The Immaculate Conception is the work of God. God doesn’t have a timeline like we do. Mary was forgiven the original sin BEFORE she was born since God and Jesus could see IN ADVANCE that she would be the mother of Jesus. thus providing a pure womb to house the Ark of the New Covenant.
You are thinking as people think — have you ever tried to think as God thinks. He could see into the future — Christ dying on the Cross — when the origianl sin of Mary was forgiven IN ADVANCE of her conception. Thus we have the Immaculate Conception.
I'm thinking as God has chosen to tell us to think because he told us in Holy Scripture. I don't think something important would have been omitted from it and only revealed to fallible humans. I also do not think that Mary's virginity wasn't enough to provide the type of womb God would use to be born as a man. Her womb was a human woman's womb, probably having experienced her monthly cycle for years - shedding the blood lining of her uterus just like most all women do. If it is only her "womb" that had to be pure, then why did her human soul also have to be kept from ever sinning? She was born as any other human was born, with a human father and mother, doing what men and women do to get pregnant. Those born that way inherit the sin nature from Adam. There's no getting around that and there was no need for the mother of the Messiah to be "sinless", just a virgin of the house of David.
See, this is the real problem I have been talking about. We get in these long convoluted discussions over something God NEVER said happened. If you want to believe in it, go ahead, just don't tell those who don't that they are wrong, not "real" Christians, "missing out", "misguided" or "misinformed". There is no real ground to stand on.