They check out all the little bits of your baby including spine, heart, valves, kidneys functioning, lips , gender if they can and if you want to know. It called the annomaly / morphology scan.
They really like to them betwwen 19 & 20 weeks as they need to look at a particular part of the heart during that gestation stage and this window offers them the best view, before the baby grows to much bigger, I think by memory ( I have had 3 ) they can only see the part of the heart during this stage before the valve closes / opens ....
Hope it goes well for you
Em