This post (from a source I find sometimes interesting but often wrong) appears to take the simplistic (but common) position that the state of personhood which defines murder is discrete
In addition to agreeing with commenter Alex Heyworth that “logical consistency is not a necessary condition for moral codes, nor even a common one”, I would also claim that logic, even when it does apply, does not require that the marker used to separate what is legal from what is illegal should correspond to anything other than agreement by the majority (or by whoever has the deciding power in a given culture).
With regard to the legal question, there may well be agreement on many cases. But in the continuum between what certainly should always be allowed and what certainly should always be forbidden there is a range of cases on which people may disagree. In the absence of any universal agreement on the moral issue, what the law does is attempt to strike a balance between competing moral positions. Often this means that it just draws one or more lines of convenience without actually claiming that they separate the moral from the immoral. So a legal distinction at twelve weeks of gestation, or twenty, or whenever, does not need to imply that any particular state of the fetus changes at any of those times.
Even for the purpose of an individual moral decision, there may well be competing values at stake and I have never seen a good argument for the existence of a common scale on which such competing values can be compared. We each decide on what seems right at the time. In many cases we have no qualms about the choice, and the vast majority agree. But there is no guarantee that all cases will be this simple, and we are often faced with situations where a choice we have made in good conscience may later be felt to be wrong (and maybe later right again). Sometimes we do fall into indecisive mental “churning”. But even if we do make a choice, that does not imply the existence of a truly “best” moral decision. What tips the balance towards our actual judgement at any given time may depend on past experience and current neuro-chemistry rather than any absolute prioritization of competing values. However, despite the fears and fear-mongering of some, this does not deny the possibility of *any* absolute moral principles. There are lots of cases where we do have essentially universal agreement on what is right, and even more where careful consideration leads always to the same answer even among those who might initially disagree with it. But I suspect that any project to find a *complete* set of absolute moral principles will fail.
With regard to the personhood of a fetus I doubt that anyone believes in a magical discrete change of status at any particular time. But many people see the progression from fertilized ovum to conscious infant as a gradual process where the attitude towards killing should range from negligible concern at the start to absolutely abhorrence at the end. And although I don’t *require* logic in morality, I see no lack of it in such a position.