Perhaps I'm overthinking it, but I too find it not that complex.
In my conversations with conservatives, it's about the sanctity of life. A fetus/baby is alive and is afforded rights, and all the protections one can give from a doctor going in and murdering the baby. This is all well and good, so long as the baby was conceived in consenting copulation. Otherwise, the sanctity of life is a little less important, and the rights of the mother to not have to carry to term an unwanted baby from an unwanted copulation session. In this case, the sanctity of life is contingent on the consent of the mother during sex.
If the sanctity of life can be contingent on the mother's consent in one case, why couldn't that be extended to the ongoing consent of the mother to carry the fetus/baby to term? Perhaps once the fetus achieves viability, the terms change in that the mother has an obligation to extract the baby and provide an option to live outside of her until such a time as adoption could be set up, and this again makes the sanctity of life relative to the level of scientific advancement in a society.