Mary referred to Our Lord has her Savior—but why would a sinless woman need a savior?
To make her sinless.
3. How could Mary be sinless if in the words of the Magnificat she said that her soul rejoices in God her savior?
The Church does not hesitate to profess that Mary needed a savior. This should be the first issue to address if this question arises. It was by the grace of God—and not the work of Mary—that she was saved from sin in a most perfect manner. By what is called “preservative redemption,” Mary was preserved from sin at the time of her natural conception. John the Baptist was sanctified in the womb prior to his birth (Luke 1:15), and Mary was sanctified at her conception. It is no difficulty that Christ distributed the grace of Calvary some forty-five years or so before it happened, just as he bestows it upon us two thousand years after the fact. The Catechism of the Catholic Church states that this gift was given to Mary, making her “redeemed in a more exalted fashion, by reason of the merits of her Son” (492). She has more reason to call God her Savior than we do, because he saved her in an even more glorious manner!
God can “save” a person from a sin by forgiving them, or by providing them the grace never to fall into that particular sin. An ancient analogy is often useful to explain this: A person can be saved from a pit in two ways; one can fall into it and be brought out, or one can be caught before falling into it. Mankind is saved in the first manner, and Mary in the second. Both are saved from the pit of sin. If Jesus wished to save his mother from the stain of sin, what is to prevent him?
http://www.catholic.com/thisrock/2001/0102sbs.asp
Mary was UNITED with Jesus physically in her womb and Jesus would have imperfection physically in Him if Mary had original sin in her