Sometimes you can get an early sacn (around 7 weeks) to date the pregnancy but if you are fairly sure of your dates your Dr may not get it done. They also do early scans if you have bleeding to see what's going on. I hope you won't need one of them. Sometimes if women are anxious about a pregancy they will also do a scan just for reassurance.
At about 12 weeks you can get an NT scan which along with a blood test checks the odds of downs syndorme. If you wouldn't terminate a pregnanacy based on the results there may not be much point getting it.
At around 18-22 weeks they do a quite detailed diagnostic scan to check for a whole range of things (amount of amnio fluid, position of placenta, cleft palate, growth of baby, babies organs).
Depending on the findings of this scan they may scedule more for later. For example I had a low lying placenta at my 18 week scan so today I have one to see if its moved up.
Some obs will do a quick scan at every visit but they are not so well trained in using the ultrasound as the technicians who do it at the diagnostic imaging places so mainly its just to check that every thing looks like it is progressing.
Although there aren't really any known side effects from US quite a few people prefer to keep thier use to a minimum whilst others get choose to get non-medical extras done.
Not all Drs follow the same pattern but at an absolute minimum they will all send you for the 18-22 week one.
HTH.