I worked at a Clinic for 3 years and they did ALOT of Ultrasounds and I found out the following and it was 90% accurate:
Little girls tend to keep their legs closed or try to hide the important parts!!!
And little boys tend to "let it all hang out!!"
My daughter had her ultrasound and of course the DR. wanted to find out so they tried and no go- no show- get up walk around- try again- no show- for about an hour.. turned out to be a girl.
My son legs wide open.