Circulating catecholamines are increased in both mother and fetus in response to the stress of labour. This increase in catecholamines helps prepare the fetus for transition to life outside of the uterus by promoting the absorption of fetal lung fluid into the pulmonary interstitium, stimulating the release of pulmonary surfactant, triggering essential metabolic changes and non-shivering thermogenesis. The placenta has a high capacity for inactivation of maternal catecholamines, limiting transfer to the fetus. At delivery once separated from the placenta, a catecholamine surge occurs to initiate non-shivering thermogenesis, enabling the infant to immediately produce heat in response to the lower environmental temperature. Despite this fetal preparation, unless immediate thermal attention is given to maintain the infant’s temperature, the infant will be at risk of hypothermia.

Thermoregulation is controlled by the hypothalamus. It receives thermal information from the body skin, deep thermal receptors and receptors found in the preoptic area of the hypothalamus. The hypothalamus compares this information with what is described as the ‘set point’ (temperature it wants to maintain the body at) and responds by stimulating the pituitary or the sympathetic nervous system (SNS) to allow the infant to conserve, produce or lose heat.

Heat is conserved by:

• Peripheral vasoconstriction
  
• Flexing extremities towards body.

Heat is produced by:

• Normal metabolic activity
  
• Muscular activity
  
• Shivering (at low environmental temperatures only)
  
• Non-shivering thermogenesis or the metabolism of brown fat/brown adipose tissue (BAT) in the infant.

BAT starts to develop in the late second trimester and can be found down the vertebral column and around the scapulae, neck, sternum and the adrenal glands. It is well innervated by the SNS and has a rich blood supply resulting in its dark appearance.

Heat is lost by:

• Vasodilatation
  
• Stretched posture
  
• Sweating
  
• Lying very still
  
• Breathing faster.

In addition to physiology, the environment plays a role in how an infant loses or gains heat. Infants lose/ transfer heat by means of the following four modes of heat loss/ transfer:

• Immature hypothalamus
  
• Sweat glands initially do not work, and are functional by day 14
  
• Larger body surface area to weight ratio
  
• Produce less body heat per unit surface area
  
• No shivering
  
• Will metabolise what brown adipose tissue is available, depleting valuable energy stores
  
• Thinner skin with less subcutaneous fat
  
• Skin may only be two or three cells thick or less
  
• Skin has less keratin, therefore leaks water
  
• Evaporative heat loss exceeds ability for heat production
  
• Blood vessels just below skin surface
  
• Posture.

As a result of mainly the physical characteristics of the premature baby, heat loss/ transfer are different for the term and preterm infant. The full term baby will lose heat by radiation, convection and lastly evaporation. The preterm infant is most vulnerable to evaporation, convection and then radiation. As the surfaces babies are placed on are usually covered with, for example, a blanket or single-use cover, conductive heat losses are less of an issue today. Heat loss in very immature infants is mainly due to evaporation of water from the skin surface. Evaporative heat losses increase with decreasing gestational age, but then decrease as the skin matures and achieves near-term maturity by 2 weeks of age. Therefore, the midwife should remember the gestational differences between term and preterm when addressing thermal care needs at delivery and while delivering postnatal care.