Look, I don't want to alarm you at all but the reason they take note of a sudden jump in percentile bands for head circumference is in case of something going on in the brain, like fluid or a tumour. If it jumps again they MIGHT send her off for a CAT scan. A friend of mine had her MCHN send her DD off for a scan after her DD's head circumference suddenly went off the charts - but she was absolutely fine, just had a big head and a growth spurt.

That anything is "wrong" is HIGHLY unlikely though - both of my kids' heads jumped percentile bands, from 75th to 97th. But DH's family all have massive heads, it is far more likely there is a genetic component involved and you just have a nice healthy big bub. This would be why the MCHN was so vague, a problem would be very rare and she wouldn't have wanted you freaking out unnecessarily.

My DS in particular grew all out of proportion, at some checks his length really increased, other times his weight increased without him growing much lengthways. He even went down percentile bands for his weight.