The most Blessed Virgin Mary was, from the first moment of her conception, by a singular grace and privilege of almighty God and by virtue of the merits of Jesus Christ, Saviour of the human race, preserved immune from all stain of original sin. 135
Catechism number 504
Jesus is conceived by the Holy Spirit in the Virgin Mary's womb because he is the New Adam, who inaugurates the new creation: "The first man was from the earth, a man of dust; the second man is from heaven."162 From his conception, Christ's humanity is filled with the Holy Spirit, for God "gives him the Spirit without measure."163 From "his fullness" as the head of redeemed humanity "we have all received, grace upon grace."164