Recently I was involved in an on-line discussion (it was more of an argument, really) regarding whether pro-life people should be using the term "unborn" in our discussion or "preborn" exclusively. The short answer is it really doesn't matter what terms you use. Arguing over terms is really just unhelpful -- it's best to use terms that both sides are comfortable with so that you can get on with the real discussion: abortion is immoral because the procedure kills an innocent human being. Arguing over terms is not helpful.
To make matters worse, I've even been told it's immoral to use the word "unborn" over the word "preborn," because the word "unborn" is an incorrect term and if you can save more children by using the term "preborn" over "unborn," then it would be immoral to use the term "unborn." But there is no possible way someone can support the proposition that using one term will save more children than the other. The reality is that everyone knows what we mean when we use the term "unborn." Most people may not understand what you mean when you say "preborn" because it's not a word that comes up very often, except usually from a pro-life person.
I sometimes do use the term "preborn." After all, the prefix "pre-" means "before." So "preborn" literally means "before birth." Pre- denotes movement, to me. Conversely, the prefix "un-" just means "not," so "unborn" literally means "not born." It's still a correct term, but it conjures up an image of stagnation to me, as if it's unborn and will remain that way unless acted upon. So I sometimes use "preborn" just because it conjures an image of the children progressing toward birth. But either term is correct, and using the term "preborn" may actually require more time because I have to explain what I mean by it, whereas anyone will know what I mean by "unborn."
So that's an even longer answer to say it really doesn't matter which term we use. Arguing over which term is better is an argument that we really shouldn't be engaging in, because it detracts from the overall issue: we are working to end abortion because it kills innocent human children. Arguing over terms is beside the point.