I had my first at 5.5 weeks. Only because i had really bad cramping and they had to rule out an ectopic pregnancy but im pretty sure the next is the 12 week scan for the NT scan to rule out downs syndrome and other abnormalities.

I too couldnt go until 18 weeks without having an ultrasound.

You should talk to them about having one in the next couple of weeks, even just to see the babys forming the way it should be.