Matthew 1:23 "Behold the virgin shall be with child and bear a son, and they shall name Him Emmanuel, a name which means 'God is with us' "
Luke 1:35 "The angel answered her: The Holy Spirit will come upon you and the power of the Most High will overshadow you; hence, the holy offspring to be born will be called Son of God"
Simple logic dictates that Mary is the mother of this Son of God. Even Luther, Calvin and Zwingli agreed on this point.
Martin Luther: "In this work whereby she was made the Mother of God, so many and such good things were given her that no one can grasp them....Not only was Mary the mother of Him who is born [in Bethlehem], but of Him who, before the world, was eternally born of the Father, from a Mother in time and at the same time man and God." Weimer, The Works of Luther, Vol. 7, page 572.
John Calvin: "It cannot be denied that God in choosing and destining Mary to be the Mother of His Son, granted her the highest honor.....Elizabeth calls Mary Mother of the Lord, because the unity of the person in the two natures of Christ was such that she could have said that the mortal man engendered in the womb of Mary was at the same time the eternal God."Calvini Opera, Corpus Reformatorum, Braunschweig-Berlin, 1863-1900, Vol. 45, page 348 and 335.
Ulrich Zwingli: "It was given to her what belongs to no creature, that in the flesh she should bring forth the Son of God." Zwingli Opera, Corpus Reformatorum, Berlin, 1905, Vol. 6, I, page 639.