IVF (FET) - Due date changed due to the baby's measurements

Hi I had a successful FET (5 days) and if based on the embryo transfer date, i should be 6 weeks along. However, after my 1st scan yesterday, baby is only measuring at 2mm and they dated the pregnancy as 5 weeks instead. There was a heartbeat and everything else seemed fine. Gynae, however, did not sound too positive but he did arrange for another scan in 3 weeks to see if fetus grows . I cant help but to feel anxious :(