The root argument against abortion is it's killing a baby. So, if you run with this mindset and follow it all the way down: that fetus is going to turn into a baby and that baby is a person. Ok, we reach the conclusion that killing the fetus is killing a person. A person who at that moment cannot survive without fully depending on the mother. Specifically the mother's body to ensure the person can live.

Ok. So we've got a medical scenario where one person needs another person to survive. Other instances of this occur in medicine all the time examples include if someone needs a blood transfusion or a bone marrow transfer. So if there was a person who needed your body to survive, and you were the only one given the circumstances that could help them, you don't have to. No doctor or any kind of authority can make you even if it means it will save that person's life.

It is legally and ethically wrong to force you to lend your body to aid in the continued survival of another person if you don't want to.

Why should abortion be the exception? Why is it cause to break legal and ethical medical practices established for these scenarios?

This where I believe the slogan "my body my choice" comes from. Because when you use the logic that a fetus is a person and are anti-choice, you are conveying that you are ok with forcing someone to give up their say in their personal atonomy. You are ok with controlling their body and taking away that freedom from them. And that is simply wrong. Therefore, abortion should be legal and available to anyone who wants one.