"Viability outside the womb" is the legal line in the sand ... the point at which the mother is (in a sense) held accountable for harming the life she is carrying, because it can survive without her. Prior to the point of viability outside the womb the fetus is legally considered a clump of foreign cells living off the woman.
Oh, I don't know about "the" legal line in the sand. Certainly some will argue so. Some would argue that when they are old enough to make their own peanut butter sandwich they are "viable". Perhaps a better argument might be when they are old enough to survive on their own. Probably 9 or 10.
However, none of that negates the idea that they are human beings.