<P> Viability, as the word has been used in United States constitutional law since Roe v. Wade, is the potential of the fetus to survive outside the uterus after birth, natural or induced, when supported by up - to - date medicine . Fetal viability depends largely on the fetal organ maturity, and environmental conditions . Another definition for viability, as used in the medical phrase limit of viability, is the expectation that a fetus has an equal chance of surviving and not surviving outside his or her mother's womb . </P> <P> According to Websters Encyclopedic Unabridged Dictionary of the English Language, viability of a fetus means having reached such a stage of development as to be capable of living, under normal conditions, outside the uterus . Viability exists as a function of biomedical and technological capacities, which are different in different parts of the world . As a consequence, there is, at the present time, no worldwide, uniform gestational age that defines viability . </P> <P> There is no sharp limit of development, gestational age, or weight at which a human fetus automatically becomes viable . According to studies between 2003 and 2005, 20 to 35 percent of babies born at 23 weeks of gestation survive, while 50 to 70 percent of babies born at 24 to 25 weeks, and more than 90 percent born at 26 to 27 weeks, survive . It is rare for a baby weighing less than 500 g (17.6 ounces) to survive . A baby's chances for survival increase 3 - 4% per day between 23 and 24 weeks of gestation and about 2 - 3% per day between 24 and 26 weeks of gestation . After 26 weeks the rate of survival increases at a much slower rate because survival is high already . </P> <Table> <Tr> <Th> Completed weeks of gestation at birth </Th> <Td> 21 and less </Td> <Td> 22 </Td> <Td> 23 </Td> <Td> 24 </Td> <Td> 25 </Td> <Td> 26 </Td> <Td> 27 </Td> <Td> 30 </Td> <Td> 34 </Td> </Tr> <Tr> <Th> Chance of survival </Th> <Td> 0% </Td> <Td> 0 - 10% </Td> <Td> 10 - 35% </Td> <Td> 40 - 70% </Td> <Td> 50 - 80% </Td> <Td> 80 - 90% </Td> <Td>> 90% </Td> <Td>> 95% </Td> <Td>> 98% </Td> </Tr> </Table>

Percentage of babies born at 24 weeks that survive