ultra sounds. first pregnancy.

I have a question for you ladies. 
Okay so at my first appointment at 7 weeks I had an ultra sound done.
But when I went in at 12 weeks to get checked they just checked the heart. And told me we won't be doing another ultra sound till in 18-20 weeks.
Is that normal??
This is my first pregnancy I'm 21years old and well Im not sure what to expect and I'm just curious.
Like how is he going to know that my baby is growing properly if he doesn't do an ultra sound to see the baby's developement.  I understand you can hear the heart beat but still I would just like to know that everything is fine.