“Therefore the Lord himself shall give you a sign; Behold, a virgin shall conceive, and bear a son, and shall call his name Immanuel” (Isaiah 7:14).
While Christendom speaks of the “virgin birth of Christ,” according to today’s Scripture, a more accurate term would be the “virgin conception of Christ.” There was nothing unusual about Christ’s birth; it was His conception that was unique because there was no human father!
Interestingly, today’s Scripture has been the point of controversy for over a century (to Satan’s delight!). Some modern Bibles (RSV, NRSV, et al.) translate the Hebrew word here translated “virgin” as the vague “young woman,” thereby leaving room for the heretical idea that Joseph was Jesus’ biological father (and denying Christ’s deity)! If someone ever tells you almah (the Hebrew word translated “virgin”) can mean “young woman” or “virgin,” they are right, but point out that the key to choosing the right translation is not up to a translator, but rather the Holy Ghost!
The author of Matthew’s Gospel, filled with the Holy Ghost, knew which translation—“young woman” or “virgin”—was what God had intended in Isaiah 7:14. If we want to know what God meant in Isaiah 7:14, why not ask God?
“Now all this was done, that it might be fulfilled which was spoken of the Lord by the prophet, saying, “Behold a virgin shall be with child…” (Matthew 1:22,23a). The Greek word translated “virgin,” parthenos, can only mean “virgin,” not “young woman.” Isaiah was prophesying a virgin, indicated by the words “firstborn son” (Matthew 1:25; Luke 2:7) and “Joseph knew her not till she had brought forth her firstborn son” (Matthew 1:25). Isaiah 7:14 meant “virgin,” as indicated by Luke 1:34, for Mary “knew not a man.” Again, the Bible is clear that Joseph was not Jesus’ biological father.
Matthew 1:23 indisputably proves that almah in Isaiah 7:14 did not simply mean a “young woman,” who may or may not be sexually pure, but “a virgin,” a woman who never had any sexual intercourse. The Holy Ghost, not Joseph, was the Father of Jesus’ body (Matthew 1:18-20). source