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declared brain dead, then that is no longer a viable life (despite their heartbeat) and will have medical treatment withdrawn.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.

Living cells start at conception. A human life, by definition, cannot start until their is enough brain development to sustain consciousness, which is around 26 weeks.