CCC 488 “God sent forth his Son”, but to prepare a body for him,1 he wanted the free co-operation of a creature. For this, from all eternity God chose for the mother of his Son a daughter of Israel, a young Jewish woman of Nazareth in Galilee, “a virgin betrothed to a man whose name was Joseph, of the house of David; and the virgin’s name was Mary”:2
The Father of mercies willed that the Incarnation should be preceded by assent on the part of the predestined mother, so that just as a woman had a share in the coming of death, so also should a woman contribute to the coming of life.3

1 Gal 4:4; Heb 10:5.
2 Lk 1:26-27.
3 LG 56; cf. LG 61.