The Jews who speak Hebrew explain it in detail on this site (and after all, this is their shit), but, in a nutshell:
Lets just say that, unless Miryam was preggers for 700 years, the 'young girl' who was 'with child' is not a reference to Yeshua (Jebus) or Miryam (Mary):
http://www.hebrew-streams.org/works/...shayah714.html