Scientifically speaking it is impossible to conceive without male seed or human intervention. However, God is able to transcend the laws of nature and perform miracles.
Mary became pregnant by a direct act of God, as the Bible foretold (Isa. 7:14; Mt. 1:23) hundreds of years earlier. God and sin are opposites. Jesus is God, and he would never have been able to remain for nine months in the womb of a sinful person. Mary received a special grace from God which allowed her to remain without sin throughout her life.
