So, as you can see, growth requires the smooth working together of many systems throughout the body. The specific causes of failure to thrive are quite numerous, with problems in every major organ system represented. The most common causes for failure to thrive vary with age. In the first three months of life feeding difficulties, infections, gastroesophageal reflux, inborn errors of metabolism, cystic fibrosis, and milk-protein intolerance top the list. Simply switching to formula is not a sufficient solution if your child doesn't respond with steady growth.
Many infants regain their birth weights by one week of age.
Most regain their birth weights by two weeks of age and have a steady weight gain thereafter. A child who has gone seven weeks without regaining her birth weight deserves a thorough work-up to determine the cause. This work-up should begin with a detailed feeding history and a careful physical examination. Given your child's unusual stools, it might be useful to have the stool examined for evidence of a malabsorption syndrome. If all of these studies come back normal, and your daughter is still not growing, then a further round of tests, possibly including thyroid function tests, liver enzymes, ammonia, lactate, pyruvate, a sweat chloride test, urine organic and amino acids, and perhaps x-ray studies, would be indicated.