Catechism of the Catholic Church §497 — excerpt
The Gospel accounts understand the virginal conception of Jesus as a divine work that surpasses all human understanding and possibility: "That which is conceived in her is of the Holy Spirit", said the angel to Joseph about Mary his fiancee.
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.