For ill or preterm infants, the limits of the immature gastroenteral function and fluid volume limits make immediate optimal nutrition more difficult in the first several days of life. Usually, TPN can be started by 2-4 days, and tiny, slowly advancing feeds can be started within a week. The schedule may be delayed by severe illness, electrolyte imbalances requiring frequent adjustments, and insults to the GI tract such as infection or asphyxia, potentially leading to NEC . Subsequently, the need to gain weight is again the primary guideline. Any stimulus to the basal metabolic rate increases the caloric requirements Repair or recovery increases the requirement Infants with BPD sometimes need enormous caloric support to gain weight.
Weight gain is related to survival.
Failure to thrive despite your best efforts is unusual.