18 week gender scan

We have booked a private gender scan for tomorrow! I will be 18 weeks and 1 day. Will the gender be accurate? What can I expect to see? They do it in 4d as well as normal 2d and I've never had a 4d one before so don't know what to expect? Has anyone got any pics or stories of when they had there's at 18 weeks or so? 
Thanks!