One of the early doctrines of the Church teaches us, and requires our belief, that Our Blessed Lady was conceived immaculate, free from all stain of sin, even original sin.  Since her body was to conceive and bear the Son of God, it would have been unbecoming for her even to have the slightest taint of sin, even although original sin is not a personal sin.

Picture: La Purísima Inmaculada Concepción by Bartolomé Esteban Murillo, 1678, now in Museo del Prado, Spain.