8 weeks pregnant with twins and no heartbeat

i am worried sick... I went for a scan last week and I was 8 weeks 3 days pregnant (from last day of Period) the doc told me i had identical twins but no heartbeat for either and measuring only 6+ weeks... she said it's possible I conceieved later and could be 6+ weeks but wouldn't i still expect to hear or see the heartbeats? I know it's just a waiting game until I go back next week to see if any change but should I be preparing for the worst?? Xxx