The BAR article, “How the Septuagint Differs,” BAR 02:02, states as follows:

In the first chapter of the Gospel according to St. Matthew, an angel comes to Joseph in a dream, telling him that his betrothed Mary is with child, conceived of the Holy Spirit, that she will bear a son who will save his people from their sins. “All this took place,” the Evangelist tells us “to fulfill what the Lord had spoken to the prophet (Isaiah): ‘Behold a virgin shall conceive and bear a son and his name shall be called Emmanuel’ (which means, God with us)” (Matthew 1:22–23) The passage which Matthew quotes is Isaiah 7:14 as it appears in the Septuagint, rather than in the Hebrew Bible. The difference in this passage from Isaiah between the Septuagint and the Hebrew Bible is explained by Père Benoit:

“The thought of the Evangelist is not in doubt: for him the oracle of the prophet (Isaiah) foretells that very virginal conception the story of which he has just told. And indeed the text of the Septuagint which he quotes and which contains the word parthenos (i.e., virgin) fully justifies his line of argument. But does the same apply to the Hebrew text? It is common knowledge that the term ’almah (the Hebrew word) does not mean specifically a ‘virgin’. For that, Hebrew has a special word, bethulah. The word ’almah designates a girl who is marriageable but not yet married, hence normally a virgin although this qualification is not expressly asserted.