We had our NT scan at 12w3d, and the tech goes, "I do about 300 of these a year, and I get about 90% of them right... but I don't like to say unless I'm pretty sure." Then he went on to explain what he was looking in determining the baby's sex, and it was pretty darn interesting.
I can't remember exactly how he put it, but said you have to be able to get a good profile view, then look for the genital tubercle, which is present in both sexes at that point. Then he explained how he draws an imaginary line perpendicular to the baby's spine (can't remember what he called the line -- the mid-sagittal line, or something?), and goes, okay, if that line is 90 degrees, how many degrees is the genital tubercle at? If the tubercle is aligned with the spine (0 degrees), or pointed as high as 30 degrees, it's probably a girl. With a boy, it's usually pointed somewhere between 30 and 90 degrees. He said sometimes it's too close to call, so he won't make a guess.
I was just blown away! I thought you had to wait until a penis did or didn't develop to be able to tell, I had no idea there was this whole thing with genital tubercle angles. Wacky!
Anyway, the tech said our baby's was at basically 0 degrees. "I say 90% chance it's a girl," he said, and referred to the baby as a "her" for the rest of the ultrasound.
Of course I'm trying to take it with a grain of salt, but I just thought that was so cool!