I'm 13 weeks currently, but when I had my first ultrasound at 5 weeks, all we were able to see was my uterus slightly enlarged and it looked like a black jellybean on the screen, lol. No sign of fetus or yolk sac..I was worried, but my doc assured me that I'd need to be further along to see more. You'll be okay, because I am! :) Congrats, btw!
It depends because fetal ultrasound is frequently done during the first trimester to verify and date the pregnancy and also in the second trimester which is between 18 and 20 weeks, when anatomic details can be seen. Consult first with you doctor, and you may find more details here http://www.mayoclinic.com/health/fetal-ultrasound/MY00777.