Catechism of the Catholic Church §509 — excerpt
Mary is truly "Mother of God" since she is the mother of the eternal Son of God made man, who is God himself.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.