6 week scan no heartbeat

Hi all, I did an abdominal scan at the gynae at what I believe was 6 weeks based on LMP but the scan measured the gestational sac as 5 weeks, doctor said there is a yolk sac but no heartbeat found so will have to return 2 weeks later for another scan. It's measuring one week behind and now I'm worried that this is a missed miscarriage. Doesn't help that I'm 39 and this is my first ever pregnancy after trying for 2 years. Is it normal to measure behind time and no heartbeat at 6 weeks scan?