A reasonably easy to remember guide is that the survival rate is about 40% for all babies born at 24 weeks' gestation, 50% for those born at 25 weeks, 60% for those born at 26 weeks, 70% for those born at 27 weeks, and 80% for those born at 28 weeks.3
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Typically, prior to 24 weeks they won't try every medical intervention to save a baby. So that is really considered viability. True viability is of course dependant on the baby itself, and the circumstances.
Re: what week is viability?