Actually, a 12 or 13 week fetus does not have fully formed genitalia, and you can't tell if it's a male or female by seeing if something is dangling between the legs - in fact, a something hanging down would be more likely to be a female at this point. At this stage of development, both sexes have a genital tubercle. The only difference is in the angle at which it points. The female tubercle is more in line with the spin, pointing down. The male tubercle is pointing more, well, up - just more towards the belly. There's a really good explanation here, with pictures and everything, just scroll down: http://www.baby2see.com/gender/external_genitals.html Note that the pictures at the very bottom are of a 12 week old fetus - 14 weeks pregnant, not 12 weeks pregnant. There are definite differences at that stage, but they are much too tiny to be seen on ultrasound. The only way you can tell at that stage is by the angle.
Then this page on that site actually gives a summary of a study on predicting gender using the angle (since you can't do it based on other features at that point, although you probably could if you had the fetus in your hand - just not with u/s): http://www.baby2see.com/gender/study_ultrasound.html
Anyway, interesting stuff, but don't get your hopes up for a boy if something is pointing downward right now. lol Honestly though, I can't see it. I keep looking, but I don't see it. Now I'm going to go look again. :)