It’s just a baby that’s not gaining weight according to the percentages they should be at certain ages. The cause is numerous - lactose intolerance, inability to suckle properly, mother not producing enough milk, etc. Generally if the baby is taken to the pediatrician regularly and being monitored it shouldn’t cause any problems unless the mother is refusing to heed the doctor’s advice on what to do.
it’s a variety of things to food allergies (to formula mainly) to gastric problems (reflux etc) to not having normal interactions (mom smokes crack all day rather than care for baby for instance)
As they grow they tend to be under weight, under size and have cognitive and/or emotional impairments.
My son was diagnosed with FTT but i didn’t agree with it. he was 9 months old dn had just suffered from a stomach bug that my at the time 4 year old brother gave him. so he lost weight. that happaned o be right before his 9 month check up. he was in the 5th percentile and dropped two lines. anyway, he is now a VERY VERY smart 2 1/2 that talks like a 5 year old who is almost 37 inches and 28.8 pounds
My daughter was diagnose with Failure to thrive. Her weight gain is below the growth chart. Her labs are abnormal and she has iron defiance anemia. She just refuses solids. We are still trying to figure it out. Not gaining weight according to the growth chart doesn’t bother me but her labs does. It can cause development problems esp in the brain. Ultimately, failure to thrive can lead to death. I’ve encountered many adults, mostly senior citizens who are diagnose with failure to thrive and in their case they are too sick to eat or have a condition that hinders eating.