But some babies are thrust before they are ready into an environment that they cannot handle without help. Babies born under 38 weeks after conception are considered premature. The smallest babies that can survive outside their mothers' bodies are born at 22 to 23 weeks after conception and weigh less than a pound. About 90 percent of those babies die. They usually go home 48 hours after birth, just like babies who are born full-term.
Doctors divide premature babies into categories according to weight.
Below 2 pounds is considered extremely low birth weight. Some babies who are small and survive, apparently otherwise unscathed, end up with problems when they reach school age. Vision problems among premature babies have been common because the blood vessels in the backs of their eyes are not mature when they are born. The vessels stop growing for a while. When growth resumes, the vessels may grow abnormally.