The best evidence of the virgin birth is to be found in the Scripture record.
Consider first of all the immediate reaction of Mary to Gabriel's announcement that she would conceive a son whose name would be Jesus (Luke 1:31). Mary responded: "How can this be since I am a virgin?" (Luke 1:34). Mary may have been a very young woman (probably about 13 years old), but she knew how a baby was conceived, and she therefore knew it was not possible for her to have a child. That's why Gabriel responded by declaring, "Nothing is impossible with God" (Luke 1:37).
Further evidence of Mary's innocence is to be found in the first action she took after she became pregnant (Luke 1:39-40). She "went with haste" to share the news with some relatives! Now, how many unwed pregnant girls have you ever seen behave in that manner? They are usually so overcome with guilt and shame that they want to hide. They certainly don't want to share the news with relatives!
Even more significant is the kind of relatives Mary selected. The head of the household was a priest by the name of Zacharias (Luke 1:5). Keep in mind that although Mary was only betrothed to Joseph and had not yet consummated the marriage, she was considered married to him. If she became pregnant during the betrothal period, she was considered an adulteress and was thus subject to stoning or divorce. In other words, she faced death or humiliation. Yet, she ran to a priestly family to share the news of her pregnancy. If she had been guilty of adultery, it would have been the responsibility of Zacharias as a priest to report her. This evidently did not concern Mary because she went in innocence.
Further evidence of Mary's innocence is found in the description of her arrival at the house of Zacharias. His wife, Elizabeth, who was six months pregnant with John the Baptist, felt the baby leap in her womb with joy when Mary entered the house! (See Luke 1:41 & 44.) Also, Elizabeth was "filled with the Holy Spirit" (Luke 1:41) and immediately spoke a supernatural word of knowledge. She cried out: "Blessed among women are you, and blessed is the fruit of your womb!" (Luke 1:42). She then declared Mary to be "the mother of my Lord" (Luke 1:43).
The final bit of evidence of Mary's innocence is presented by Luke in verses 45 through 55. He records a glorious song which Mary sang to the Lord in the presence of Elizabeth. It begins with the words, "My soul exalts the Lord, and my spirit rejoices in God my Savior." How many unwed pregnant girls have you ever heard sing a song like that?