Maybe not a precise definition, but certainly a much more informed definition. If you are claiming, for instance, that the experts on this can't say that there is a difference regarding the personhood between a 1 week old fetus and a baby, I'd say you're outright lying. This holds true for a 10 week old fetus and several further weeks out as well.
The general argument I use with things like this is "Just because we don't know everything, it doesn't follow that we don't know anything."
It seems to me that is exactly what you are doing: claiming that because it's impossible for science to know everything about the personhood of a fetus, that they can't know anything.
From what I've seen, this is a fairly standard trick of 'religious' conservatives. "There are holes in evolution, therefore 'intelligent design' is an equivalent alternative." (A great deal of these 'holes' aren't even true either, but are also 'religious' conservatives misrepresenting the science, or stating outright falsehoods.) "Evolution is just a theory."
It amazes me how many people who claim to be religious repeatedly and shamelessly violate the 8th Commandment.
My point was different from the argument to which you responded here. It was not that we have incomplete scientific knowledge of the fetal development process ergo science cannot adequately determine when personhood begins. It was that there is no scientific basis for personhood as a concept whatsoever. As far as science is concerned we may just as well all be naught but atoms and the void. Science does not prove we're naught but atoms and the void, yet such a question is in principle unanswerable by science.
Contrary to what some people think, there is, necessarily so, more to knowledge than what can be ascertained by science. To understand how we can be absolutely certain of this, consider the proposition that only what can be ascertained through the scientific process is true. Well, that statement can't be ascertained through the scientific process. Thus science is a subset of philosophy and simply because a concept cannot be scientifically verified does not mean it can be dismissed out-of-hand. Now, you can likely then think of many ludicrous ideas that cannot be proven by science and could then claim I should accept them since I don't think all ideas that cannot be proven by science should be dismissed out-of-hand. Simply because I accept some ideas unprovable by science doesn't mean I accept all ideas unprovable by science (which is a position literally everyone holds whether they admit it or not).
Personhood is a question of ethics because it's not a scientific term. That, however, also raises the issue of whether 'personhood' is even a valid part of any debate on abortion as opposed to a (conservative) religious construct.
I should have challenged the use of this term right away on those grounds, but I didn't think the terminology here was that big a deal.
The exact question of "personhood" is unavoidably a question that has to be addressed in some fashion in order to have a political system of any kind (not necessarily answered in the context of abortion but necessary to have in general). For instance, without some abstract concept of personhood, a government has no idea who its citizens are or who has rights. That is why a concept that there are persons at all is a pretty important one. You are of course free to deny altogether that "person" is a meaningful category at all, but that's probably not where you're going. Thus I object to calling in only a "religious construct" unless you're willing to defend reductive or eliminative materialism.
As for abortion, it is, I think, incredibly obvious that the question of whether or not a fetus is a person (and thus should be protected under law) is very much a relevant one. Indeed it's rather hard to imagine any honest consideration of the issue at all without recognizing the relevance of that question.