Of God and His Creatures

"Enough," says St Thomas, "that the body is of the mother, though the soul is not. Suitably therefore is the Blessed Virgin said to be the mother of the Word of God, and even of God, though the divinity of the Word is not borrowed from a mother. There is no need of a son borrowing all his substance from his mother, but only his body" (above, n. 13) -- Mary is the mother of Him who is God. She is not mother of the Godhead, or divine nature. Neither is any man's mother mother of his soul.

Of God and His Creatures: 4.34