It is something like the probablity of winning the lottery. Each time they call a number that matches yours, your potential to win increases. You do not win until they call that last number. Up until that last matching number is called you have nothing. After it is called you have everything. If you walk away after the first number is called, you have not lost a lot of potential. If you walk away just before the last matching number is called, you are losing a lot of potential.
It is the same with the fetus. The closer you come to the moment of birth the higher the potential and less you will want to abandon that potential.
I am not sure what a partial birth abortion is, but I will say this. If I had to terminate a fetus moments before it was to be born in order to save the mother's life, I would. The mother is living human with soul. The almost birthed baby is still only a potential human.
I hope this satisfies your morbid curiousities.