Let's put the dilemma in a slightly different form and see how you handle it.

It is a fact that the majority of fetuses which are conceived are spontaneously aborted before the women carrying them are even they've conceived. suppose a drug came onto the market which would ensure that, say, 10% of those spontaneously aborted fetuses could be carried to term: would you desire to have all women who were in a position to conceive be forced to take it? To sharpen the dilemma, let us also assume that the drug had severe and unpleasant side-effects, some of which were irreversible: would you maintain your insistence on all women taking it still? If you can answer "no" to either of these questions, then your posturing doesn't really set you apart from the rest of non-absolutist humanity, and all we're left arguing is questions of degree.

Oh, and by the way, if you really do believe zygotes are actual "human beings" even before they've developed brainstems, why do you think it acceptable for a woman to abort a fetus if she's been raped? I don't see why the fetus ... ahem, sorry, "human being" growing in her womb should suffer for the sins of his father. 

Obviously, spontaneous abortion is very different from actively killing.

I question the value of thought experiments/hypothetical situations such as these. You can always come up with a borderline case that supposedly ridicules the others' position no matter where a line of legality is drawn.

Both sides are paranoid of the slippery slope but I think to most people, abortion is a pragmatic issue, not an idealistic one. 

A sensible person knows that human zygotes are human beings. While there is a chance that a human zygote may spontaneously abort (abiola says greater than 50%) there is zero chance that a human zygote could develop into a fox or a chicken. For that you need a fox zygote or a chicken zygote.

Now, about abiola's "sharpened dilemma." We have enough ethical dilemmas without fantasizing more. Paul N is right about this. Let's consider the case of rape. Some women do actually become pregnant as a result of rape. With great sadness I support a rape-exception. Sadness because of the evil of the act of rape itself and sadness because the exception permits the killing of an innocent life.