Luke 1:47, . . . and my spirit rejoices in God my Savior. .
How can she call him "my savior" as in the present tense unless she was Immaculately concieved(saved from sin) to bear the divinity who became man. Remember the angel did not rip the curtain after Christ expired on the cross. Not yet. That action signified absolutely the old testament works system was over. But it did not happen until later from this statement.
So the old testament system was still in effect until that moment with the angel. She had to be sinless to bear the most High and Holy God that people died from being sinful in the past.
Thus she speaks" My Savior" as in the present tense being it already happened.
http://www.scripturecatholic.com/blessed_virgin_mary.html#the_bvm-IIhttp://www.scripturecatholic.com/blessed_virgin_mary.html#the_bvm-II
Why? Where is that in Scripture?
The only requirement is that she be virgin until the birth of Christ.