«The verses surrounding Isaiah 7:14 tell how Ahaz, the king of Judah, is told of a sign to be given in demonstration that the prophet's promise of God's protection is a true one. The sign is that an almah will give birth to a son who will still be very young when Judah's enemies will be destroyed. Most Christians identify the almah of this prophecy with the Virgin Mary. In Isaiah 7, the almah is already pregnant, and modern Jewish translators have therefore rendered almah here as "young woman". The Septuagint translation of the Hebrew Scriptures, which was completed in the late 2nd century BCE, translated almah into Greek as παρθένος (parthenos). Many scholars render parthenos into English as virgin. However, the Septuagint also describes Dinah as a parthenos even after she has been raped and hence is no longer a virgin.»Almah - Wikipedia, the free encyclopedia
If the word Almah really meant B'tulah, then, the traditional understanding IMHO would have been that a B'tulah gave birth as a result of her first co-habition - which in Rabbinic literature is unusual due to the interference of the B'tulim.
It would typically NOT imply that she was still a B'tulah AFTER the pregnancy. And I don't think the ancient Jews would have seen it that way at all.