Humans are warm-blooded and able to maintain their own body temperature independent of their outside environment. This is called 'thermoregulation' and refers to a natural physical balancing of heat loss, with heat production in our bodies. As adults, when we are hot our bodies sweat to compensate and cool down. When our body temperature drops, we shiver and move around to increase our metabolism and therefore our body heat.
During pregnancy the baby's temperature is kept fairly constant inside the mother's body at about 37.7oC. The baby relies on their mother to sweat and shiver for them (as they are unable to do these things themselves) to keep their own body temperature regulated.
Once born, babies still have only a limited ability to regulate their own body temperature. Babies can move a little but are unable to shiver. They can sweat, but only the glands in their head, neck hands and feet are active (being about 25 to 30% of their total body size). To keep warm a baby may try to curl up into the fetal position, move or cry (if not covered warmly with clothes or wraps). However, a baby's main source of heat production is their special body fat, known as brown adipose tissue (or 'BAT'). BAT starts to be produced at 26 to 30 weeks of the pregnancy and makes up about 2 to 7 % of the baby's total body weight at birth. BAT is similar to fat tissue found in hibernating animals.
BAT plays an important role in providing body heat for babies. Unfortunately a disadvantage of the baby using their BAT to warm up is that metabolism of the fat to produce heat requires extra oxygen and glucose. This can result in the newborn baby's system becoming physically stressed, as they try to maintain their body heat, if allowed to become too cold (ideally the baby's temperature should be no lower than 36.0oC). Often the baby is reluctant to want to feed when their temperature is very low (in an effort to conserve energy) compounding the problem.