Is the beginning of "personhood" a subjective opinion?

Here's the original question:

I take the position that the age at which we feel a human being should be granted the status of "personhood" is not a simple matter of religion, but of individual conscience. Even if religious people could agree that personhood should be granted from the point of a fertilized egg onwards, which they don't, we could hardly expect religions to adjudicate between questions of whether personhood should start at the moment of implantation, vs. at 2 weeks, 3 weeks, beginning of second trimester, beginning of third trimester, at partial birth, at full birth, when the umbilical cord is cut, when the first words are spoken (if ever, since some are born mute), when the child knows the right hand from the left (alluded to in the Bible), etc. For one person to feel it should begin at the beginning of third trimester and another to feel it should begin at full birth is not an issue that any religions I know of debate over, so I don't see why it should be any more religious to argue an earlier age at any certain point either. Scientifically, there are infinite graduated points of development along the way, and no exact way to declare personhood through science at any of those points along the way to adulthood, if we could ever even have a perfect consensus about when adulthood begins. Should someone stop being a minor at age 13 (barmitzvah), age 16, age 18, age 21? At an earlier time, a later time or some point in between these ages? Just as there is no exact age for determining the beginning of adulthood, I don't think there can be an exact point for determining when personhood should begin - it is equally subjective whether someone is religious or not, and all views are basically equal on this. So my question is: Would you be okay with granting legal personhood to the unborn if there were a general and generous type of "stand your ground" or "self-defense" clause built in to allow the mother to kill the person she is carrying for even a fear of danger to herself or her own well-being? Would this be a good compromise?