The only reasonable line is fertilization
A proper interpretation of the 14th Amendment would apply equal protection to unborn babies and make abortion illegal nationwide.
Roe v. Wade is bad law. The "right" to abortion exists nowhere in the Constitution, and the states had the legal authority to restrict abortion and had done so until the Supreme Court created a "right" to abortion by stretching the actual text beyond recognition.
Planned Parenthood v. Casey is even worse. The decision's psychobabble about each person's "the right to define one's own concept of existence, of meaning, of the universe, and of the mystery of human life" is an utter embarrassment and has nothing to do with the law, the Constitution or logical interpretation of the text. It is also wildly inconsistent with the decision's establishment of "viability" as the line where abortion can be restricted.
The "viability" standard, applied logically, could be used to legalize killing newborns or even toddlers, not to mention the severely disabled. These people are not "viable" in that they cannot care for themselves and have little understanding of the world around them. Advances of medical technology have proven that arguments about "viability" are inherently unstable, and the law should never be tied to outside forces. The entire purpose of law is to provide a clear, easy to understand framework of rules and limitations, and tying the law to an outside force negates that.
The only logical line to draw, then, is fertilization. When sperm joins with an egg, a new human being is created, with a separate DNA code. That new being will grow and develop until the natural process of gestation is completed. The fetus is not part of the mother's body - he or she resides inside of and attached to the mother's body. The decision to kill a fetus is not a decision of what a woman does with her body, but a decision over what she does to her child's body.
Can we please stop using the "forced birth" meme to describe this process? Birth is the end of a natural process that starts with the free choice to have sexual intercourse. Birth is no more "forced" on a woman who gets pregnant after sex than digestion is "forced" on us after we consume a meal. Abortion is an artificial intervention to stop a natural process.
So both Roe and Casey should both be overturned. The best standard, though, is not to send it back to the states and allow each state to decide for itself what its abortion laws should be. Basic human rights - including the right not to be dismembered by a so-called "doctor" - should not be decided by state legislatures any more than slavery should have been decided by the states. Instead, the Supreme Court should apply the 14th Amendment's guarantee of equal protection under the law to unborn babies and make abortion illegal nationwide. That is the only just outcome here.