But, the developing human is not to blame for rape or incest. Thus, once we select the point at which all rights would be extended to the unborn, it should apply to all pregnancies.
Otherwise: (1) The gov't/court system/etc. will have to judge each case based on whether or not the woman had sexual relations willingly. That's not protecting the unborn; it's just penalizing women for their sexual behavior (which may indeed be what some people want to do). And, (2) such a system may give women the incentive to lie and claim they were raped, which means the men involved would be penalized wrongly.
However, your position of protection at "the beginning of the fetus stage" is much more lenient than the one I offered (i.e. when the heart starts beating).
There is a reason to hold a seemingly dichotomous position regarding embryonic life and easing restrictions in special cases. We are also in a battle to protect embryonic life from experimental exploitation. By acknowledging that embryonic life is to be protected from murder with the exceptions clearly stated, the problem of supporting the life of embryonic humans is more tenable.