Fundamentalists are sometimes horrified when the Virgin Mary is referred to as the Mother of God. However, their reaction often rests upon a misapprehension of not only what this particular title of Mary signifies but also who Jesus was, and what their own theological forebears, the Protestant Reformers, had to say regarding this doctrine.
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. There is no way out of this logical syllogism, the valid form of which has been recognized by classical logicians since before the time of Christ.
(source: http://www.catholic.com/tracts/mary-mother-of-god)
When we fail to receive the gift of Mary as Mother we can also miss the call of every Christian to bear Jesus for the world as she did. It is time to re-examine the deeper implications of the treasure that is found in the life example and message of the little Virgin of Nazareth. This wonderful title, Mary, the Mother of God, “Theotokos”, reveals a profound truth not only about Mary, but about each one of us. We are now invited into the very relationship that she had with her Son. We can become “God-bearers” and bring Him to all those whom we encounter in our few short days under the sun.
(source: http://www.catholic.org/mary/theo.php)
