They assume what they are trying to prove.
Advocates of elective abortion must show that the unborn are not fully human for their case to succeed. But instead of proving this conclusion with facts and arguments, many simply assume it within the course of their rhetoric. We call this begging the question, and it's a logical fallacy that lurks behind many arguments for abortion.
A person begs the question when he assumes what he is trying to prove. Suppose federal prosecutors confronted you with this question: "Have you stopped cheating on your taxes?" Obviously, the question is unfair. It assumes that you have broken the law, which is in fact the very point the prosecutors are trying to prove.
Arguing that abortion is justified because a woman has a right to control her own body assumes there is only one body involved---that of the woman. But this is precisely the point abortion advocates try to prove. Hence, they beg the question.
Some argue that no one knows when life begins, and that abortion must remain legal through all nine months of pregnancy. Here we have the assumption that life does not begin until birth---the exact point abortion advocates are trying to prove. This is hardly a neutral position. It's a clear case of begging the question.
Consider the coat hanger/back alley argument, which states that women will once again be forced to procure dangerous illegal abortions if laws are passed protecting the unborn. Why should the law protecting the unborn be faulted? Should we legalize bank robbery so that it's safer for felons? Even abortion advocate Mary Anne Warren points out, "The fact that restricting access to abortion has tragic side effects does not, in itself, show that the restrictions are unjustified, since murder is wrong regardless of the consequences of forbidding it." Again, the issue isn't safety, but rather the status of the unborn.
In short, if you think a particular argument begs the question regarding the status of the unborn, simply ask if this justification for abortion also works as a justification for killing toddlers or other humans. If not, the argument assumes that the unborn are not fully human. It may be the case that the unborn are not fully human, and abortion is therefore justified. But this must be argued with evidence, not merely assumed by one's rhetoric.
Proceed to the Mistake #4