Mary understood that the angel Gabriel was not talking about the eventual children that she would have after her marriage to Joseph. She knew that the angel was talking about something miraculous which was birth without an earthly father being involved in the process. The angel explained that the baby would be conceived miraculously by the power of the Holy Spirit. So that even greater then being the Son of David, the Messiah would be the Son of God. The angel points to Elizabeth miraculous pregnancy in her advanced age to encourage Mary’s faith, as God is a God of miracles.