Pediatricians and parents often use growth charts at a baby's well child checkups can help to determine how well they are gaining weight.
Failure to Thrive
While most babies gain weight well, even if they are moving up or down on their growth chart a little bit, some babies lose weight or who aren't gaining weight well enough. These babies have what is termed failure to thrive (FTT) and according to the Nelson Textbook of Pediatrics will usually be found to have growth that is "growth below the 3rd or 5th percentile or a change in growth that has crossed two major growth percentiles."
If you think that your baby isn't gaining weight well, be sure to talk to your pediatrician, who can look for an underlying cause. If you pediatrician suspects that your baby has failure to thrive, among the conditions that your baby may be tested for may include, but are not limited to:
- gastrointestinal problems - reflux, celiac disease, food intolerances and allergies, malabsorption, liver diseases
- endocrine problems - hypothyroidism, diabetes, growth hormone deficiency, and adrenal disorders
- infections - tuberculosis, AIDS, gastrointestinal parasistic and bacterial infections
- heart and lung disorders - asthma, congestive heart failure, cystic fibrosis, obstructive sleep apnea
- miscellaneous conditions - metabolic disorders, chromosomal abnormalities, congenital syndromes, lead poisoning, cancer, chronic kidney problems
In addition to these medical conditions that can lead to failure to thrive, children can also have weight loss or poor weight gain when they are simply not given enough to eat (psychosocial failure to thrive).
Sources:
Failure to Thrive. Behrman: Nelson Textbook of Pediatrics, 17th ed.
Failure to thrive. Krugman SD - Am Fam Physician - 1-SEP-2003; 68(5): 879-84.

