Its a boy...wait, its a girl?

It is really common. I had an ultrasound at 13 weeks and the tech told me that she wouldnt even guess what it might be because at that stage of development, all the parts look the same. The oris for a female and a for a male at that time look the exact same. At 20 weeks though they are usually right about the gender determination. You always do have to keep in mind though that ultrasounds are just tools that are used by humans and humans do make mistakes. That is why no ultrasound is 100%.