Descriptions of Failure to Thrive
There isn't one neat and tidy definition of failure to thrive (FTT) in medicine, though it is usually defined using one of two parameters. The first is normative. It compares a particular child's growth with the growth of other children his age. This is what is meant by the percentile rank on a growth chart. A rank of 25 percentile in length for a seven-month old means that if 100 babies where lined up side by side from the shortest to the longest, that seven-month old would be 25th from the short end, with 75 babies being longer.
The other method is longitudinal and compares the child to himself over time. Knowing the expected changes in length, weight, and head size through infancy, we ask: Is this child meeting those expectations? So a small baby when compared to others might be growing well over time while remaining "small," while a baby who was quite large at birth might still be in the middle percentile a few months later because of sluggish growth due to a serious illness.
Roughly speaking, the concern of failure to thrive comes up when a child falls two or more percentile groupings on a growth chart over a fairly brief time span, is on the very bottom percentile, or grows inadequately over time. Faced with one of those three situations, it is now up to the health care provider to explain FTT. Which children are small but perfectly healthy? Which children were born large and to small parents and are falling in the growth charts as they slowly assume their smaller genetic destiny (which happens in the first few years of life)? For which children does poor growth reflect a serious illness? As opposed to extensive lab testing, most of the clues usually come by asking a series of questions and performing a thorough physical exam.