Medicolegal Viability Thresholds
The medicolegal threshold for fetal viability is generally established at 20 completed weeks of gestational age or a birthweight of 350 grams or more when gestational age is unknown. 1
Gestational Age Criteria
- Viability begins at 20 0/7 weeks of gestation for legal definitions of stillbirth versus miscarriage 1
- The periviable period extends from 20 0/7 to 25 6/7 weeks of gestation, representing the transition zone where outcomes range from near-certain death to likely survival with significant morbidity 1
- Delivery before 20 completed weeks is classified as miscarriage, while fetal death at or after 20 weeks constitutes stillbirth 1
Weight Criteria
- A birthweight of 350 grams or more can be used as an alternative viability threshold when gestational age is unknown 1
- This weight threshold serves as a practical medicolegal marker for distinguishing viable from non-viable births in cases where dating is uncertain 1
Clinical Viability Context
While the medicolegal threshold is set at 20 weeks, actual survival prospects differ substantially:
- Before 23 weeks: Survival is 5-6% with 98-100% serious morbidity among rare survivors 1, 2
- At 23 weeks: Survival rates are 23-27% 1, 2
- At 24 weeks: Survival increases to 42-59% 1, 2
- At 25 weeks: Survival reaches 67-76% 1, 2
Important Medicolegal Distinctions
Live birth is defined as complete expulsion or extraction after 20 completed weeks showing any evidence of life (breathing, heartbeat, cord pulsation, or voluntary muscle movement), regardless of whether the infant subsequently survives 1
Stillbirth requires the same gestational age threshold (≥20 weeks) but with no signs of life after separation from the mother 1
Critical Caveats
- The 20-week medicolegal threshold is administrative and legal, not a reflection of realistic survival expectations 1
- Viability is not an intrinsic fetal property but depends on both biological maturation and available technological support, which varies by location and institution 3
- Individual factors including birthweight, fetal growth restriction, plurality, and congenital anomalies significantly affect actual viability beyond gestational age alone 1
- Wide institutional variation exists in resuscitation practices at 22-23 weeks, which substantially influences survival outcomes at these borderline gestational ages 1, 2