The relationship between the mother and fetus is the most important consideration, and the rights of the mother to have control over her own body is paramount. Giving rights to the fetus over the mother's own rights is asking the mother to sign her right to her own body to the fetus. In effect, the mother becomes the bodily slave of the fetus. Is it not clearly unethical to make one sign a contract enslaving themself?
Furthermore, this contract would not be between two equal parties. The relationship of the mother to the fetus is NOT one of commensalism between two organisms. It is a relationship of parasitism (take away any negative connotations you have with this word, and keep its simple scientific implications). The fetus depends solely on the mother to live, and gives nothing back in return. The mother should have the right to terminate such a relationship, because the fetus is physiologically dependent on her and withdrawing her energy and resources.
Note that, even should the fetus be conscious (which is not clear), the rights of the mother supercede the (presupposed) rights of the fetus. Why? Because of the "parasitic" nature of the relationship between the mother and the fetus. The fact that the mother helps create the fetus is irrelevant. Say, hypothetically, that there was an 80 year old man who temporarily needed a blood, oxygen, hormones, water, proteins, antibodies, and so on to live. Could we in our right mind force a woman to be "hooked up" to the man in order to give him a chance to live? The answer is no. And the reason is because this is a parasitic relationship, where we cannot ethically make the mother enslave her bodily rights to the man.
For these reasons alone, the mother has any and all rights to do what she wants with the fetus, since it is as much a part of her body as her left hand.