They usually do it a bit later because not all the organs are big enough to see at that point. It's called the "anatomy" ultrasound not just because you learn the gender but they make sure everything is working properly in baby's body. Most doctors send you later because they want the most accurate results and don't want to just send you for another one two weeks later in case they can't see the images properly.
I just think it's retarded like seriously I have an appt tomorrow and I will ask her if it's ok which it should be
I know my doctors office doesn't do it until 20+ weeks! It is usually accurate after 18, but each clinic has its ways and I know most doctors/sonographers just want the best view possible so that they don't miss anything important!
I got mine done at exactly 19 weeks. My doctor said it's pretty accurate from 18 weeks and over