Because your baby is not a complete stranger who walked up to you and just randomly jumped into your body. You put him there, he has no choice but to be there, and he's your child. You can't cite self defense against someone who isn't attacking you, who is only doing what you made them do. Also, it's your fucking child, you have a duty to it.
The duty someone has to their child is not one which concerns bodily autonomy, and if someone does not live up to their duties, the child is taken away and put into foster care, so it is not without limitations.
The fact remains that the legal and moral precedents which our society have set dictate that the right to bodily integrity is one which can be defended at the cost of someone else's life if the two are forced to come into conflict. If a rapist is forcing themselves upon someone, and the victim kills the rapist during the struggle, the law takes the side of the victim.
The issue of intent is irrelevant. If, hypothetically, the rapist was mentally ill and couldn't help themselves, it wouldn't change the line our society would draw. If we apply this precedent to abortion, then it's pretty clear that the pro-life stance on the issue can't be supported.
Hunt for the word "consciousness" in the declaration of independence, in Locke, in any text originating the philosophies of Natural Rights. You will not find it. Human rights are bestowed by virtue of Humanity, not by virtue of consciousness. You have rights when you are sleeping. You have rights when you are in a coma. You have rights when you are a fetus. You even have rights when you are a corpse. If you exist in any capacity at all and you are human, you have rights.
The people who wrote the Declaration of Independence couldn't even decide if "humanity" included black people, and you want me to dogmatically defer to them on the matter of what constitutes personhood? We've come a long way since the 18th century, you know, and I think you'll find that even back then, the general attitude towards abortion was not in line with what modern pro-lifers suggest should be policy.
Early Christian thought on the subject of abortion was mostly influenced by the Aristotelian concept of delayed ensoulment, and the writings of Christian scholars like Thomas Aquinas and St. Augustine reflect this. The view they took was that before the very late stages of pregnancy, an embryo/fetus was ontologically on the same level as plant life: alive, but without a soul. It wasn't really until the middle of the 19th century that the modern pro-life movement began, after the Catholic church finally took a firm line against abortion, having decided that personhood begins at conception.
If we're going to look to history for vindication, my view on the subject is actually far closer to historical precedent than yours is.
Back in modern times, however, there's been a lot of development in the last century in areas such as animal rights which support my view on the subject of personhood. Dolphins and chimps, for instance, don't contain human DNA, but they are generally regarded to have rights above and beyond other animals because of their cognitive capacity.
