Adam and Eve our first parents opened the door with their first sin and tainted all of humanity with Original Sin. Well, God had a plan and that was to send His Son Jesus to save us. He chose to accomplish this through a new Eve: Mary. " Therefore the Lord himself shall give you a sign. Behold a virgin shall conceive, and bear a son, and his name shall be called Emmanuel." Isaiah 7:14 . One must ask how could the Lord of all Creation take up a home in the womb of a person with the stain of Original Sin? Jesus hadn't yet come to save us so there was no baptism available to wash away Mary's Original Sin. But never fear there was a greater plan and that was The Immaculate Conception. Her conception in the womb of St. Anne was miraculous because St. Anne was barren and quite old to begin with, but there was more. God choose to wipe away the stain of Original Sin in Mary at the moment of her conception...she was conceived without Original Sin. Not because she was without the need for a savior but that Our Savior chose to save her first.
A favorite book of mine is titled The Mystical City of God by venerable Mary of Agreda. A very detailed private revelation about the life of the Virgin Mother of God. Click the image below to listen to an audio version or download the book. Only the first book is currently in audio at the moment, but that's great because a story of the Immaculate Conception is in that part!