Therefore the Lord himself will give you a sign: The virgin will conceive and give birth to a son, and will call him Immanuel. Isaiah 7:14
“Were she not a virgin, the birth would not have been a sign. A sign is something that differs from the normal way things happen, that is outside the natural manner. A sign is so unusual and unexpected that someone who sees it or hears of it sees that it is out of the ordinary. It is called a “sign” because it is significant. Were the birth to be like normal births, it would not have been significant.”
John Chrysostom (349-407) was one of the Four Doctors of the Eastern Church and Archbishop of Constantinople. Commentary on Isaiah 7.5. SC 304:314. ACCS.I 64.