HOW?

And Mary said to the angel, “How will this be, since I am a virgin?” And the angel answered her, “The Holy Spirit will come upon you, and the power of the Most High will overshadow you; therefore the Child to be born will be called holy—the Son of God. And behold, your relative Elizabeth in her old age has also conceived a son, and this is the sixth month with her who was called barren. For nothing will be impossible with God.” (Luke 1:34-37)