In fact, 'sentience' is generally the difference between humans and animals. Didn't you ever read C. S. Lewis?
You speak of the embryo as if it possesses a conscious human will. Yet all of your commentary in this paragraph can just as easily be applied to chipmunks as to human beings. None of the processes that you name are what makes us human.
The period during your lifetime when your brain stem and body became coordinated to the point of directing your functions is but an arbitrarily chosen period before which you have decided already there is no human being there. You are wrong, both scientifically and regarding the soul.
I don't usually raise the soul in these discussions, but in your case, the cold-blooded dehumanization you've proposed needs to be exposed via the spiritual bankruptcy within it. Your dehumanizing transactional approach to human life is something now abhorrent to me. Reading through this thread, it is apparent that you either have a long way yet to travel to get humanity in perspective, or you have a whole lot to learn regarding the truths of human life, or you just have a need to view human life as a commodity prior to a certain arbitrarily chosen age; try absorbing more data, with a mind and heart open to the truth. Goodnight
I would advise you that your proposed solution is no better than the original Roe v. Wade decision. It settles nothing if enacted.
Why do I say that?
First of all, your proposal is a activist rational solution. Judge Bork in The Tempting of America explains better that most that activist Supreme Court Justices used just such an approach to get us to the unresolvable conflict we have today.
Certain types of matters are meant by our nation's structure to be settled by law within the legislative branch or left alone. This one should have been left alone, but failing that, should have been settled through long work, compromise and legitamacy imparted by the legislative process. It wasn't and your proposal is no different.
Judges, wanting rationality to prevail over the Constitution and the process of legislation, wrote law rather than adjudicated it. Lacking the process of political compromise or adherence to the Constitution, which is silent on issues like abortion, the decision lacks, and will continue to lack, legitamacy in the minds of much of the citizenry.
There have been other tough issues, Civil Rights' Acts come to mind, that have achieved legitamcy solely because they went through the process prescribed by our system of government.
You are trying to use rationality rather than Principle to "achieve" an end best left to the proper process. It won't work precisely due to peoples adherence to Principle when faced with overly expediant ratioanality.
I too, three years ago during the primaries, started a thread to discuss just the points I make to you now.