Period of Viability
The periviable period is defined as 20 0/7 weeks to 25 6/7 weeks of gestation, with viability representing a continuum rather than a fixed threshold. 1
Gestational Age-Specific Survival Rates
The likelihood of survival increases dramatically with each additional week of gestation during the periviable period:
Before 22 weeks: Fetuses are considered previable and would not survive outside the uterus; they are not candidates for life-sustaining interventions 1
22 weeks gestation: Survival rates are approximately 5-6%, with nearly universal significant morbidity (98-100%) among rare survivors 1, 2
23 weeks gestation: Survival to discharge ranges from 23-27%, with 40% of survivors experiencing moderate-to-severe neurodevelopmental impairment 1, 2
24 weeks gestation: Survival improves to 42-59%, with 28% of survivors having moderate-to-severe neurodevelopmental impairment 1, 2
25 weeks gestation: Survival reaches 67-76%, with 24% of survivors experiencing moderate-to-severe neurodevelopmental impairment 1, 2
Critical Factors Beyond Gestational Age
Viability is not determined by gestational age alone but represents a physiological continuum influenced by multiple clinical factors. 1
Additional determinants of viability include:
- Estimated fetal weight: Lower birth weights decrease survival probability 1
- Multiple gestations: Twins and higher-order multiples have different outcomes 1
- Fetal genetic diseases and anomalies: Congenital abnormalities significantly impact viability 1
- Location of delivery: Tertiary care centers with neonatal intensive care capabilities achieve better outcomes 1, 2
- Administration of antenatal corticosteroids: Improves survival rates 2
- Institutional practices regarding resuscitation: Variation in practices at 22-23 weeks explains differences in survival between hospitals 1, 2
Clinical Decision-Making Framework
For births before 23 0/7 weeks, neonatal death is virtually certain (94-95% mortality), and life-sustaining interventions are generally not appropriate. 1
At 23-24 weeks, outcomes vary greatly and require individualized counseling incorporating patient values, with consideration that most infants will either die or survive with significant morbidities. 1
At 25-26 weeks, active intervention including cesarean delivery when indicated and neonatal resuscitation are recommended, as most infants will survive and most survivors will not be severely disabled. 1
Important Caveats
- The definition of viability shifts according to available medical technology and varies between institutions, particularly at the earliest gestational ages 1
- Outcomes at the extremes of each gestational week may be closer to those of the adjacent week 2
- Wide variation exists in institutional practices regarding resuscitation at 22-23 weeks, which explains some between-hospital differences in survival 1, 2
- Historical context matters: from the 1950s through 1980, death was virtually assured at or before 24 weeks, but progressive improvements have occurred over the past three decades 1, 2