Adam Clarke on Matthew 1:20

That which is conceived (or formed) in her - So I think γεννηθεν should be translated in this place: as it appears that the human nature of Jesus Christ was a real creation in the womb of the virgin, by the power of the Holy Spirit. The angel of the Lord mentioned here was probably the angel Gabriel, who, six months before, bad been sent to Zacharias and Elisabeth, to announce the birth of Christ's forerunner, John the Baptist. See Luk 1:36.