Here is a summa styled article I wrote on Mary the Mother of God. I've rearranged it to make it easier to read for your Catholic in the pew. Rather than using Thomas' method of objection, objection, sed contra, respondio, reply, reply, I'm beginning with a respondio to the question at hand and am then posing objections and answering them immediately.
Thoughts?
Whether Mary is the Mother of God
Yes, Mary is the Mother of God. A woman is a man’s mother either if she carried him in her womb or if she was the woman contributing half of his genetic matter or both. Mary was the mother of Jesus in both of these senses, because she not only carried Jesus in her womb but also supplied all of the genetic matter for his human body, since it was through her—not Joseph—that Jesus “was descended from David according to the flesh” (Rom. 1:3).
Since Mary is Jesus’ mother, it must be concluded that she is also the Mother of God: If Mary is the mother of Jesus, and if Jesus is God, then Mary is the Mother of God.