16 weeks and up is the most accurate. You can make an educated guess between 12-13 weeks using nub theory if you get just the right angle... But for actually being able to determine (reliably) the gender by the baby's bits, it's 16 weeks. Some elective ultrasound offices will try at 15 weeks, but it isn't guaranteed. I think most people say 20 weeks because at their routine medical scan around that time, the tech will often take a guess. But private scan start earlier.