Isa. 7:14 (ESV)The interesting thing - if you read Hebrew - is that Isa. 7:14 does not use the common word specifically denoting a "virgin." Some say that this ambiguity shows that Isaiah didn't really have a miracle in mind. He wasn't thinking that a real virgin would conceive because he used a word that simply means "young woman." In fact, there was quite a fuss back when the RSV came out and translated Isa. 7:14 with "young woman" instead of "virgin." The translation itself was undermining
Therefore the Lord himself will give you a sign.
Behold, the virgin shall conceive and bear a son, and shall call his
The problem is that the words for "young woman" and "virgin" in Hebrew and Greek overlap quite a bit in their semantic fields. The Hebrew word for "young woman" in Isa. 7:14 is 'almah. The more typical word that appears to mean "virgin" more specifically is betulah. The Septuagint (LXX) translated Isa. 7:14 using the word parthenos "virgin" - the more usual equivalent for betulah. So, the connotation of virginity in Isa. 7:14 entered the text more explicitly with the LXX, which was used in turn for the quotation in Matt. 1:23.
In an attempt to understand the semantic fields of 'almah and betulah more fully, I examined every occurrence of each in the Hebrew Bible and noted its rendering in the LXX. The first occurs only 7 times; the second occurs 50 times.
The LXX renders 'almah in two different ways - 4x with neanis "young woman", 1x with neotes "youth", and 2x with parthenos "virgin." On the other hand, betulah is rendered by parthenos 43 out of 50 times. Of the remaining seven, several are left out in translation and some use yet another word such as korasion "girl."
The usage of these two Hebrew words doesn't provide enough information to draw a line between them and say betulah implies virginity and 'almah is ambiguous. Both words are used to refer to unmarried young women who are either betrothed or eligible to be betrothed. The fact that they are eligible for marriage implies they are assumed to be virgins. We can't say betulah inherently implies virginity because the Hebrew writers felt compelled to make that explicit at times, following betulah with a phrase clarifying "who has not known a man" (see Gen. 24:16 and Judges 21:12 for examples).
The ESV, which seems to want 'almah to be "virgin" consistently in its translation, often uses "young woman" or "maiden" to render betulah. This usage highlights a similar overlapping semantic range in English. If you look up "maiden" and "virgin" in an English dictionary, you find that the primary meaning of "virgin" is a "person who has never had sexual intercourse", but the secondary meaning is "an unmarried girl or woman." Likewise, the word "maiden" refers to a "girl or unmarried woman" in its primary meaning, but "virgin" is a secondary meaning.
The problem is that most of the uses of 'almah and betulah don't provide enough context to determine whether a distinction was intended between "young woman" and sexual "virgin." My sense is that 'almah implies virginity because of the positive overtones of eligibility for marriage versus the shame and censure (for a woman) of extramarital sex. I finally found a pair of references indicating that 'almah and betulah are more or less synonymous. We think betulah is more specifically "virgin" simply because it occurs more frequently than 'almah.
Genesis 24 tells the story of how Abraham's servant found a wife for Isaac - Rebekah. Immediately after the servant had prayed that God would show him the right woman, he sees Rebekah coming out for water.
Gen 24:16 (ESV)After the servant meets Rebekah and gets invited into her home, he retells the story of their meeting to her father and brother.
The young woman (na'arah) was very attractive in appearance, a maiden (betulah) whom no man had known. She went down to the spring and filled her jar and came up.
Gen 24:43 (ESV)I think it's telling that this story presents the same event twice and uses both betulah and 'almah to refer to Rebekah. Both terms referred to an unmarried virgin woman.
Behold, I am standing by the spring of water. Let the virgin ('almah) who comes out to draw water, to whom I shall say, "Please give me a little water from your jar to drink"
In Isa. 7:14, the LXX is simply drawing out a logical connotation of the meaning of 'almah. From the perspective of Matthew, that explicit detail was precisely the right interpretation.
Update: Ben Witherington also posted on the Virgin Birth on Dec. 12 and addressed some of the same issues of terminology that I've raised. I skimmed his post back then but hadn't looked at it again before finishing my post. If you're interested in more on this issue, go there.