S.S.
In my understanding, it's virtually unheard of to find out gender with any kind of certainty at 16 weeks. Personally, I was told at six months that I was having a girl. I found out at 8 months that I'd need to replace all the pink with blue.
I was mad, and I did some reading - something like 85% of fetuses called girls will be girls, and 90something% of the ones called boy are boys. Everything, apparently, depends on position and technical skill.
Good luck.