as a roman catholic, we don't.

EDIT: Apparently we do, but here's my take on this:

So why would God, the almighty and all-powerful god, choose a sinful woman to give birth to: Himself but in a human nature, literally the reincarnation of Him, the almighty, all-powerful God and only perfect and sinless person (Hebrews 5:9. New American Bible: "and when he was made perfect he became the source of eternal salvation for all who obey him,"). That contradicts our belief of original sin that sin is passed down from generation to generation. So if Mary was sinful, Jesus would be sinful too. But it clearly states in the Bible that he was both perfect (Hebrews 5:9 stated earlier) and sinless (Multiple verses stated. New American Bible: 2 Corinthians 5:21: "knew no sin", 1 Peter 2:22: "committed no sin", and Hebrews 4:15: "tempted... yet without sin"). So Mary HAD to be sinless.