18This is how the birth of Jesus Christ came about: His mother Mary was pledged to be married to Joseph, but before they came together, she was found to be with child through the Holy Spirit. 19Because Joseph her husband was a righteous man and did not want to expose her to public disgrace, he had in mind to divorce her quietly.
20But after he had considered this, an angel of the Lord appeared to him in a dream and said, "Joseph son of David, do not be afraid to take Mary home as your wife, because what is conceived in her is from the Holy Spirit. 21She will give birth to a son, and you are to give him the name Jesus,[c] because he will save his people from their sins."
22All this took place to fulfill what the Lord had said through the prophet: 23"The virgin will be with child and will give birth to a son, and they will call him Immanuel"[d]—which means, "God with us."
24When Joseph woke up, he did what the angel of the Lord had commanded him and took Mary home as his wife. 25But he had no union with her until she gave birth to a son. And he gave him the name Jesus.
http://www.biblegateway.com/passage/?book_id=47&chapter=1&version=31 the true story