The Lord himself will give you a sign:  A virgin will be with child and bear a son and she will call his name Immanuel.  Isaiah 7:14  written 700 b.c.

In the sixth month, God sent the angel Gabriel to Nazareth, a town in Galilee, to a virgin pledged to be married to a man named Joseph, a descendant of David.  The virgin's name was Mary.  The angel went to her and said, "Greetings, you who are highly favored!  The Lord is with you."  Mary was greatly troubled at his words and wondered what kind of greeting this might be.  But the angel said to her, "Do not be afraid, Mary, you have found favor with God.  You will be with child and give birth to a son, and you are to give him the name Jesus.  He will be great and will be called the Son of the Most High.  The Lord God will give him the throne of his father David and he will reign over the house of Jacob forever.  His kingdom will never end."  "How will this be," Mary asked the angel, "since I am a virgin?"  The angel answered, "The Holy Spirit will come upon you and the power of the Most High will overshadow you.  The Holy One to be born will be called the Son of God.  Luke 1:26-35  

This 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.  Because 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.  But 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.  She will give birth to a son and you are to give him the name Jesus because he will save his people from their sins."  Matthew 1:18-21

Even Mary wondered how it was possible for a virgin to become pregnant.