depending on your doctor many don't do the early scan unless there have been previous problems or you are unsure of your dates. The Nuchal scan is not mandatory and again depends on your thoughts, your doctor, and your age. I believe if you are in the high risk category (over a certain age) they will recommend it but under that it depends - I don't know that the public system does it unless you are high risk. Then the morphology scan at 18 weeks (or there abouts) is I believe a standard where they check absolutely everything they can out to make sure bubs is growing correctly - in reality the only "have to have" so to speak (not that you have to have anything but I'm sure you know what i mean. Any scans after that are dependent on yours and babys health - if everythings is fine you won't get anymore. If you are with an OB that has a scan machine in their office quite often they will do one at a lot of visits just to give you a peek and see how things are going.

Hope this makes sense someone else may be able to clarify further - especially the "requirements" for the NT scan