Maybe the most clear case comes in this way:
Christians believe that human beings are chosen by God just as much as they choose God (both must be true to be saved). This is most clear in Romans 8:29-30.
> For those whom he foreknew he also predestined to be conformed to the image of his Son, in order that he might be the firstborn among many brothers. And those whom he predestined he also called, and those whom he called he also justified, and those whom he justified he also glorified.
So God does decide who to save, he "foreknows" them, whatever that means. Now there are several examples of people being called by God, many of which happen before they are born.
Jeremiah is called by God before his birth in Jeremiah 1:5:
> Before I formed you in the womb I knew you, and before you were born I consecrated you; I appointed you a prophet to the nations.
Paul is called by God before his birth in Galatians 1:15:
> But when he who had set me apart before I was born, and who called me by his grace, ...
These are just some examples, there are more references to unborn in the Bible. The argument goes like this: if it's wrong to kill another being imbued with the image of God, fully capable of being foreknown by Him, called, predestined, justified and glorified, it is equally wrong to kill them before birth as opposed to after birth and the Bible makes no distinction between the two in terms of how this process works, evidenced by the above passages.