Teachings on the Immaculate Conception

Dogma:

Mary was conceived without stain of original sin. (De fide)

By conception is to be understood the passive conception. The first moment of the conception is that moment of time in which the soul was created by God and infused into the bodily matter prepared by her parents.
The essence of original sin consists in the lack of sanctifying grace, in consequence of the fall of Adam. Mary was preserved from this defect, so that she entered existence in the state of sanctifying grace.
Mary's freedom from original sin was an unmerited gift of God, and an exception from the law which was vouchsafes to her only.
The efficient cause of the Immaculate Conception of Mary was Almighty God.
The meritorious cause was the Redemption by Jesus Christ. It follows from this that even Mary was in need of redemption, and was in fact redeemed. By reason of her natural origin, she, like all other children of Adam, was subject to the necessity of contracting original sin, but by a special intervention of God, she was preserved from stain of original sin. Thus Mary also was redeemed "by the grace of Christ" but in a more perfect manner than other human beings. While these are freed from original sin present in their souls, Mary the Mother of the Redeemer, was preserved from the contagion of original sin. Thus the dogma of the Immaculate Conception of Mary in no way contradicts the dogma that all children of Adam are subject to original sin and need redemption.
The final cause of the Immaculate Conception of Mary is her Motherhood of God.*