Catechism of the Catholic Church §723 — excerpt
In Mary, the Holy Spirit fulfills the plan of the Father's loving goodness. With and through the Holy Spirit, the Virgin conceives and gives birth to the Son of God.
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.