Confused

Angelica

I went for my 20 week check up my dr said I'm 15 weeks, how could I be 15 weeks if my due date is December 4th??? Could she have been mixed up with my last visit a month ago? I also have a anatomy scan next week. They don't do anatomy scans before 18 weeks so I think shes way off. Should I call tommrow and make sure she knows how far along I am ??