At 12 weeks they don't generally look for a vagina or testicles/penis, they use a technique where they determine the angle between the genital "nub" and the direction of the lower portion of the spine. If the sonographer is experienced the rate of success is a little over 80% at 12 weeks.

My husband found out our last baby was a girl at our 12 week scan and had it confirmed at 16 weeks and 20 weeks (and probably again at 24 weeks, haha), and she certainly is a little girl I had no idea during the entire pregnancy if she were a boy or girl though. Hubby is good at keeping secrets

Oh, our sonographer told hubby he has a success rate at 12 weeks of 80% so warned him that 1 in 5 times he gets the gender wrong. Lucky for DH he got it right hehe