Anxious...7+2, but doctor says fetus is 6+1

Went to see my gynecologist and she said the fetus is no more than 4,1mm(Which is 6+1 days), when it should have been around 10-11mm.. Didn't see any heartbeat either.. I am scared to death that it could be a missed abortion. Anyone experienced something like this?? And it came out well..? Could it be that my doctor just calculated wrong. I know 100% that i had sex on 27th of desember, same day as my ovulation. And maybe concieved around 30dec-1-2jan.. Calculated from that date, it seems right that the fetus only is 6 weeks and something.. But still... I have to wait a whole week for the result of HCG levels and a new ultrasound..☹ been trying to get pregnant for two years..... All words are appreciated❤️