+ Mt 1: 18-24
Now this is how the birth of Jesus Christ came about. When his mother Mary was betrothed to Joseph, but before they lived together, she was found with child through the holy Spirit.
Joseph her husband, since he was a righteous man, yet unwilling to expose her to shame, decided to divorce her quietly.
Such was his intention when, behold, the angel of the Lord appeared to him in a dream and said, “Joseph, son of David, do not be afraid to take Mary your wife into your home. For it is through the holy Spirit that this child has been conceived in her.
She will bear a son and you are to name him Jesus, because he will save his people from their sins.”
All this took place to fulfill what the Lord had said through the prophet:
“Behold, the virgin shall be with child and bear a son, and they shall name him Emmanuel,” which means “God is with us.”
When Joseph awoke, he did as the angel of the Lord had commanded him and took his wife into his home.
Source: New American Bible
Catechism of the Catholic Church
497 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. The Church sees here the fulfillment of the divine promise given through the prophet Isaiah: “Behold, a virgin shall conceive and bear a son.”