The second person of the trinity, the Logos, existed prior to the incarnation. He existed in His fullness, 100% intact, omnipotent, omniscicent, and omnipresent. There is not one aspect of that fulness about which it can be said that Mary mothered. She gave birth to that incarnation, she gave birth to the flesh, she did not give birth to the Logos, she did not give birth to that which already existed, which is the fulness of the Godhead. The Word became flesh, she gave birth to the flesh not the Word. This is a unique situation and our normal definitions do not work.