Youcat commented through CCC - YOUCAT Question n. 82.

YOUCAT Question n. 82 - Isn’t it improper to call Mary the “Mother” of God?

(Youcat answer) No. Anyone who calls Mary the Mother of God thereby professes that her Son is God.    

A deepening through CCC

(CCC 495) Called in the Gospels "the mother of Jesus", Mary is acclaimed by Elizabeth, at the prompting of the Spirit and even before the birth of her son, as "the mother of my Lord"(Lk 1:43; Jn 2:1; 19:25; cf. Mt 13:55; et al.). In fact, the One whom she conceived as man by the Holy Spirit, who truly became her Son according to the flesh, was none other than the Father's eternal Son, the second person of the Holy Trinity. Hence the Church confesses that Mary is truly "Mother of God" (Theo-tokos) (Council of Ephesus (431): DS 251).   

Reflecting and meditating 

(Youcat comment) As early Christianity was debating who Jesus was, the title Theotokos (“God-bearer”) became the hallmark for the orthodox interpretation of Sacred Scripture: Mary did not give birth merely to a man who then after his birth “became” God; rather, even in her womb her child is the true Son of God.This debate is not about Mary in the first place; rather, it is again the question of whether Jesus is true man and true God at the same time.

(CCC Comment)

(CCC 509) Mary is truly "Mother of God" since she is the mother of the eternal Son of God made man, who is God himself.    

 (The next question is: What does the “Immaculate Conception of Mary” mean?)