There's a lot of debate over the point of viability in pregnancy, or the length of a pregnancy at which the baby could survive if born prematurely. The usual number is 24 weeks. At that point somewhere around 40% of babies are able to survive, although the experience is certainly nothing to desire -- very early premature babies must spend months in an intensive care unit, during which the parents must hold a highly emotional day-to-day vigil that may potentially involve a lot of tense moments wondering whether or not the baby will even make it through the night.
A few babies as early as 22 weeks have survived, although these babies are even less likely to make it. Doctors frequently will not even initiate intervention for babies that early, given the low odds of success and relatively high odds of long term problems in babies that do survive. Early premature babies face a high odds of having at least some sort of minor disability in the long term -- and more than 20% may have a severe disability. And it's also important to note that other factors beside gestation length play a role in whether or not a premature baby survives and what the other outcomes are, such as birth weight and any medical factors that played a role in the early birth.
As an aside, the point of viability is an interesting question that often gets tied up in political debates, such as legal abortion limits and how early lifesaving intervention should be offered considering the odds of disabilities -- all of which are interesting and evolving questions that inspire a lot of strong views. More to come on these at some point in the future, as these questions can affect the rights of parents who unexpectedly give birth to very premature babies.
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