My Baby S 56 Days Old And Weighing 3.9kgs His

my baby s 56 days old and weighing 3.9kgs his birth weight was 3kgs and after a week 2.9 kgs in between had to give him similac formula milk once from 7-26 days I am worried He s not gaining enough weight his height s though increased.....now he s only on breastfeeding when he was on both formula and breastfeeding he had only out on 400gms....I am worried for his weight gain