Here's a really well done video that shows why you cannot tell the difference between boys and girls until the 12th week of gestation. And let me just stress that just as the video says, the penis does not even really start to elongate till during the 12th week, making the 13th week much much more reliable for nubs.

YOU CANNOT TELL GENDER FROM ULTRASOUND IN THE 9TH, 10TH, OR 11TH WEEK. Genitalia is identical at this stage of gestation!!

Inside pregnancy':' Girl or boy'?' '|' Video | BabyCenter