During the first week of life, healthy neonates experience a physiologic reduction in body weight primarily due to a decrease in which compartment?

• A. Equivalent volumes of extracellular and intracellular water
• B. Extracellular water
• C. Intracellular water
• D. Total body water

Answer: (B)

In the first week after birth, a healthy neonate loses weight mainly because extracellular fluid decreases. At birth, a large portion of body water is in the extracellular compartment (plasma and interstitial fluid). After birth, the kidneys begin diuresis and there are insensible losses that reduce this extracellular fluid, while the intracellular water inside cells remains relatively stable. Since the extracellular compartment contributes most of the fluid lost during this early transition, the net weight decline largely reflects a drop in extracellular water rather than intracellular fluid or total body water per se.