While a woman has the right to choose what to do with her body in most cases, it is not clear that she has the right to use her body in ways that harm other people. There is a difference in cases where we donate organs. Lots of people can donate organs and save lives. An infant, however, is only able to survive within one person. Because of that, a mother has a greater level of responsibility towards the infant. Furthermore, whenever a woman has sex voluntarily, she is accepting the chance of her becoming pregnant. As that is the case, she is, in a sense, volunteering her body to be used by a fetus. To continue with the organ analogy, most people would consider it wrong to suddenly decide you will not donate an organ after promising an individual to give them what they need.