Slavery is banned by the U.S. Constitution. Infants are persons also protected by the Constitution. The Supreme Court or a Constitional Amendment would have to clarify whether the unborn are person subject to the protection of our laws. Currently we recognize a distinction between the born and the unborn. Even in S.D. the penalty for abortion is 5 years for the doctor, and none for the mother who cannot be prosecuted. A woman who has an abortion performed on her the day before her due date cannot be charged; a mother who has a doctor kill her day old newborn can be charged with murder.
But it wasn't at one time.
Infants are persons also protected by the Constitution.
As are the unborn. Abortion was illegal everywhere at the time of the nation's founding.