Newborns typically do not shiver in response to cold like older children and adults do. Shivering is a form of heat generation accomplished through rapid, involuntary muscle contractions. If a newborn exhibits shaking or trembling, it may indicate distress, such as low blood sugar, rather than an effective attempt to warm up. Infant temperature regulation relies on a completely different, specialized biological mechanism.
The Absence of Shivering
Adults and older children rely heavily on shivering because their large muscle mass can generate a significant amount of heat through contraction. Newborns, however, possess a relatively small muscle mass, which makes shivering an inefficient way to produce warmth. Furthermore, shivering rapidly consumes the body’s reserves of glucose and oxygen, which are limited in a newborn. This high metabolic cost could lead to complications like hypoglycemia and metabolic acidosis without providing sufficient heat.
The physical structure of a newborn also predisposes them to rapid heat loss. Infants have a high surface area-to-volume ratio compared to adults, meaning they lose heat to the environment much faster. Their skin is thinner, and they have less insulating subcutaneous fat, further reducing their ability to retain warmth.
Non-Shivering Thermogenesis: The Newborn’s Heater
The primary way a newborn generates heat is through a process called Non-Shivering Thermogenesis (NST). This mechanism bypasses muscle movement entirely, relying instead on a specialized type of fat tissue. NST can increase a newborn’s metabolic rate and heat production by two to three times its resting value in response to cold exposure.
This specialized tissue is called Brown Adipose Tissue (BAT), often referred to as brown fat. Brown fat is distinctly different from the white fat that stores energy; its sole purpose is to produce heat. BAT cells are densely packed with mitochondria and are richly supplied with blood vessels.
When a newborn’s core temperature begins to drop, the nervous system signals the brown fat to activate. The stored triglycerides within the BAT are broken down into fatty acids. A unique protein called Uncoupling Protein 1 (UCP1) within the mitochondria then essentially short-circuits the normal energy production process. This “uncoupling” allows the energy from the fatty acids to be released directly as heat, rather than being used to produce adenosine triphosphate (ATP). Brown fat is strategically located to quickly warm the central circulation, primarily around the neck, shoulders, and kidneys.
Recognizing and Preventing Cold Stress
Because newborns cannot shiver to self-regulate, they are highly susceptible to cold stress, which is defined as a drop in body temperature below 97.7°F (36.5°C). If the newborn’s temperature continues to drop, it can quickly lead to hypothermia. Hypothermia depletes energy reserves and increases the risk of hypoglycemia and respiratory issues. A rapid temperature drop of 2°C to 4°C can occur within the first twenty minutes of life without proper thermal protection.
Parents should be vigilant for signs of cold stress, which include pale or mottled skin, cool extremities, lethargy, and decreased feeding or a weak cry. The most reliable way to monitor an infant’s temperature is by taking an axillary (armpit) reading. A normal range is between 97.7°F and 99.5°F (36.5°C and 37.5°C). If the temperature is below 96.8°F (36.0°C), immediate medical attention is necessary.
Preventative measures focus on minimizing heat loss and promoting heat retention. Immediately drying a newborn after birth and replacing wet linens is crucial, as evaporation is a major cause of heat loss. Maintaining a stable room temperature and dressing the baby in appropriate layers is also important. Skin-to-skin contact, where the infant is placed directly on a parent’s chest and covered with a blanket, is a highly effective method for regulating the newborn’s temperature through conduction.