How do we know Mary was a virgin at the time of Jesus' conception?

[Luke 1:26-26](http://www.biblegateway.com/passage/?search=Luke%201-2&version=NIV) : > **26** In the sixth month of Elizabeth’s pregnancy, God sent the angel Gabriel to Nazareth, a town in Galilee, **27** to a virgin pledged to be married to a man named Joseph, a descendant of David. The virgin’s name was Mary. Many other Bible verses talk about Mary's virginity, even to the point that the Catholic church teaches she was a virgin her entire life*. What evidence exists (either Biblical or extra-Biblical) to support the claim that she was indeed a virgin at the time of Jesus' conception? *This question is _not_ asking whether Mary was a perpetual virgin; only for evidence that she was a virgin at the time Jesus was conceived.