Interesting point. I never thought about this. It does raise some tricky issues. There are some practical and moral reasons for wanting the unborn child destroyed:
- If allowed to live, it may regard itself as the woman's husband's heir, creating considerable strife.
- The Old Testament recognized that it is sometimes proper for children to suffer for the sins of their parents.
- Some adulterers might be hoping to pass on their seed to future generations; making it clear that children conceived in adultery will not survive would thus serve to discourage this.
Under Old Testament morality, I think there is no problem whatsoever with a rape victim destroying any child sired by her rapist. Under New Testament morality, things are a little more complicated, but that's a subject for another post.