Is this thinking about abortion correct or do I need to rethink this?

I believe now that a child is made upon conception which makes sense to me because when a child is concieved, it has intentions to live and grow one day get out of the womb. I believe that the mother/child relationship during pregnancy is a parasitic relationship and that the baby is not just a part of the mother but its own entity. (IE a fetus). How can people say that "women can do what they want with their bodies" when an abortion is taking someone out of their bodies that they put in there in the first place. I have also heard that abortions are justifiable because they are feasible. That sounds irresponsible to me. More people use abortions as a form of birth control than not which is why half of abortions are by people who have already had a previous abortion. We are all simply a group of cells, so how can you consider a group of fertilized cells in a woman's body not a baby when it intends to be so? How is this constitutional? How is this legal?