I don't think you have it quite correct.
It is, as far as I know, impossible to "prove" when a "soul" comes into being.
I know of no scientifically valid test that could be used (and recognized) to demonstrate the point at which a "soul" comes into being an enter a human body.
I believe very strongly in the existence of souls, and I also believe that there is a triune God of the universe who is a Spirit who created and sustains the universe.
Can I offer a scientific proof for trhe existance of a God whos is three in one? I have several pieces of evidence, but I am quite sure they would not withstand the most rigorous scientific scrutiny. That is not the same things as saying that God does not exist as a triune diety. It only say that science will not prove for you.
Same thing with souls and when they enter the human body.
I would suggest, however, that it is really up to you (or, if not you, then those who support abortion-on-demand (as long as an "understanding phsyician feels that the mother's health could be in jeopardy if the pregnancy were not aborted) to show why aborting a human life (for certainly human zygotes, human embryos, and human fetuses are all human. And they are all life.
They are, from a scientific point of view, not too much different from you or me. In fact, both you and I were, at one point in our respective lives, a zygote, an embryo, a fetus, a two-year-old child, a sixteen year old, and a twenty-one-year-old.
And, if we are both lucky, you and I will someday be 80 year old human. Our eyesight, hearing, and mental capacity may not be the same at 80 as it was for each of us when were were 20, But the lack of hearing in an 80 year old does not negate his or her humanity.
Similarly, I wojuld suggest that the fact that a fetus does not have, for instance, a fully formed opposable thumb, does not render him or her something "less than human" It seems to me that one is only able to posit the situation that unborn children are not "human life" or not "human beings" if one injects into the argument a notion that "human life" or human beings" require something more than (1) having the identifiable characteristics of human, as opposed to say, feline life, and (2) being alive.
> I know of no scientifically valid test that could be used (and recognized) to demonstrate the point at which a "soul" comes into being an enter a human body.
None exists at this time. This is not to say that none will exist in the future (measuring the mass of the moon was at one point an impossibility; now we can measure the mass of planets around other suns). However, it is not good jurisprudence to base important laws upon things that cannot be proven, or even demonstrated.
>They are, from a scientific point of view, not too much different from you or me.
Actually, not true. Early enough in embryonic development, the embryo has no nervous system. Prior to a nervous system, there is no pain, there is no awareness. Whether there is a "soul" or not is something I leave to the sort of people who like to argue how many angels can dance on John Edwards pinhead.
> And, if we are both lucky, you and I will someday be 80 year old human.
It is perhaps unwise to automatically assume that reachign 80 years of age would be "lucky." Death is hardly the wors tthing that can happen to a person. A life wasted and devoid of success, family, respect and love would be incredibly sucko... and merge that with a body being consumed by disease... death would be a boon.