What might be the reason behind a 4 month baby being just 3.5kg even after a proper formula milk?

A. Hi momma, it depends on the baby's birth weight. Ideally, babys should double the birth weight by 5 months and they must triple birth weight by 1 year. So an increase of 250 to 450 grams every month is ideal, after initially losing a bit of weight soon after birth and recovering it in few weeks time. Please check with your baby's pediatrician for any specific recommendations.