I understand. For you, what you term "the existence of a human being" is the significant and over-arching principle.
Others call conception a condition of human potential and seek to balance it with the right of the mother to privacy (choice), and the protection of a woman's health. The balance shifts as the pregnancy progresses.
This fits with my model of the universe in which absolute truths are never sustainable and the existence of time means that almost all equilibria are temporary.
Human sanctity doesn't seem to be conferred by the universe. It is something we like to grant to ourselves. Accordingly, we are free to define terms as we wish. At the moment, I think more people accept the current definition than not, which is that human potential and the existence of a human being are different things. For me, during the first four weeks of a pregnancy, the egg-sperm combo deal is a cellular event, and once sentience occurs I am thinking more like "ecce homo". Personal choice occurs unadulterated within the borders of pre-sentience.
In this way, my view is distinct from Roe vs Wade. I see less significance in viability. What matters is suffering.
I do agree that one major role of government is to protect the weak and powerless. This is one reason I am keen on a universal health service mandated by government and that I am willing to accept some level of freeloading in order to ensure the protection of the poor through welfare. Bad luck happens.