Final answer:
Premature infants and those with intrauterine growth restriction (IUGR) are at the highest risk for hypothermia due to compromised regulation of body temperature and potential lack of brown adipose tissue that is needed for heat generation.
Step-by-step explanation:
Infants at the highest risk for hypothermia are premature infants and those with intrauterine growth restriction (IUGR). Premature infants and those with low birth weight due to factors such as maternal undernutrition are particularly vulnerable because their bodies are less capable of regulating temperature effectively. Premature infants may also have issues such as respiratory distress syndrome (RDS) which can compound their difficulties in maintaining body temperature. The use of nonshivering thermogenesis in newborns, which involves the breakdown of brown adipose tissue, is critical for generating heat, but premature infants and those with IUGR might have less brown fat, making them more susceptible to hypothermia. Additionally, factors such as environmental exposure and lack of adequate postnatal care can further increase the risk of hypothermia in these infants.