The genitals can look the same at 12 weeks for both sexes, they say 16 weeks is the earliest semi accurate way to tell.

For my 12 week scan, I went to a place that was one of the pioneers of NT scanning in Australia, they radiographer was showing me the screen saying it was the best machine in Australia at that time, but still they wouldn't tell me the sex.

I asked why, she said that no matter how good someone is, or no matter how good the machine is, it is way too early to give a guess and as policy they don't do it. She said she's seen it end in tears too many times and the best anyone should give at that stage is a 50/50 guess on sex. They even had a written policy saying that they will not devulge the sex until 17-18 weeks.

I have heard of people getting an accurate guess at 12 weeks, but I've heard of probably more that are told the wrong sex. I found out the sex of my bubba at 16 weeks, but really didn't think anything of it until I got the morph scan at 21 weeks and I still needed Dr O'Callaghan himself to tell me that I was having a boy for me to take it too seriously.

When it comes down to it, no matter the scan, the only 100% accurate way is through amnio's or CVS's, there is still a chance they get it wrong no matter how early or late the scan.