Reposting here a comment I made on a thread at PrawfsBlawg called, "Why Does it Matter if a Fetus is a Person?"
So what, really, is different about the state of the law if we call fetuses persons for legal purposes?
It makes a difference because in one case, you're balancing the rights of two people against each other, and in the other case, you're balancing a woman's rights simply against society's preferences about what she does. When there are two people involved, it can be legitimate to constrain the action of one with respect to the other. However, if the fetus is not a person, then how do proponents justify restrictions on abortion (other than what might be considered consumer protection measures)? Are those restrictions justified on the grounds that society has enough of a stake in reproduction that it gets to override personal decisions? If so, then on what basis can women be protected from being legally forced to have abortions, or to use contraception, or not to use contraception?
It may not make a difference to the unborn whether or not they are protected because they are legal persons, but it makes a huge difference to the rest of us.
[I'd add to that comment that there are other plausible legal rationales for restricting abortion in the case of fetal non-personhood besides the one I posited above, but I can't think of any that don't have implications I find unacceptable. This is why I do not support, for instance, efforts to overturn Roe v. Wade on the grounds that the right to privacy is not enumerated in the Constitution.]