Is someone on a ventilator not human because they are not viable with advanced medical equipment?
If someone on a ventilator still has higher brain function and still has a possibility of survival, then that person will receive full medical treatment.
If someone on a ventilator has been declared brain dead, then that is no longer a viable life (despite their heartbeat) and will have medical treatment withdrawn.
If a fetus has higher brain function and has a possibility of survival when they are born, then they will receive full medical treatment.
If a fetus has no higher brain function and will not survive even with advanced medical equipment, then it is not a viable life.
Life starts at conception, not at viability.
Living cells start at conception. A human life, by definition, cannot start until there is enough brain development to sustain consciousness, which is around 26 weeks.
Again, viability does not define human life. Viability refers to chance of survival.
The start of human life begins at the formation of a zygote.
What you might define as "living" begins a philosophical discussion, not a scientific one, so there are reasonable disagreements and subjective interpretation.
Do we consider living to be sentient or conscious? If that were the case, that means we are are no longer alive while sleeping?
A human life is still human life even if it is not sustainable without the mother's womb. Abortion is ending human life.