There is a concept called body autonomy. Its generally considered a human right. Bodily autonomy means a person has control over who or what uses their body, for what, and for how long. Its why you can’t be forced to donate blood, tissue, or organs. Even if you are dead. Even if you’d save or improve 20 lives. It’s why someone can’t touch you, have sex with you, or use your body in any way without your continuous consent.
A fetus is using someone’s body parts. Therefore under bodily autonomy, it is there by permission, not by right. It needs a persons continuous consent. If they deny and withdraw their consent, the pregnant person has the right to remove them from that moment. A fetus is equal in this regard because if I need someone else’s body parts to live, they can also legally deny me their use.
By saying a fetus has a right to someone’s body parts until it’s born, despite the pregnant person’s wishes, you are doing two things.
1. Granting a fetus more rights to other people’s bodies than any born person.
2. Awarding a pregnant person less rights to their body than a corpse.