according to your findings the possibilities are,
- as your doctor told, if you had irregular periods
there is a chance that your ovulation may delayed leading to wrong estimation of gestational age
in this case you wait for two more weeks as your doctor told and then undergo ultrasound.
or you can go for transvaginal ultrasound
- may be it is a case of blighted ovum
where sac without fetal pole
will be present.
Don't worry you better wait for 1 to 2 weeks then undergo ultrasound which will rule out the cause, don't take tension which will affect your health and fetal condition.