Is a nuchal translucency ultrasound at 12 weeks 5 days too early to tell the gender of your fetus?

Hi I am 12 weeks, 5 days pregnant with fraternal twins. The nurse today said it looks like I am having two girls. She pointed at the bottom one and said, "See there's nothing in between the legs". I just don't understand how it is possible to determine it this early! I am kind of irritated that she even said anything because now I'm going to be really anxious for the next 6 weeks until they have a more accurate ultrasound. Are nuchal translucency ultrasounds more advanced then normal? I'm wondering if that was why she was able to predict so early. Well they said there is a very low risk that my babies have down's syndrome. So that is a plus!!!!! :-) I am just so anxious to find out sexes now.