My baby birth weight is 4.3 kg ,, now she is 4 months old and her weight is 5.8 is under weight or normal????

A. It should actually be 6.3 kg by completing the age of four months because the basic funda behind calculating weight gain for a child about half a KG gain in a month so look at the birthweight and now by four months completion it should be 2 KG plus the birth weight of the child