The doctrine of the Immaculate Conception originated out of confusion over how Jesus Christ could be born sinless if He was conceived inside of a sinful human female. The thought was that Jesus would have inherited a sinful nature from Mary had she been a sinner. In contrast to the Immaculate Conception, the biblical solution to this problem is that Jesus Himself was miraculously protected from being polluted by sin while He was inside Mary's womb. If God was capable of protecting Mary from sin, would He not be able to protect Jesus from sin? Therefore, Mary being sinless is neither necessary nor biblical.
The Roman Catholic Church argues that the Immaculate Conception is necessary because without it, Jesus would have been the object of His own grace. The thought goes like this – for Jesus to have been miraculously preserved from sin, which itself would be an act of grace, would mean God essentially “graced Himself.” The word grace means “unmerited favor.” Grace is giving someone something he or she does not deserve. God performing a miracle in preserving Jesus from sin is not “grace.” In no sense could Jesus possibly be infected with sin. He was perfect and sinless humanity joined with sinless divinity. God cannot be infected or affected by sin, as He is perfectly holy. This same truth applies to Jesus. It did not take “grace” to protect Jesus from sin. Being God incarnate, Jesus was in His essence “immune” from sin.
So, the doctrine of the Immaculate Conception is neither biblical nor necessary. Jesus was miraculously conceived inside Mary, who was a virgin at the time. That is the biblical concept of the virgin birth. The Bible does not even hint that there was anything significant about Mary’s conception. If we examine this concept logically, Mary’s mother would have to be immaculately conceived as well. How could Mary be conceived without sin if her mother was sinful? The same would have to be said of Mary’s grandmother, great-grandmother, and so on. So, in conclusion, the Immaculate Conception is not a biblical teaching. The Bible teaches the miraculous virgin conception of Jesus Christ, not the immaculate conception of Mary.
http://www.gotquestions.org/immaculate-conception.html