From The Encyclopedia of Mormonism
Author: Colton, Eleanor
Mary, mother of Jesus Christ, was a virgin at the time of Jesus' birth. Of Old Testament prophets, isaiah alone foretold this circumstance (Isaiah 7:14), but Book of Mormon prophets also foresaw the virgin birth. Nephi 1 described Mary as "a virgin, most beautiful and fair" and "mother of the son of God, after the manner of the flesh" (1 Ne. 11:15, 18). Alma declared that Christ "shall be born of Mary a virgin who shall conceive by the power of the Holy Ghost and bring forth a son, yea, even the Son of God" (Alma 7:10).
In fulfillment of these prophecies, Gabriel "was sent from God to a virgin and the virgin's name was Mary," and Gabriel announced to her that she would "bring forth a son, and call his name Jesus." To her question, "How shall this be?" Gabriel answered, "The Holy Ghost shall come upon thee therefore [the child] born of thee shall be called the Son of God" (Luke 1:26-35). Thereafter, Joseph married Mary but "knew her not till she had brought forth her firstborn son" (Matt. 1:25). Thus, Jesus was born of a mortal mother who was a virgin. [See also Immaculate Conception; Mary, Mother of Jesus.]
McConkie, Bruce R. The Promised Messiah, pp. 465-66. Salt Lake City, 1978.