Ultrasound waveform question.

Natalie • 2 losses and 1 beautiful daughter. Pregnant with #2 and praying to make it all the way.
Help! I should have asked this at the appointment but didn't and now have myself worried. Had a 6 week and 8 week ultrasound and saw the little flickering heartbeat. (Tears of joy). However, the waveform for the heart beat was pretty much flat and not like a typical waveform that has deep peaks and valleys. Also, we didn't hear a heartbeat. What does that mean? It was measured at 166bpm which is great, but the flat waveform makes me worried. Anyone know anything about ultrasounds?