Sovereignty, is, in and of itself ownership, over that self. It is the exercise of power over that body. It is the exercise of volition. In the most aggressive sense of the term, it would be fully 'theirs' upon their assumption of the cultural age of majority. I would argue, however, that sovereignty goes far beyond that... that it is also a power of defense of self against others. It is the exercise of self-will.
If you are asking when the soul enters the body, I cannot tell you. I cannot with honesty tell you when volition enters the body, outside of instinct. I am not certain anyone knows, or can know.
If you ask ten men on any of these issues, you will get at least ten, and sometimes more different answers. It is a matter of belief, not something that may be proven.
I can sympathize with your argument that life begins at conception, however, volition, and self-will cannot exist within those simple cells. We do know that the mother has, and maintains volition, so long as she is not a vegetable. At that point, her caretakers take over. So long as her mind is her own, she is hers.
That sovereign right insists that it is her volition that should determine her belief. That no force may be justly exercised upon it without trespass upon that sovereignty. The fetus.. cannot exert that right. The position that she cannot make the choice attempts to place society in that position.
As much as I wish it were possible to transfer those fetuses to artificial wombs, it is not practical or possible. The fetus, itself, cannot exercise volition below a certain complexity. It cannot exert those rights, for good or ill, until after it is born, or capable of birth. It is neither sovereign nor subject, and the only limited rights it has, are the rights we establish in its name, maintain and preserve in its name. It has no property in its opinion, no standing to suit in a court. It is, in the most absolute sense, a ward of the mother, an absolute ward in that it cannot, even in the most limited sense, survive without her.
If it were otherwise, it could later sue for the choices the mother made... and criminal charges would ensue for miscarriage. It could be charged with in-utero cannibalism, absorbing a second fetus. Along with that sovereignty comes responsibility.
The issue is between her, her doctor, and their collective conscience.
This is why laws for matters of opinion are almost never good laws. We tend to create the issues, the factions, and then create the causes to move behind, and ignore the costs, realities, and issues created.
We all have to suffer the consequences of our actions. That is the right of conscience, and the cost of volition.
It is by that right we all are tried, is it not? I will answer for mine when that day comes.
