Isaiah had taught over one hundred years earlier: “Behold, a virgin shall conceive, and bear a son, and shall call his name Immanuel” (Isaiah 7:14). The Book of Mormon is an additional witness that Mary, the mother of the Son of God, was indeed a virgin, and that the means by which the Son of the Father was conceived and born was miraculous and transcendent.
“Can we speak too highly of her whom the Lord has blessed above all women? There was only one Christ, and there is only one Mary. Each was noble and great in preexistence, and each was foreordained to the ministry he or she performed. We cannot but think that the Father would choose the greatest female spirit to be the mother of his Son, even as he chose the male spirit like unto him to be the Savior.”
(The Mortal Messiah 1:326-27, n. 4.)