Is it 100% confirmed?

At 19 weeks, I get to find out the sex during an ultrasound. 
Is it 100% confirmed at that time? Or can the sex end up 'changing' after 19 weeks? 
This will be my last ultrasound, & I just don't want any surprises when i give birth lol. 
I know it sounds silly but I've heard a lot of stories about the sex not being fully developed during the ultra sound so the sex ends up changing.  I'm not sure how far along those ladies were. 
Do you think 19 weeks would be too early for a definet confirmation?