Pope Bl. Pius IX solemnly defined the traditional belief of the Church: the Blessed Virgin Mary was conceived free from all stain of Original Sin.

Her Immaculate Conception is the most beautiful fruit of the work of redemption accomplished by her Son, Jesus Christ. Chosen to be the Mother of the Savior, God did not allow sin to contaminate her, which makes her a model of holiness of life for all Christians.
~ from the Proper of Saints, Daily Roman Missal