Baby gender determination on a sonogram?

I am 16 weeks pregnant and I go to the Dr on my 20th week to get a sonogram and they will determine the then. I have been worried that they would tell me "it's a boy" and have it be a girl or they would tell me "it's a girl" and have it be a boy. So, this leads to my question. If they by some weird chance do say the wrong , will they be able to tell later on after the baby is more developed? As much info as you can give would be appreciated. Thanks :)