Looking at the verse Isaiah 7:14, when the Lord gave Ahaz a sign:
Therefore the Lord himself will give you a sign. Behold, a young woman shall conceive and bear a son, and shall call his name ImmanueIsaiah 7: 14
Jerome inquires as to the exact meaning of the term, young woman or ‘almah:
Gambero, L., 1999. Mary and the Fathers of the Church 1st ed. Translated by Buffer, T., San Francisco: Ignatius Press.