Baby’s heartbeat

Hi I went for a scan at 5 weeks 6 days and doc said it’s too small to hear a heartbeat, scheduled to go back at 8 weeks 1 day..(am currently 6 weeks 2 days). However, am experiencing symptoms such as headaches and stomach cramps (moderate) and no bleeding... Usually when can a heartbeat be detected? And at how many weeks if there is no heartbeat, it is confirmed a loss..?