This is a legal matter, not an ethical/moral matter. a Legal Person is a person that has the protection of the law and is under the jurisdiction thereof (at least, one definition)... A fetus, by default can not be under the jurisdiction of the law, and therefore can not be protected by it. The moral matter of whether or not it should reach the level where it would become a legal person is another matter. Generally speaking, it should, unless the person (mind you, the legal person) carrying it does not want to carry it anymore. The argument isn't on whether the baby should be killed. It's that the Mother does not want to carry it. After some point, you can grow a fetus outside the mother's body -which is what I believe should be done, if the option is available) or just fucking deliver it -after the 12th week, iirc). The Fetus is not a legal person, because legally speaking, it has no personage. it's just a mass of cells, alive? sure. but a Dog is also alive and doesn't get to have legal personhood. Why should an entity with no ability to function under the law get personhood before it has that ability?