My brother and SIL are finding out tomorrow! I'm 13 1/2 weeks shes 15 weeks and a few days! If you can afford to pay for an elective scan you can find out at 15 weeks' it's very accurate- as long as the ultrasound place is experience and certif... Otherwise, the 20 week period is just as fine!
I found out at exactly 17 weeks. It was crystal clear that my sweetie is a girl. :) Just make sure to drink a sugary drink 30min before, or if you havent gaven up caffiene then a cup of coffee will do the trick. It worked perfect for me! :) Good Luck I hated the wait but it is worth it.