But while he thought on these things, behold, the angel of the Lord appeared unto him in a dream, saying, Joseph, thou son of David, fear not to take unto thee Mary thy wife: for that which is conceived in her is of the Holy Ghost. 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 Luke 1:36.
Related Commentaries of Matthew 1:20
Matthew 1:20
20 But while he thought on these things, behold, the angel of the Lord appeared unto him in a dream, saying, Joseph, thou son of David, fear not to take unto thee Mary thy wife: for that which is conceivedb in her is of the Holy Ghost.