Viability is defined as the ability of the fetus to survive outside the womb. You are right that it may vary according to circumstance.
But how is that determined? If you remove the fetus early in the pregnancy, he / she (on the proper medical equipment) could survive a few hours or a few days or could possible survive and eventually grow into a health adult. Other than actually checking to see if the fetus could survive, there is no way to actually check viability. You can't say he / she wasn't viable yesterday but is viable today.
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I would say most libertarians support legalized abortion, but most libertarians are able to work with each other on common ground, rather than make a political enemy out of someone because they disagree on that particular issue.