Bible Stories: The Christmas Story
God Becomes Man 
Now, Joseph, her husband found out that Mary was with child. Since he was a just man, he was going to break off the marriage quietly and without much fuss, but an angel appeared to him in a dream.
"Joseph, do not be afraid to take Mary as your wife. For she is with child of the Holy Spirit. She will bring forth a Son, and you shall call His name Jesus, for He will save His people from their sins."
So Joseph obeyed God and took Mary as his wife. And she stayed a virgin until after she gave birth to her firstborn Son.