that this child has been conceived in her.
She will bear a son and you are to name him Jesus,
because he will save his people from their sins.”
All this took place to fulfill
what the Lord had said through the prophet:
Behold, the virgin shall be with child and bear a son,
and they shall name him Emmanuel,
which means “God is with us.”
-- Mt 1:18-25
The Gospel accounts understand the virginal conception of Jesus as a divine work that surpasses all human understanding and possibility: "That which is conceived in her is of the Holy Spirit", said the angel to Joseph about Mary his fiancee. The Church sees here the fulfillment of the divine promise given through the prophet Isaiah: "Behold, a virgin shall conceive and bear a son."
-- CCC 497