Catechism of the Catholic Church §973 — excerpt
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.
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.