Contrary to what one might think, the term 'womb' is actually already a technical term in Trinitarian theology. A fairly standard usage is in the Symbol of Faith of the Eleventh Council of Toledo (675):
We must believe that the Son is begotten or born not from nothing or from any other substance, but from the womb of the Father, that is from His substance. Therefore the Father is eternal, and the Son is also eternal. If He was always Father, He always had a Son, whose Father He was, and therefore we confess that the Son was born from the Father without beginning. We do not call the same Son of God a part of a divided nature, because He was generated from the Father, but we assert that the perfect Father has begotten the perfect Son, without diminution or division, for it pertains to the Godhead alone not to have an unequal Son. This Son of God is also Son by nature, not by adoption; of Him we must also believe that God the Father begot Him neither by an act of will nor out of necessity, for in God there is no necessity nor does will precede wisdom.
De Patris utero, id est, de substantia ejus. We find a similar usage in a sermon on Mary (fourth century, I think) misattributed to Chrysostom:
You have found a Spouse who will protect your virginity instead of corrupting it; you have found a Spouse who wants to become your Son because of His great love for men. The Lord is with you! He who is everywhere is in you; He is with you, and He comes from you, the Lord in heaven, the Most High in the abyss, the Creator of all, Creator above the cherubim, Charioteer above the seraphim, Son in the womb of the Father, Only-begotten in your womb, the Lord--He knows how--entirely everywhere and entirely in you. Blessed are you among women!
There is a connection to the Vulgate translation of Ps. 110:3, which is, roughly, 'From the womb, before the daystar, I have begotten you'. Compare Augustine:
But this is put off, this will be granted afterwards: what is there now? "From the womb I have begotten Thee, before the morning star." What is here? If God hath a Son, hath He also a womb? Like fleshly bodies, He hath not; for He hath not a bosom either; yet it is said, "He who is in the bosom of the Father, hath declared Him." But that which is the womb, is the bosom also: both bosom and womb are put for a secret place. What meaneth, "from the womb"? From what is secret, from what is hidden; from Myself, from My substance; this is the meaning of "from the womb;" for, "Who shall declare His generation?"
I have seen, once or twice, an argument that the Spirit could be considered 'the womb of the Father', but this just messes things up; the Son is not born from the Spirit, and the Spirit is not the divine substance. So it seems no-go, at least without the sorts of explanation you can't stop a church service to give.
It's similar with the name 'Mother'. Julian of Norwich applies the label to the Trinity, in a completely orthodox way; but she's thought it through enough to recognize which person of the Trinity is most fittingly called 'Mother'. Just a few such passages:
The Second, most precious, Person, who is our substantial mother has now become our sensual mother, for we are double by God's making, that is to say, substantial and sensual. Our substance is the higher part that we have in our father, God Almighty.
The Second Person of the Trinity is our mother in nature, in our substantial making. In him we are grounded and rooted, and he is our mother by mercy in our sensuality, by taking flesh.
Thus our mother, Christ, in whom our parts are kept unseparated, works in us in various ways. For in our mother, Christ, we profit and increase, and in mercy he reforms and restores us, and by virtue of his passion, death, and resurrection joins us to our substance. This is how our mother, Christ, works in mercy in all his beloved children who are submissive and obedient to him.
And identifying the Second Person as Mother is far easier than identifying the First Person as Mother, because the Second Person bears distinctive mother-like characteristics toward us (e.g., by his pains upon the cross we are birthed into new life), whereas the Father does not. This is not intra-Trinitarian language, as the Father-Son-Spirit language is supposed to be; but it is orthodox, thought out well, and easily incorporated into liturgy in a way that Mother-Child-Womb language is not. (As most people are saying in response to it, What in the world does that mean?) Whereas Julian's route is only obscure when you jumble it all up with other language without explanation (as I do in the title to this post).
