21month old baby eating really well, not fussy with food and taking bottle milk but not gaining weight as I expect. should I be worried? he's very active, no health issues.

A. hi dear When growing kids don't gain weight as they should, it is called "failure to thrive." Failure to thrive is not a disease or disorder itself. Rather, it's a sign that a child is undernourished. In general, kids who fail to thrive are not getting enough calories to grow and gain weight in a healthy way.