While I personally have no problems with either of these two opinions
1) Abortion should be illegal even in rape for it is life, and life is sacrosanct
2) Abortion should be legal for it is a women choice

There can be a philosophical reason for a middle ground. One possible reason is that people believe that a person's actions should have consequences. If a women has sex and get pregnant she should not have an abortion for it was her decision to have sex in the first place. Now she may not have wanted to get pregnant, maybe she was on birth control and she got unlucky, maybe the condom broke, maybe she didn't know it was near her time of ovulation and has sex without protection etc. You may not have planned to have a child but you shouldn't have an abortion for your decision's have consequences.

But rape is different, the women did not choose to have sex. She did not choose to be violated and that what is rape is the ultimate violation. The ultimate removal of choice, rape is about pure power and using that power to subjugate another.

In such a situation some people believe that a women should have the choice to continue the pregnancy or abort it. She did not make the choice to have sex but at least she had some choice on what happens with her body months after the heinous act. To force her to carry a rape baby to term is to continue to subjugate that women again for a period of nine months.

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Full disclosure one of my close friends and coworkers is the product of his mother being raped and her making the decision to raise the baby as her own son.