doctors wont normally order a ultarsound to determine se until you are 20 weeks.
Very few women find out the sex and it be right at 12-14 weeks. At that stage the genitals are still growing and for the most part look the same. Average time is 20 weeks, when your doctor also has the measurements and baby checked out but some find out as early as 15 weeks. There WILL be people who find out at 13 and 14 weeks, but that's going to happen regardless. It can only be one or the other, so they're bound to guess right sometimes.
Most women can't find out until 16 to 20 weeks. Some doctors will "predict" the gender between 12 and 14 weeks but it is alot more accurate later. I am 14w 5d and my doctor won't tell me until 18w. I can't wait!!