Why was Jesus born from a virgin? Why not just a woman who had all ready given birth to a child, or did Mary pray for it perhaps?
The mortal mission of Jesus Christ was of such a nature that it could never have been performed by a mere human. It required the power of a God to complete it. In the beginning of the story there was a great war in heaven. It was the desire of the great God of heaven, the Father of the spirits of all mankind, to bring back into His presence following their mortal sojourn all who could qualify to do so, and so He called from among His spirit children for a plan for its accomplishment. One of the spirits, a son of the morning, whose name was Lucifer (Light Bearer) offered a plan to bring all of God’s children back into his presence, without losing even one. He proposed—
Behold, here am I, send me, I will be thy son, and I will redeem all mankind, that one soul shall not be lost, and surely I will do it; wherefore give me thine honor (Moses 4:1).
A counter proposal was offered by Jehovah, the first born Son of God in the spirit world, H e said—
Father, thy will be done, and the glory be thine forever. (Moses 4:2)
The Father chose the plan of His First Born Son, and Lucifer, or Satan, rebelled—
Wherefore, because that Satan rebelled against me, and sought to destroy the agency of man, which I, the Lord God, had given him, and also, that I should give unto him mine own power; by the power of mine Only Begotten, I caused that he should be cast down;
And he became Satan, yea, even the devil, the father of all lies, to deceive and to blind men, and to lead them captive at his will, even as many as would not hearkenunto my voice. (Moses 4:3-4)
Among the qualities of God are both justice and mercy. If He were to extend his mercy and bring back into His presence all His children, regardless of the kind of lives they had lived, it would be unfair to the righteous, who had lived according to His will and therefore deserved His blessings, and justice would not be serve. If He were to exercise justice, then all those who had violated any of the laws of God could never return to His presence. However, the plan of the First Born Son was to offer Himself as a sacrifice for sin in order to satisfy the demands of justice, thus allowing the Father to extend His mercy to all those who had sinned, but who had overcome their sins by repentance and good works. Thus, justice would be satisfied and the Father could extend His forgiving mercy to all who had repented from their sins, and bring them back into His presence.
So God became the very literal Father of the babe in Bethlehem, and thus Jesus was imbued with the power of God from his Eternal Father, and with the constraints of mortality (the requirement of death) from His mortal mother. Thus He took upon Himself all of the sins of all mankind, and suffered for them sufficient to satisfy the demands of an eternal justice. Therefore, because of the infinite number of our Father’s children, His suffering had to be infinite in its extension, as recorded in the scriptures—
And lo, he shall suffer temptations, and pain of body, hunger, thirst, and fatigue, even more than man can suffer, except it be unto death; for behold, blood cometh from every pore, so great shall be his anguish for the wickedness and the abominations of his people. (Mosiah 3:7)
For behold, I, God, have suffered these things for all, that they might not suffer if they would repent;
But if they would not repent they must suffer even as I;
Which suffering caused myself, even God, the greatest of all, to tremble because of pain, and to bleed at every pore, and to suffer both body and spirit–and would that I might not drink the bitter cup, and shrink–
Nevertheless, glory be to the Father, and I partook and finished my preparations unto the children of men (D&C 19:16-19)
Elder Neal A Maxwell spoke of that suffering in these terms—
“Several scriptures describe the essence of that glorious and rescuing Atonement, including a breathtaking, autobiographical verse confiding how Jesus would that I might not drink the bitter cup, and shrink (D&C 19:18). Since the infinite atonement required infinite suffering, the risk of recoil was there! (2 Ne. 9:7; Alma 34:12). All humanity hung on the hinge of Christ’s character! Mercifully, He did not shrink but finished [His] preparations unto the children of men” (D&C 19:19)..(Elder Neal A. Maxwell, Plow in Hope, Ensign (CR), May 2001, p)
The virgin nature of Jesus’ birth was required because His Father was the very God of Heaven and He could not be defiled.