Obvious snipe at the abortion debate notwithstanding, I hope that the baby and mom continue to be healthy!
Do you struggle this hard with questions like legal age requirements for consent, alchohol, prison, cigarettes, joining the military etc? Although science can give some objective ballpark data from which to decide reasonable limits (just like viability) they have no magic line either.
Imo, as an aside, I've never found the question of personhood relevant to the abortion debate. I would side with women to end a pregnancy literally any time, since I put value on body autonomy more than any person of any age (even if it were an adult in there). Point of viability just changes the process. After viability through induction of labor or c-section.
And when does it cease being a fetus?
When born. That is the beginning of the infant developmental stage. Terms like "baby" are non-technical colloquial terms, and you can call your pet cat a baby and still have the same weight as calling a six week embryo like the one in this dish: