Regarding the argument that the moment of birth is an arbitrary distinction as it pertains to the endowment of rights, proponents of that argument would need to resolve the discrepancy between that stance and the Constitution. Is it fair to only espouse the Constitution when it fits your agenda? The framers of the 14th Amendment certainly felt that birth was the earliest point at which a person should be afforded the rights and protections of a citizen. They felt it necessary to begin the first section of what would become the most important part of the Constitution with the assertion that "all persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside. No State shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any State deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws."
So, before the moment of birth, I don't see how anyone can assert that the rights of the mother are equal or subordinate to the rights of the unborn.