finding out the sex of the baby

We are finding out if we are having a boy or girl tomorrow at 16 weeks 4 days. I know people say it's easier to see if it's a boy. If it turns out to to look like a girl, could it change? I am just so excited and I would hate to have to wait to confirm 100% at 20 weeks. Is 16 almost 17 weeks a good place to tell the sex for sure?! Please give me some help. Thanks😀