Need reassurance - no heartbeat at 5w4d

My hcg levels more than doubled within 48 hours, which is great. I only saw the sac and yolk sac when i went for my first ultrasound. I am reading so many conflicting statements online on when you should see it for the first time. My doc said we should see it this week at 6w4d. I can't sleep. I am so worried. Had 3 miscarriages, but never made it so far. Do you have positive stories to tell with no heartbeat at 5.5 weeks, but a heartbeat a week later? Thank you!

Update: saw the heartbeat today at 6w4d (127 beats per minute). Thank you all for your reassurance. ♡♡♡