I, a 47-year-old man, am clearly a person and can clearly express my will to live.
Despite this, I'm not entitled to so much as a drop of my mother's blood or even a hair off her head, regardless of the consequences for me if I don't get these things.
If she doesn't consent to provide whatever blood or tissue I need to stay alive, then I don't get it, period. Even if I'll surely die without it and even if she gives no reason at all for her refusal, she has the absolute right to her own body.
If you thought that fetuses were human beings, this is the standard you'd apply for them, too.
... but you don't. You argue that fetuses should have different rights than people, which is how I know you consider a fetus to be something different from a person.