Do you believe this because of something from scripture or because of oral tradition?
Becky
Both Scripture and Tradition tell us that Mary was "full of grace" or "perfected by grace," or sinless. Typology (Mary as the Ark of the New Covenant, Jesus) also tells us that Mary must have been conceived without sin. Since Christ's death on the Cross redeems everyone including those who lived before the time of Christ, it is only logical to conclude that Jesus' redeeming obedience to His Father, his death on the Cross, saved Mary from the effects of original sin.
Put another way, there is no logical reason not to believe that Jesus couldn't have saved Mary from the effects of original sin "before the fact."