9 weeks pregnant, how soon can Dr hear the heartbeat w/ a fetal doppler?

I saw the little heartbeat on an ultrasound at six weeks, and i went in today, but they couldn't hear anything w/ the doppler. (9 1/2 ) weeks. The doctor told me to come in Thursday for another sono to see if he can visually see it at least. I'm still freaking out though. I have very little fat on my abdomen, as I've heard that plays a role too.... has this happened to anyone? How soon did u hear it?