This is a half-formed thought I'm jotting down so I don't forget it.
So, in Catholicism, when you hear the term the Immaculate Conception, they're talking about the Virgin Mary's conception, not Christs. (Though His, too, would be termed Immaculate, but the whole Virgin Birth thing takes precedence, I suppose...)
Anyway. The concept is that, at her conception, Mary was free from all sin, which would have to be Original Sin, since at that point, she was incapable of having committed personal sins. Not a lot of opportunity in the womb, one supposes.
So later on, at the Annunciation, Mary was already perfect and sinless when she said yes to God, through Gabriel. She was at a pre-Fall state of being.
However, it is my understanding, though I could be wrong here, that in Orthodoxy, the 'stain' of Original Sin doesn't....doesn't exist, for lack of a better word, here. There are consequences of Original Sin - death and the propensity for personal sins, but like, an infant who dies isn't going to be stuck in limbo/hell because of the stain of Original Sin, right?
So, because of that, there's no doctrine of the Immaculate Conception in Orthodoxy. Rather, and again, my understanding, it was only at Mary's acceptance, at her 'yes', that she was cleansed, I'm assuming because she'd accepted Christ, even if she maybe didn't understand the details at the moment? I don't know, but she was made immaculate and sinless at that point, and never sinned for the rest of her life.
The point is- the *point* is, mind, that I think the second makes more sense. If Mary was *already* perfect and sinless, then *of course* she was going to say yes. However, if she was just like everybody else, then her yes was a free choice, see? She was at the same state of being as all the rest of us, and in that flawed state, chose God. Which makes the choice more important and meaningful, see?