Isn't that the crux of the legal issue though (at least, preliminarily, and in a way that sets the ground rules for the legal consideration) - whether an embryo or fetus is a human life rather than merely "the potentiality of human life"? I'm not suggesting that it isn't, but when it comes to considering the matter, isn't assuming that all embryos and fetuses are life just begging the question?
I think it's safe to say that, to the extent they are clearly regarded legally as human life, there would be a compelling government interest in protecting that life which is, in the vast majority of cases, sufficient to overcome the appreciated privacy right. If the Roe majority had specifically found that embryos and fetuses were human lives, in the same way legally that the born are, the result would have been much different I think.
That's the crux of the problem even with just the legal consideration of the issue, let alone a particular conclusion. That question - whether an embryo (especially) or a fetus is a human life - isn't, by its nature, a legal one. It's a philosophical one, maybe a moral one. If there's to be an objective answer, perhaps a medical one. Whatever, but it isn't really a legal question. Yet, at the same time, in our society, it has to be.
If that's a separate human life that a woman is carrying around in her belly, she doesn't have a right to murder it, except in self-defense. She certainly doesn't have the right to murder it after it's capable of surviving without her. On the other hand, if it isn't a separate human life, she has a pretty strong right to do what she wants with her body. That's the conundrum the Supreme Court faced. Pretend though we might that the legal issue is simple, it is not - because it isn't a legal issue at all, just one demanding that it be treated as one. If anything, the Supreme Court should be most criticized for skirting that question somewhat - but, had it not, I suspect the controversy would be even sharper and its 'talk past each other' tone would be in even greater force.
EDIT: Oh, forgot - yes, Griswold was the 'penumbras' decision.