Ultrasound date vs. OPK/timing date

So I was trying to explain the nub theory to a friend who just had her 12 week ultrasound. She is sure of when she ovulated and said the would have been 11+5 (maybe 6 days) based on charting/using OPK's but that at the ultrasound baby measured 12 weeks +3 days. I know the nub theory goes off of what baby was measuring, but does this hold true when people are sure of their dates? Her nub looked quite girly (which is what shes hoping for) but when I tried to explain it she was so sure even though baby was measuring bigger she was only 11 weeks + 5 days and that she didn't want to get her hopes up.
Hoping someone has some insight as to what they have seen happen when people are so sure of dates?

Thanks!

Even 12 w 3 days is too early for a reliable guess. Nubs rise even after that. I have seen a very girly nub at the start of the 13th week even that ended up to be a boy (coralsky and I believe you can look it up in the ultrasound forum if you were interested)

If the tech is good, I put a high level of confidence into measurements. Things happen and charting and OPK are not always reliable. It's more than just measurements, they use actual stages of development and these are somewhat set in stone. But some techs aren't good, may have been having a bad day and such and so it's one of those gray areas - measurements and development are quite reliable (more so than charting and OPK), but there is "pilot error" sometimes and then that changes things. If the tech has actually made a mistake, then all bets are off.

I agree with your friend, she should be cautious with assuming girl, sometimes baby is just a bit big, but has not reached certain developmental stages anyways. Most of my babies have measured large, and I was 100% sure when they were conceived. They were all big at birth. But developmentally they were probably at an earlier stage than their size iykwim. So, your friend has, for now, an early but girly nub, meaning that things can change. It's better for her not to get too happy in order to protect herself from potential disappointment. So I advise you not to try to convince her it's a girl, just let her doubt the nub. It sounds like she needs to doubt her girly nub in order to protect herself.