The Catechism in Questions & Answers

122. What events preceded the birth of Jesus?

The angel Gabriel brought the virgin Mary the message: “And behold, you will conceive in your womb and bring forth a Son, and shall call His name Jesus. He will be great, and will be called the Son of the Highest; and the Lord God will give Him the throne of His father David. And He will reign over the house of Jacob forever, and of His kingdom there will be no end” (Luke 1: 31-33).
The angel also explained to Mary that she would become pregnant by the Holy Spirit: “The Holy Spirit will come upon you, and the power of the Highest will overshadow you; therefore, also, that Holy One who is to be born will be called the Son of God” (Luke 1: 35).