"Missing" Weasley Children

[gideoner4]

No: HPFGUIDX 37322

--- In HPforGrownups at y..., "Bernadette M. Crumb" <kerelsen at q...> 
wrote:
> I know that we're having a lot of fun speculating horrible
> reasons for that nine year gap between the oldest two Weasley
> children and Percy, but has anyone thought that maybe one reason
> for that is that Arthur might not have been home to conceive a
> child during that time?
> 
> 

Please enlighten me. Where is it mentioned that the gap between the 
two eldest Weasley children and Percy is nine years? If not 
explicitly mentioned in canon, how did people come to a conclusion? 
I think I missed that part. Thank you. :D