My first pregnancy ,I had the normal 2 Ultrasounds.... 12 weeks & 20 weeks,No problem but apart from abit of etc fluid which they were not worried about.

Second pregnancy I had Gestational Diabetes,so they had to keep on eye on the baby by ultrasound and I had one at 28 weeks because baby was big.

This pregnancy I had to have extra GTT test because of my previous Diabetes and my Final test at 31 weeks have come back clear,but even though it was clear and i was worried about how big i was still,the doctor said he would put me in for an Ultrasound to put my mind at rest...good job he did because they found i am carrying excess fluid again,but alot more then my first pregnancy and need to keep an eye on it because there COULD be risks.

So yes they only do Ultrasounds after 20 weeks,if they feel they need to keep an eye on the baby for lots of reasons:

Baby too small
Baby too big
Excess Fluid
Not enough Fluid
Gestational Diabetes
Previous miscarrieges or abnormalities
Placenta too low etc.