The Virgin Birth Story

Where do you think the virgin birth story comes from? Do we have New Testament authors making this up to fulfill what they think a prophecy says when it actually doesn't? Is it a sort of narrative that maybe already exists in an oral tradition that's put into writing and then subsequently people look at this passage in the old in Isaiah and say, oh, well, maybe that's what it's referring to. One option is that somebody came up with it to say that it fulfills Isaiah 714. There are all of the options are really pretty interesting.