Any other cell is just as much of a "being" as the zygote and none of these are "human beings". The term "human being" is a combination of "a human" and "being"/that exists.
A zygote is not "a human".
The zygote is not a Homo sapien lacking the traits necessary to gain membership into that club.
Biology is the "domain science", not medicine, and doctors take very little biology that relates.
So many citations from anti-abort doctors never give "the why".
Here are the comments of a "Biologist"
Note that this expert gives "the why", something you never see from the anti-aborts.
http://en.allexperts.com/q/Biology-6...pien-cells.htm
Here is a peer reviewed Journal article that gives a detailed explanation issues related the first after conception. "without the inflammatory language"
http://www.jci.org/articles/view/23549/files/pdf
Note the argument that a zygote is "a human" is not considered ... Because there is no valid argument.