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Key Points

  1. Fetal urine production begins at 10 to 13 weeks gestation, and although urine production increases thereafter throughout pregnancy, glomerular filtration rate (GFR) is always lower in preterm infants.

  2. Renal function changes quickly in the fetus and newborn with an increasing GFR and tubular maturity leading to enhanced concentrating ability.

  3. Calculation of maintenance fluid requirements is size dependent; however, practically, calculations are more typically based on body weight rather than body surface area.

  4. Premature infant fluid requirements are different from term infant requirements in both total fluid volumes and electrolyte content.

  5. To avoid serious neurologic injury, sodium abnormalities should not be corrected quickly.

Renal Embryology

The development of the human kidney is a complex process that requires precise timing. Three sets of excretory organs develop: pronephros, mesonephros, and metanephros. During the fourth week of gestation the pronephros appears and rapidly regresses but the ductal structures are utilized by the mesonephros that consists of tubules and glomeruli, which are a simple version of those seen later. The metanephros, the permanent kidney, develops in the sixth week of gestation after regression of the former 2 primitive kidneys. As this appearance and regression process occurs, the collecting system and tubules contact each other leading to the final functional unit or nephron. Upon final formation, the fetal kidney begins to produce urine by 10 to 13 weeks of gestation, and the number of nephrons continues to increase to approximately 1 million by 36 weeks of gestation. Initially, the fetal kidneys are in close approximation in the pelvis with the hilum facing ventrally. During fetal elongation, the kidneys ascend in the abdomen and rotate 90° medially, and by the ninth week, they are in the normal anatomic position. The blood supply to the developing kidneys is initially from surrounding vessels but during the cranial ascent regress, and formation of the renal vessels is seen by the ninth week as well. The complex interaction required for normal kidney development and position allows for a large number of malformations to occur.

Renal Function

Urine produced by the fetal kidney is excreted into the amniotic fluid, and by 20 weeks, it becomes the major portion of the overall amniotic fluid volume. Fetal urine production is initially brisk at 5 mL/h, reaching 40 mL/h by term. As a consequence, fetal anuria/oliguria manifests with oligohydramnios by approximately 24 weeks. While the fetal kidney does excrete urine, the majority of electrolyte balance is regulated by the placenta. During the first postnatal days, the kidney begins the transition from fetal to neonatal function. When compared to adults, the neonatal kidney has decreased renal function owing to the low mean arterial pressure and high vascular resistance, and this is the primary reason for the decreased glomerular filtration rate (GFR) of neonates. The initial GFR in term infants is approximately 20 mL/min × 1.73 m2 and doubles within ...

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