Hi clo,
But you make viability the measure, and that is as random as birth or conception. Or suffering. Or a complex set of carved out compromises. Or whatever.
At the end of the day, I choose some compromise between conception and birth as the proper division. Or between the duties to the unborn child and the rights of the mother. But I cannot argue the point from principle as there is none that is measurable and therefore none that is other than arbitrary or personal. We can assign these kinds of values as we choose.
Rule of the majority resolves some of these kinds of issues, even if we don't like the outcome. Others are governed by the protections in the constitution: hence your Roe and Wade. Looks to me like this ends up going along the latter path.