Roisin CF is getting picked up earlier and earlier as it is now being incorporated into the heelpreek tests that newborns have in many areas (with a view to this being all areas of the UK asap.) The sooner the diagnosis the better. Many children with CF will have aborbtion problems from the word go and therefore fail to thrive which is a sign of CF.. (although probably the LEAST likely reason for a baby failing to thrive!) This is due to pancreatic insufficency.
Some will have some degree of pancreatic function however and least initially and will gain weight/grow normally for a while therefore masking the issue until other symtpoms of CF start to show up, ie respiratory.
Good luck I hope your nephews tests are clear.
SJ x