It seems clear as a matter of scriptural fact that Mary was immaculately conceived: Gabriel, who is in a position to know, said as much, in Luke 1:28. He could not have noticed or said that she was full of grace if there had been a jot of sin in her anywhere; for, being a defect of being, sin is an emptiness – a defect of fullness of grace. OK, so far so good.
But that I submit is not the real nub of the issue. It is, rather this: stipulated that Mary was indeed free of Original Sin from her very conception, why was that necessary? Why was it necessary that the Mother of God should be without spot or stain of sin? As Gabriel went on to say in Luke 1:37, all things are possible with God; so, why couldn’t God have raised up his Son from a sinful woman, or for that matter from some stone? Matthew 3:9.