according to IVF timing the gestational age
should be around 6 weeks,
but it is more on scan, so the possibilities of twins or abnormal pregnancies like molar pregnancy
are there but as HCG
levels are within normal range there is possibility for multiple gestation like twins.
by this time cardiac activity should be noticed, but it is not appeared yet.
so you can repeat the scan in the next week to rule out above possibilities, transvaginal scan will detect heart beat earlier than transabdominal scan.