Hi Ellie, it's lots and lots of measuring!! Mine went for 1.5 hours... and believe it or not, I got bored....I never thought that would happen!! But they check all the parts of the heart, brain, blood flow etc etc. Limb, bone measurements etc. They also found cysts on Coco's brain, which I now know is perfectly normal, but at the time I didnt' and there was no one to talk to us afterwards, so I strongly suggest you have someone to sit with after and go through the scan with. There are usually Dr's there to do that but for some reason we ended up at a place that they didnt' do that, even though it was a very prestige hospital in Syd. I am still astounded as to why we didnt' get seen after.
Anyway, it's a very special time and if you request to see the sex, they will do their best but it's not always possible to see (and on the other hand sometimes it's way obvious, so if you don't want to know tell them that too)!!!
Good luck, you'll be fine and it will be amazing!!! xoxoxo