Personhood is a complicated issue. The biggest problem being it is rather arbitrary. The point about human life, and human life with a very good chance to become a person, is i feel the more important one.
I disagree. "Having a very good chance to become a person" isn't a very important metric. Yes, all people were necessarily once proto-people, but that doesn't mean all proto-people should be people. A fine block of marble has a 'good chance' of being a world-class sculpture, but it very much is not a world-class sculpture. The moral issue of cutting up a fine piece of marble is very different from cutting up a world-class sculpture.
Like the sculpture, the fetus will only become a person if it is finely crafted over the course of months into personhood. And yeah, once it becomes a person, it gains a new dimension of moral significance. Fetuses only have a 'good chance' of becoming a person if a series of very careful steps are taken to make it become so. But, in truth, we
force a fetus into sentience (depending upon perspective), it just so happens that the necessary steps in forcing this development takes place fairly instinctively. But, like the marble, the fetus is crafted. The moral status of a 'living human being' changes with time, as the physical status of that human being changes.
Now, the mistake (imo) is to put some type of 'intention' into the fetus. To give it, prematurely, personhood. I write that the mother 'forces' the fetus to develop, and the intuition is that the fetus 'forces'
itself to develop too. Now, this is loosely true (the fetus is obviously living), but there is no intention in the fetus. There is no intent that requires consideration. We don't need to take its 'feelings' or 'desires' into our calculations, because the fetus has no 'feelings' or 'desires'.
Upthread, you mentioned that the pro-choice camp had 'no science' justifying their stance. But this just isn't true. A lot of this discussion is regarding perspectives, and our intuitions regarding gestalt organisms. Some people see the 'embryo' as an important organism, because they perceive it to be, essentially the same organism as the later person. And yes, the fusion of the oocytes is a real event that creates a new organism. Remember, though, that these oocytes are actually a chain-of-life going back billions of years. Deciding it's a 'new organism' is reasonable, but still arbitrary. There are
other things that also occur during the development of a person that are just as definitive in being a 'start' of personhood. (For example, the formation of the primitive streak, or the beginnings of sentience). Choosing embryo fusion as the morally important stage IS somewhat arbitrary. And embryo has almost no traits that we consider to be morally important in person.
It's my position, one that I think is much more reasonable, that we actually consider sentience and sapience and cognition to be the most important attributes of personhood. As well, it is my position that we consider people as they are
in the actual dimension of time when making moral decisions regarding them.
The anti-abortion side will seize on anything, twist anything, to make their point. It's impossible to reason with them. 
