usually by 6 weeks in trans vaginal ultrasound
fetal cardiac activity can be seen and in trans abdominal scan it has to be noted by now usually, if you undergone trans abdominal scan you can wait for one more week as your doctor told for safe side,
but according to your gestational age
by last menstrual period there is lag of 3 weeks in scan findings,
so if your dates are accurate there is possibility for growth of the fetus stopped by 7th week due to demise, but if you are not sure of dates then you can take the scan findings into consideration and wait for one week.
for more details on antenatal scan you can read my article,