I don't think my definition would change. The definition would be the same, but the age at which a baby can survive outside the womb is, and probably will continue to change. The point is not whether that "fetus" could survive if it were born in the year 2435, but whether or not it could live outside the womb TODAY. So the definition will stay the same forever, but the science/skills will change.
But at that point, my definition of "human" might come into play!
I'm thinking of Sci-Fi-Future movies, where all the "humans" are born from test tubes, and there are no more interpersonal relationships, and everything is done by genetic engineering: We need more factory workers, so we "breed" "beings" who can stand the monotony of working on an assembly line all day, etc.
At this point in time, our super-human efforts to save a premature baby (20-weeks gestation), result in a relatively normal human being. But I can't forsee the future, and I don't know (a) how long it will take to develop the skill/technology to keep an 8-week-gestation "baby" alive, and (b) whether the resultant "product" of that effort will be "fairly normal" or not.