thanks for your query.
As per your last menstrual period, you are right.
The gestational age
should have been more than 8 weeks.
But there are two possibilities :
One is that you had a late conception due to delayed ovulation, which would show up in future on repeat scans as a continuing normal viable pregnancy
, apart from the discrepancy with the menstrual age.
The second is that this is a failed pregnancy, which is not growing normally.
This would also show up if the serial growth of the fetus does not occur on repeat scanning.