Like all other descendants of Adam, she was subject to the necessity of contracting original sin. But by a special intervention of God, undertaken at the instant she was conceived, she was preserved from the stain of original sin and its consequences. She was therefore redeemed by the grace of Christ, but in a special way—by anticipation.
The Catechism of the Catholic Church states that she was "redeemed in a more exalted fashion, by reason of the merits of her Son" (CCC 492). She has more reason to call God her Savior than we do, because he saved her in an even more glorious manner!