Transition to extrauterine life involves changes in body water, early weight loss, limited renal handling of water, and insensible losses.
Body water and early weight loss
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Neonates are born with excess total body water (TBW), primarily in the extracellular fluid (ECF) compartment. Diuresis during the first week reduces ECF and consequently TBW.
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Term neonates lose about 7–10% of body weight during the first 3–5 days.
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Preterm neonates have proportionately higher TBW and may lose 10–15% of birth weight during the first week.
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SGA neonates have less TBW and lose less weight than their AGA counterparts.
Regulation and renal limitations
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The heart, kidneys, skin, and neuroendocrine system regulate fluid and electrolyte balance.
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Neonatal kidneys have limited ability to concentrate or dilute urine because of a lower glomerular filtration rate and reduced proximal and distal tubular sodium reabsorption.
Insensible water loss
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In addition to renal and gastrointestinal losses, water evaporates from the skin and respiratory tract; this is termed insensible water loss (IWL).
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IWL is higher in preterm infants because their skin is thin.
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Fever, increased respiratory rate, radiant warmers, and phototherapy increase IWL.