How accurate is fundal height?

Hi all! Just wondering how accurate fundal height is in comparison to how big baby is! I’m measuring 3 weeks ahead but won’t be having another ultrasound for at least another 3 weeks to actually know how big little one is. OB said that she’s either just a little chunker or she could be tall but I know

That doesn’t necessarily mean shes either so just looking for anyone who has a similar experience 🙂