Well, I was having a 'boy' with my first pregnancy and out came a girl!!! Yep, it was a bit of a shock. She didn't have a name for 12 hours!!! I hadn't done the whole boy thing and because I have seen sooooo many boys come out girls and vice versa I kept a fairly open mind.

It does depend on the expertise of the operator, the machine they use and of course the position of the baby. Baby girls have very prominant and swollen vulvas that have very often been mistaken for a penis and scrotum on ultrasound...

The only way to know with absolute certainty is amniocentesis or cvs - or of course to have the absolute joy of opening that little pair of legs at birth and taking a peek!!!