My baby is 15months,still not putting weight she is 8.8

A. Hello mom babies tend to triple their birth weight by the age of one year and then on they tend to gain about 2 to 4 kgs yearly .. a little variation is acceptable as far as the child is active for his age and eating healthy foods as well . If the baby was a preterm or a low birth weight baby then they tend to gain weight on a slow pace