That is a reasonable response - you can hold that birth is the bright line between "this is human" and "this isn't." That's a defensible, coherent position.

However, you may want to know that some of the pro-choice advocates (like Peter Singer, ref. above) are arguing that infanticide is the moral equivalent of abortion, and should be permitted. That is a pretty chilling position, as far as I'm concerned.

On a topical note, one very interesting thing is that the child mortality rates are calculated very differently by different countries - with the US having a relatively strict interpretation. There are many places where very-low birth weight babies are not counted as live births, while in the US they are, for instance.