According to Roman Catholic doctrine, why does Mary refer to God as her Savior if she was born without sin? In the Magnificat, Mary refers to God as her Savior.  According to Roman Catholic doctrine, Mary was born sinless and remained sinless throughout her life.

So, why does Mary refer to God as her Savior if she was sinless (and therefore would not have need of a Savior or even have one).


  And Mary said:
  
  “My soul exalts the Lord, And my spirit has rejoiced in God my
  Savior.  For He has had regard for the humble state of His
  bondslave;  for behold, from this time on all generations will count me
  blessed.  For the Mighty One has done great things for me; and holy
  is His name. And His mercy is upon generation after generation Toward
  those who fear Him.  He has done mighty deeds with His arm; He has
  scattered those who were proud in the thoughts of their heart. He
  has brought down rulers from their thrones, And has exalted those who
  were humble. He has filled the hungry with good things; And sent away
  the rich empty-handed. He has given help to Israel His servant,  In
  remembrance of His mercy,  As He spoke to our fathers, To Abraham and
  his descendants forever.” (Luke 1:46-55)
Credit to H3br3wHamm3r81 who actually found this link.



According to Catholic Answers, it was because of God's salvation that she was able to be born sinless and remain sinless. 

Paraphrasing, God saved her pre-emptively, preventing (or perhaps keeping/preserving) her from sinning in the first place, something no human could do without God's protective saving power.  In other words, in Catholic doctrine, she was sinless, but only through the Grace of God.  Without the Grace of God and His keeping power, she could not have remained sinless.

Excerpt:


  The Catholic Answer
  
  Not a few Protestants are surprised to discover the Catholic Church
  actually agrees that Mary was "saved." Indeed, Mary needed a savior!
  However, Mary was "saved" from sin in a most sublime manner. She was
  given the grace to be "saved" completely from sin so that she never
  committed even the slightest transgression. Protestants tend to
  emphasize God’s "salvation" almost exclusively to the forgiveness of
  sins actually committed. However, Sacred Scripture indicates that
  salvation can also refer to man being protected from sinning before
  the fact: