I don't know exactly. And reasonable people can reasonably disagree.
A similar question is when a child becomes an adult. You know though, some never do, and some do a lot younger than the average.
tpaine's point in #96 seems to say it as well I could. A reasonable but arbitrary point, as opposed to an unreasonably arbitrary point.
For the record, by the way, I believe a woman retains sovereignty over her body ALWAYS and can expell a fetus at any point she chooses. The question is how you treat the expelled entity -- as a human or not. Viability and other estimations of human life speak to that point -- and not whether a woman MUST bring a birth to term (there are always death risks with live birth and a woman should never be forced to risk her own life for another if she chooses not to.)