The supreme court decide that a fetus does not have 'civil rights' because it can not live independently of the mother biological systems.   It is not an independent person in the eyes of the law until it can interact in society independent of the mothers body.

I'm not saying that's going to satisfy you, but that's the legal argument that results in the mother making the decision.    The court and most supporters of 'choice', do not put a stamp of approval on abortion, we leave the decision in the hands of the pregnant woman.    

I have never discouraged anyone from making a case to go full term.   But ultimately  it's the woman's body and the woman will clearly have to live with the consequences. (emotional, physical, logistical, ect...).