You're both overlooking the idea (and viability) of implied rights.
Every government has inherent rights (intrinsic to being a government, the right to make laws and so on), express rights (clearly spelled out), and implied rights, which are not expressly stated but are understood as basic concepts upon which the more specific laws are based.
One of these implied rights is body autonomy, the idea that no one can take ownership and make demands on your personal self but you. It isn't spelled out in the Constitution, but it the basis for the protection against illegal seizure, the right not to self-incriminate, habeas corpus, and lots of others.
It's also the basis for, among other things, a case in Pittsburgh a few years back where a terminally ill man sued his cousin because he wouldn't donate half his liver for a transplant that would save his life. The case failed because the government could not violate the cousin's body autonomy, and the sick man died. This concept has been supported in jurisprudence at all levels for centuries, and just because it's implied rather than expressed doesn't make it any less binding.
The argument for legal abortion is based on the mother's body autonomy. Whether you agree with the conclusion or not, the basis for the argument is actually a real Constitutional thing.