Here are some DS quotes from that other thread:

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"An embryo is not a human being just as a sperm isn't."

That is just wrong. Except for those first few days when twinning can occur. Unless you define a human being as something that knows it is alive. In which case a 7 1/2 month old fetus and probably a 3 month old baby is not a human being either.

The point is that there is no distinction between a two month old embryo and a 7 month old fetus other than the fact that at this particular point in time one needs a human womb and one doesn't. Right now they are in different boats. But 100 years ago and 1000 years from now they are in the same boat.

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"They just do not have the ability to think, reason, feel emotions or function as a person the way we do or a more developped unborn child would."

Aha. Very Nice. You have thought of a way to get out of my trap. Push back my "gaining cognizance" date to several months after conception but before the date a fetus would presently be viable outside the womb. Doubt most would agree with this but it does work. But to be consistent you must agree that if technology allows us to remove two month old embryos and sustain them in incubators, killing them is less of a crime than killing an older baby.

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I'll be even more consistent. Killing a pre-person fetus is not a crime at all -- regardless of societal technology.

THE question is: what criteria denotes a person's existence? Or: when does personhood begin?

If you want to be consistent, you should use the same criteria to denote when a person ceases to exist. If you look at the Terry Schiavo case, you can see what science thinks we should look at: higher brain activity. Before you get it, you are't a person. After you lose it, you aren't a person. In either case, the family is the one that gets to decide what happens with the body.