Mine was determined at 16 wks, they I had monthly ultrasounds since I wasn't quite a year out from RNY to make sure the baby was growing properly and she told me two more times that he was indeed a boy!  The last time was about 3 weeks ago, and his sack had already dropped, she showed me on the ultrasound.  I will be induced tomorrow, and the verdict never changed, always a boy.

I found out at my 20 week sonogram and she moved around A LOT! The tech was even able to find her ovaries, so if I have a boy with ovaries, he/she is going right to the circus! :)