When to hear heartbeat

Hi, on my last ultrasound it said I was 6w5 days (2 days ago) and there was a yolk Sac and gestational sac but no heartbeat yet, is that normal? The doctors think I’m a bit earlier than that though like early 6 weeks, would that have been to early to hear the heartbeat? I go back next Wednesday to see if we can hear one, I’m just so anxious there’s something wrong..I should be around 7 weeks next week