The argument from self ownership thread brought to my attention some of the ways the issue of abortion is dealt with.  Semantic difficulties seems to be an area where a more thorough investigation is warranted.  The language problem is described quite accurately by Joyce Arthur on her post from the Pro Choice Action Network.

[a]… major fallacy perpetrated by the anti-choice is their interchangeable use of the word “person” with the terms “human”, “humanity” or “human being”. These terms are not synonymous. For example, anti-choicers often confuse the adjective “human” and the noun “human being,” giving them the same meaning. I’m struck by the question they often pose to pro-choicers: “But isn’t it human?” – as if we think a fetus is really a creature from outer space.

If you point out that a fetus consists of human tissue and DNA, anti-choicers triumphantly claim you just conceded it’s a human being. Now, a flake of dandruff from my head is human, but it is not a human being, and in this sense, neither is a fertilized egg. Anti-choicers will respond that a fertilized egg is not like dandruff, because the egg consists of a unique set of chromosomes that makes it a distinct human being. But with cloning, a cell from my dandruff is enough to create a new human being. Although it would have my identical genetic make-up, it would still be a unique individual, because human beings are much more than our genes. Also, both a fertilized egg and a cloned cell represent a potential, not an actual human being.

It’s a worn cliché, but it bears repeating – an acorn isn’t an oak tree and the egg you had for breakfast isn’t a chicken.  So the only objective scientific fact we have is that fertilized eggs are human (the adjective) – not that they are human beings (the noun).