Why do newborns lose weight after birth?

Why Newborns Lose Weight After Birth

Newborns normally lose weight in the first days of life due to physiological contraction of extracellular fluid compartments, which is an essential adaptation process after transitioning from intrauterine to extrauterine life. 1

Physiological Basis for Weight Loss

Newborn weight loss occurs through a well-defined adaptation process that can be divided into three phases:

Phase I: Transition Phase (Days 1-5)

• Characterized by initial relative oliguria followed by a diuretic phase
• Body fluid compartments undergo rearrangement through isotonic or hypertonic contraction of extracellular fluid (ECF)
• Continuing natriuresis (sodium excretion) that was present during fetal life 1
• Considerable insensible water losses through the skin, especially in premature infants

This phase ends when maximum weight loss has occurred, typically by day 2-5 of life.

Normal Parameters for Weight Loss

• Term infants: Weight loss generally occurs during the first 2-5 days and should not usually exceed 10% of birth weight 1
• Preterm infants: ELBW (extremely low birth weight) and VLBW (very low birth weight) infants typically lose 7-10% of birth weight due to their higher body water content 1
• The nadir (lowest point) of weight typically occurs on days 2-4 after birth 2
• Most breastfed infants regain their birth weight by 7-10 days of life 1

Contributing Factors to Weight Loss

1. Fluid shifts: Contraction of extracellular fluid with negative water and sodium balance 1

2. Limited intake: Initially small volumes of colostrum in breastfed infants

   • Milk production increases from <100 ml on day 1 to 500-600 ml by days 4-5 1

3. Feeding method:

   • Breastfed infants typically lose 6-7% of birth weight
   • Formula-fed infants typically lose 3-4% of birth weight 1

4. Insensible water losses:

   • Through immature skin (especially in premature infants)
   • Through respiration
   • Increased by radiant warmers, phototherapy, and single-wall incubators 1

5. Diuresis: Increased urine output after initial oliguria 1

Clinical Implications

• Weight loss is a normal physiological process that should be monitored but not necessarily prevented
• Excessive weight loss (>10% in term infants) may indicate inadequate fluid, sodium, protein and/or energy intake, or other pathology 1
• Close monitoring is essential during the transition phase to avoid:
  • Oliguria (<0.5-1.0 ml/kg/hour for >12 hours)
  • Electrolyte disturbances
  • Acidosis 1

Management Considerations

• A gradual increase of fluid intake is recommended in both term and preterm neonates after birth 1
• Electrolytes (Na, Cl, K) should be supplied during the contraction phase to maintain normal serum concentrations 1
• Environmental factors can significantly influence postnatal weight loss:
  • Double wall incubators reduce insensible water loss by about 30%
  • Mechanical ventilation with warmed, humidified air reduces respiratory water loss 1

Monitoring

• Daily weights until birth weight is regained
• Monitoring of urine output
• Serum electrolyte measurements as clinically indicated
• Assessment of hydration status (skin turgor, mucous membranes, fontanelle)

Weight loss in newborns represents a normal physiological adaptation to extrauterine life rather than a pathological process, and understanding this helps guide appropriate monitoring and intervention decisions.