The Scripture says, Isa 7:14 "Therefore the Lord himself will give you a sign. Behold, a Young Woman shall conceive and bear a son, and shall call his name Immanuel."
Before Christian theologians who came centuries later translated alma as virgin, the Septuagint, the rabbis' own translation of the Hebrew Scriptures into Greek, 280 years before Jesus' arrival on earth, translated Isa 7:14 almah into Greek as "parthenos" (virgin). Why would the Rabbis have done that???
According to history, the translation was done at the request of Ptolemy II Philadelphus (284-247 B.C.), he requested the "Laws" of the Jews translated into to Greek. At that time, there was a sizable Greek speaking Jewish community in Alexandria, Egypt. The translation of the rest of the Hebrew scriptures followed the Law. The "Almah" was translated to Greek word "virgin" by the Rabbis, not by Christians.
As in Genesis 24:16: "The maiden (na'arah) was beautiful, a virgin (betulah) whom no man had known" Obviously, betulah clearly and unequivocally meant "virgin" as she never had intercourse with a man. BUT it does not refer to the age of the girl which have to be supported by na'arah (young woman). The culture during 1200 B.C. is different from the modern day's usage of the word betulah. Hence Almah (young woman who is a virgin under Jewish law) is a better choice of word spoken by Jehovah God.
"Betulah" in Hebrew means "virgin" but does not refer to the age of the woman. Old woman who never married can also be betulah. As in Judges 21:12, 1Kings 1:2 & Esther 2:2 "betulah" have to be supported by another word "na'arah" (young).
Jdg 21:12 And they found among the inhabitants of Jabesh-gilead four hundred young(na'arah) virgins(betulah) who had known no man by lying with any male. ...
1Ki 1:2 Wherefore his servants said unto him, Let there be sought for my lord the king a young(na'arah) virgin (betulah): and let her stand before the king, and let her cherish him, and let her lie in thy bosom, that my lord the king may get heat.
Est 2:2 Then said the king's servants that ministered unto him, Let there be fair young (na'arah) virgins(betulah) sought for the king:
Est 2:3 And let the king appoint officers in all the provinces of his kingdom, that they may gather together all the fair young(na'arah) virgins(betulah) unto Shushan the palace, to the house of the women, unto the custody of Hege the king's chamberlain, keeper of the women; and let their things for purification be given them:
The Torahic law required that a young woman of marriageable age be a virgin. Deuteronomy 22:20-21 "But if thing(charge) be true, and the tokens of virginity were not found in the young woman (na'arah), then … the men of her city shall stone her to death." An almah (young woman) was apt to be a virgin at the time of her marriage under the Law!
Today in Israel moral standards have changed dramatically. In modern Hebrew, an alma means simply a young woman. But we are discussing Isaiah, who wrote during Biblical Hebrew's "golden age," 1200 BC to 500 BC, and so we must take the meaning that a word had at the time of Prophet Isaiah.
Moreover, Hebrew is a concise language in which much is taken from context. Since any young woman could conceive a child in the usual way and name him Immanuel, that would not be a miracle from God. However, a virgin birth would be a supernatural event.
Isaiah could have written that a betula would conceive and bear a son. The Hebrew word betula definitely means a virgin. But a betula is a virgin of any age. Up to that time, the sign of a miraculous conception had been an old woman bearing a child.
God had performed miraculous conception for old women like Sarah 90 years old & Elizabeth in her old age!!!
And so God, speaking through Isaiah, used the word alma to rule out a miraculous birth to an old woman and point instead to a miraculous birth by a (Almah)young woman in her virginity.