No embryo as yet

Fasha • Wife and Mother of 3 beautiful girls 11, 7, & 3 hoping for a boy dec 13.
So I had an ultrasound on Monday I was 6wks and it shows. Sac a yolk but no embryo? I was also measure 5wk 6days I'm not sure if I should be worried or that normal the dr still had not called me about any of the test I did or the ultrasound. I recently had a miscarriage at 18wks dec 24 of last year which almost took my life as well. And with that baby I did an ultrasound at 5wks 6days and their was an embryo with a heart rate of 95bpm idk what to thin. Has this happen to anyone of you ladies and it turns out the embryo came late?