I really liked Carl Sagan's answer to this question. He said that human life begins when the fetus begins to exhibit brain wave patterns that are recognizably human.

I spent several minutes trying to find a good source for that, and oddly enough I could only find two pro-life/anti-choice pages, both of which have on them (what I consider to be) pro-choice statements by Sagan:
http://www.bigissueground.com/philosophy/blair-whenlifebeginsclone.shtml
http://www.alysion.org/truelife/

(Unless the authors of the second page are arguing that it is wrong to destory sperm and eggs, at which point I have no idea what to say.)