My simple response to these idiots is “Ok... so when does life begin?” The squirming and rationalizations that follow are pretty entertaining.
Here’s an exchange on the Senate floor where Santorum was nailing a squirming Barbara Boxer with that very question:
http://www.nrlc.org/abortion/pba/notansweringboxersantorum.html
This got down to the partial birth abortion question - whether a baby was “born” if it had one toe still in the birth canal.
Or ask them when life ends. I got into an argument with a pro "choice" lib and eventually asked her: "If someone has a beating heart, is it OK to bury them?" No more comments from her.