When Joseph learned that Mary was pregnant, he decided to divorce her, but an angel told him to take her as his wife because it was the power of the Holy Spirit that had enabled her to be pregnant

Matthew 1:18-25

18 This is the account of what happened just before Jesus Christ was born. Mary, his mother, {had publicly promised Joseph that she would marry him} had been publicly promised to marry Joseph. Before they began to sleep together, Mary realized that she was pregnant. It was the power of the Holy Spirit that had caused her to become pregnant. 19 Joseph, her future husband, was a man who obeyed God’s commands. One of those commands was that men must divorce women who had acted immorally. So when Joseph learned that Mary was pregnant, he assumed that she was pregnant as a result of her acting immorally. So he decided to break the engagement. But because he did not want to shame her publicly, he decided to do it privately. 20 While he was seriously considering this, much to his surprise, in a dream he saw an angel whom the Lord had sent. The angel said, “Joseph, descendant of King David, do not be afraid that you(sg) would be doing wrong if you(sg) would marry Mary. Instead, begin to treat her as your wife, because the Holy Spirit, not a man, has caused her to be pregnant. 21 She will give birth to a son. Since it is he who will cause that his people will be saved from the guilt of their having sinned, name the baby Jesus, which means ‘the Lord saves people’.” 22 All this happened as a result of what the Lord told the prophet Isaiah to write long ago about what was going to happen. This is what Isaiah wrote: 23 “Listen, a virgin will become pregnant and will give birth to a son. They will call him Emmanuel.” Emmanuel means ‘God is with us’. 24 When Joseph got up from sleep, he did what the angel whom the Lord had sent commanded him to do. He took Mary home to be his wife. 25 But he did not have sexual relations with her until she had given birth to a son. And Joseph named him Jesus.