my baby 7 month old baby wait only 7.400 kg why??

A. See you have to understand this logic that from the time of birth whatever wait your child has it is expected for a child to gain half a kilo kilogram 3 1/2 kg kilogram gain would be good in this age if your child has gained it is good and if not then it is less there is no answer to why I as such it that your breast breastfeed is not enough