Fetal viability is the ability of a fetus to survive outside the uterus. Viability depends upon factors such as birth weight, gestational age, and the availability of advanced medical care. In low-income countries, more than 90% of extremely preterm newborns (less than 28 weeks gestational age) die due to a lack of said medical care; in high-income countries, the vast majority of these newborns survive.
Medical viability is generally considered to be between 23 and 24 weeks gestational age, meaning that these newborns have a < 50% chance of either dying or surviving with severe impairment if active care is instituted; this applies to most fetuses at ≥ 24 weeks of gestation, and to some fetuses at 23 weeks of gestation with favourable risk factors.