The doctrine of Mother of God is clearly defined in the Council of Nicea, but this was just a reiteration/emphasis on what was acknowledged by most (Nestorius/Arius being the exceptions) since +Ignatius of Antioch. It also falls pat into place if you consider Jesus to be wholly man and wholly God and not an adopted son or one whom the Holy Spirit possesses and then leaves on the Cross.
The question is, however, from whence comes the doctrine, if it is Catholic doctrine, that Mary did not die but is body and spirit in heaven presently.