The ability of a fetus to survive outside of the womb. Historically, a fetus was considered to be capable of living at the end of gestational week 20 when the mother had felt fetal movement (quickening) and the fetal heart tones could be auscultated with a fetoscope. In actuality, even with prompt and intensive neonatal support, a preterm fetus of less than 23 weeks’ gestation has little chance of surviving outside of the womb.