When the woman decides to get pregnant, she entered into an implicit contract with that baby...

So if at some point in the past you said it was ok for me to use your kidneys, you don't have the right to walk away from it? Not even if your plans change or you find out there will be permanent medical side effects and a risk of death?

BTW, you do have the right to walk. That's why multiple kidney transplants need multiple doctors, they have to have everyone unconscious at the same time or someone would be able to walk away.