At what gestational age is a fetus generally considered viable?

Period of Viability

The periviable period is defined as 20 0/7 weeks to 25 6/7 weeks of gestation, with meaningful survival beginning around 23 weeks (23-27% survival), though viability represents a physiological continuum rather than a fixed gestational age cutoff. 1

Defining the Periviable Period

The consensus definition from a multidisciplinary joint workshop involving the Society for Maternal-Fetal Medicine, NICHD, American Academy of Pediatrics, and ACOG establishes the periviable period as 20 0/7 weeks through 25 6/7 weeks of gestation. 1 This represents the window where outcomes range from near-certain death to likely survival with high risk of serious morbidities. 1

Viability is not based on gestational age alone but represents a physiological continuum influenced by multiple clinical factors including estimated fetal weight, plurality, genetic diseases, fetal anomalies, and availability of neonatal intensive care. 1

Survival Rates by Gestational Age

The data demonstrate a clear gestational age-dependent gradient in survival:

• Before 23 weeks: 5-6% survival with 98-100% serious morbidity among rare survivors 1, 2
• 23 weeks: 23-27% survival to discharge 1, 2
• 24 weeks: 42-59% survival to discharge 1, 2
• 25 weeks: 67-76% survival to discharge 1, 2

Neurodevelopmental Outcomes

Among survivors, the burden of moderate-to-severe neurodevelopmental impairment decreases progressively with advancing gestational age:

• 22 weeks: 43% moderate-to-severe impairment 2
• 23 weeks: 40% moderate-to-severe impairment 2
• 24 weeks: 28% moderate-to-severe impairment 2
• 25 weeks: 24% moderate-to-severe impairment 2

Long-term follow-up studies show that impairment (defined as cerebral palsy, blindness, profound hearing loss, or developmental quotient ≥2 standard deviations below mean) occurs in 45% at 22-23 weeks, 30% at 24 weeks, and 17% at 25 weeks. 1

Factors Modifying Viability

Beyond gestational age, several factors significantly impact survival and should inform counseling:

• Institutional practices: Wide variation exists in resuscitation practices at 22-23 weeks, which explains between-hospital differences in survival and survival without impairment 1, 2
• Antenatal corticosteroids: Administration improves survival 2
• Birth weight and fetal growth: Appropriately grown infants have better outcomes 2
• Multiple gestations: Affects survival rates 2
• Location of delivery: Tertiary care centers with neonatal intensive care capabilities achieve superior outcomes 2, 3

Historical Context and Evolution

From the 1950s through 1980, newborn death was virtually assured at or before 24 weeks of gestation. 1, 2 Over the past three decades, there has been progressive improvement in survival rates at 22-25 weeks, reflecting advances in perinatal and neonatal care. 1, 2

Clinical Application

The periviable period requires nuanced decision-making that accounts for:

• Outcomes at the extremes of each gestational week may be closer to those of the adjacent week 2
• Practices and outcomes become more consistent across tertiary care institutions at more advanced gestational ages (≥24 weeks) 1
• At 22-23 weeks specifically, institutional variation in resuscitation practices substantially influences outcomes 1, 2

Counseling should include accurate, individualized information regarding both short-term survival and long-term neurodevelopmental outcomes, incorporating the specific clinical factors present in each case. 2, 3