In the preterm infant, body weight decreases after birth in a
fashion similar to that seen in term infants, but in an exaggerated
fashion. Infants born modestly preterm may lose 5% to 10% of
their birth weight, whereas the youngest of premature infants may
lose 15% to 20% of their birth weight with no
apparent ill effects. Their weight loss is a result of salt and water
loss over the first week after birth, and the fluid is lost primarily
from the interstitial space. Whether this weight loss is “normal” is a
matter of definition. The spontaneous feeding and apparent health
of term newborns lends itself well to describing the observed weight
loss in term infants as normal. However, because fluid intake in
small premature infants is often determined by rates of intravenous
fluid infusions, these smallest of preterm infants cannot be considered
to regulate fluid intake in the same manner as term infants. Thus,
the range of “normal” for variables such as weight
loss in this population is somewhat arbitrary and probably should
be abandoned in favor of what is most desirable for optimal health.
In this regard, the interpretation of epidemiological and prospective
intervention studies suggest that there is a direct relationship
between morbidity and the abundance of salt and water intake, at
least in the early newborn period. Thus, the most prudent extrapolation
of this information would lead to a strategy in which fluid intake
is adjusted to allow a gradual loss of body weight over the first
postnatal week, with restriction of sodium intake until near the
time when the target weight loss has occurred. Frequent measurement
of body weight, urine output, and serum concentrations of electrolytes
help to assure this gradual transition.