This is where the argument rests.

People who believe in reproductive choice recognize that embryos and fetuses are potential human life, and that women are actual humans. That's why the last trimester is such a point of contention. At that point, the forebrain is formed, though nobody is quite sure when it's engaged, and therefore we begin to talk about two potentially actual humans instead of just one. The community, in the form of the state's legal power, begins to have a stake at that point.

People who don't believe in abortion believe that the embryo is a fully invested human from conception. This is tied not to biology, but theology: the belief that there is a soul that makes collection of increasingly organized cells a human.

A brain stem does not a human make.

In the United States, religious beliefs on this topic do not trump the legal rights of citizens. Yet.