The ones who cite "potential" to grow up as being the thing to be preserved have no reason to draw the line as late as conception. They are being inconsistent if they do so.
Sorry to paste another link, but this reminded me of an awesome post one of my contributors made, mainly against apologist JT Wartick. Schueler is an evolutionary molecular biologist and he approached personhood in a biological manner. What is the biological difference between entities at fertilisation etc which brings about claims of personhood?
Well worth reading.http://www.skepticink.com/tippling/2012/11/14/life-starts-at-conception-but-what-about-personhood/