Catechism of the Catholic Church §722 — excerpt
The Holy Spirit prepared Mary by his grace. It was fitting that the mother of him in whom "the whole fullness of deity dwells bodily" should herself be "full of grace."
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.