Update Dr appt yesterday

So I had my Dr appointment yesterday and everything was good. All test were good, but my one concern is my 20 week anatomy scan, so my doctor says I’m measuring 29 weeks and I’m only 23 weeks 😳 I’m a little confused, is this a really big baby or has he already been cooking. She says she not going to move my due date until she measures me again on 8/22. Has anyone else experienced this??? I just want to know what to expect because this is my first baby and I’m terrified…..