Luke 1:34 - Ellicott's Commentary On The Whole Bible

How shall this be? — The question of the Virgin is not altogether of the same nature as that of Zacharias in Luke 1:18. He asks by what sign he shall know that the words were true which told him of a son in his old age. Mary is told of a far greater marvel, for her question shows that she understood the angel to speak of the birth as antecedent to her marriage, and she, accepting the words in faith, does not demand a sign, but reverently seeks to know the manner of their accomplishment.