I never brought the Constitution into it, thinking that human rights are self-evident and not solely given by four pieces of paper. Since you brought it up though, that document does recognize that the rights to life and the pursuit of happiness (I.E a future not determined by others) are self-evident rights. Unless you're going to join the ranks of people who think a fetus (clearly a human in an early stage of development) is not human despite having its own DNA, growing just like the rest of us, and quite simply not capable of being anything but human, you must respect the innate dignity and rights we all possess.
Since I suspect you were referring to rape specifically, I'll be the last to say any woman who is impregnated against her will needs to just suck it up and deal with it. The fact that happens at all is an utter travesty (rape, in my mind, is every bit as heinous as murder). The reality of a pregnancy though is that a new budding human life has been brought into play, and it specifically needs the mother's body to survive (until future technology advances change that reality).
Two wrongs never make a right. You can't just say, "Sorry kid, if allowed to live you could live a great and fulfilled life, but you weren't meant to be. Prepare to be sucked down a vacuum and thrown into a biodisposal center." We need to respect the dignity of everyone and give everyone the love and support they need. That means the mother. That means the child. Everyone means everyone.