That is the argument I heard recently from a pro-choicer. That under the 14th Amendment (all persons born etc.) the fetus is not a citizen and therefore has no constitutional rights. The mother is a citizen and therefore has the right to determine what medical procedures are best for her. I havent really come up with a good rebuttal to that one yet.
This is why I would like to see a constitutional amendment stating that life begins at conception. As far as I know there has been no attempt at this.