The question is how to do it, because there are many possible paths, some short and some long, some more interesting and some less interesting.

I suppose the shortest and least problematic path is something like:

vagina, vulva → source, point of origin, birthplace → 3S pronoun → complementizer → predictive/counterfactual marker

Perhaps with an extra stage or two between "point of origin" and full-on grammaticalization as a pronoun. Like going from "source" to "source of debate/discussion, source of an issue" to "the one in question".

Mood