πŸ‘‘πŸ‘’ nub/ skull theory

Danielle
Went to my 12 week appt today and couldn't find heart rate with Doppler. I asked my doc, "at what point do we panic?" And she said, "we don't panic. We ultrasound."  This is my 12w1d u/s. I cannot find a "nub" and not too familiar with the skull theory. What are your thoughts??