First ultrasound at 6 and then not again until 12?

Annie
Hi! I just went to the doctor yesterday. I went as soon as I found out I was pregnant (1st baby and I'm 32) and I had my first ultrasound which showed only the sac, no baby yet. She measured at 6 weeks. 
They said we didn't need another appointment for another 6 weeks at week 12.  I may be a little paranoid about miscarriage but it seems like a long time to go before seeing a baby ultrasound to put my mind at ease. Is this just first timer's nerves?