Mary did not just give birth to part of Jesus, she gives birth to Jesus. And thus the title “Mother of God” speaks to us as much about Jesus as about Mary. It is a title that she has because of the Church’s insistence that Jesus cannot be divided up into two different people, and we cannot say Mary gives birth to one Jesus but not “the other.” There is only one Jesus, though he has two natures, human and divine.
It is a good thing to repeat!