"Human" is arbitrarily defined. For one extreme of the pro-life position, it is human the second the sperm penetrates the egg.
It's arbitrary, but not completely arbitrary. We can, for example, notice that nobody below a certain level of brain development exhibits behaviors or traits we associate with personhood.
Defining personhood involves a certain amount of judgement, but I think determining whether any given fetus matches the criteria we eventually settle on can be done empirically.