It's a negative right and the fetus doesn't have relevant rights. If both your kidneys fail and I have two that are a match, do you have a right to one of mine? That's the beginning and end of it. You don't have the right to anyone else's body, not even if you'll die without it. We can discuss whether it's morally virtuous to offer your body, whether or not you should, but nobody has the right to your body and you don't have the right to theirs.