At what time can you learn your babies gender??

I am a Ftm and Sometime last week I went to the emergency room and they did an ultrasound to check the baby.. I was 16w & 6d and the baby measured that exactly if not a little less.. I asked the tech about the gender and he told me it was too early to tell if it was a boy or a girl.. Next Monday ill be 18 weeks and still dont know what im having.. Can anybody tell me when ill be able too know if my baby a boy or girl!!!

** Baby wasnt very active during the ultrasound.. And when the tech ran over a part of the baby i could of swearn it was the baby private area and i think its a girl as soon as i saw that i asked about the gender and thats when i was told its too early to know anything about the gender...