We’re talking about rights, not science. If the fetus is part of the mother’ body, it has no autonomy to have rights. That’s the way it’s seen, because fetuses can’t inherit, and can’t be the beneficiary of life insurance payouts. They’re not considered to be autonomous human beings.
If the fetus isn’t part of the mother’s body, then if she doesn’t consent, it has no right to be there.
The science part would be to determine which it is: part of the mother’s body, or not part. Either way, the fetus has 0 legal rights of its own.