In Brief CCC 973
© Libreria Editrice Vaticana Source: vatican.va
By pronouncing her "fiat" at the Annunciation and giving her consent to the Incarnation, Mary was al ready collaborating with the whole work her Son was to accomplish. She is mother wherever he is Savior and head of the Mystical Body.
Read the full chapter 3. I Believe in the Holy Spirit ¶683–1065