: 30% with moderate to severe impairment at 30 months 1
• 25 weeks: 17% with moderate to severe impairment at 30 months 1

Regional and Temporal Variations

The definition of viability varies globally based on available medical resources:

• The World Health Organization sets the lower limit of viability at 22 weeks of gestation or 500g birth weight 3
• In countries with limited neonatal intensive care, viability may not be considered until 28 weeks 3
• The threshold of viability has been gradually decreasing over time with medical advances 4

Clinical Practice Considerations

Most obstetricians in the United States consider 24 weeks to be the earliest age of potential viability, which aligns with the gestational age at which they would routinely perform cesarean section for fetal distress 5.

Common Pitfalls to Avoid

1. Assuming a fixed threshold: Viability exists on a continuum and varies by individual case and available resources
2. Focusing solely on gestational age: Multiple factors beyond gestational age affect viability
3. Overlooking the high morbidity rates: Survival does not equate to absence of serious complications
4. Failing to consider regional differences: Viability thresholds vary based on available medical technology and expertise

The concept of viability continues to evolve with advances in neonatal care, with the threshold gradually shifting to earlier gestational ages over time. However, 24 weeks remains a clinically significant milestone where survival rates substantially improve.