On a biological, religious and a philosophical level, I draw a stark contrast between a 'life' and a 'potential life'. I also draw a contrast between 'alive' and 'a human life'.

I do not believe that a human life starts until a child is born. An implanted embryo or a developing fetus is a potential life, albeit I would say it's alive, so long as it's in a viable womb. An embryo without a womb is no more alive than the sperm at a donor bank.

The why's are based on a combination of Jewish law and biology. It's a much longer response than I have time to type out at the moment (impending move+impending travel means I should be packing either a box or a duffel bag at the moment), but I will come back and answer, most likely as a post.