What defines a preterm infant?

A preterm infant is specifically defined as an infant born before 37 weeks of gestation. This classification is crucial in obstetrics and neonatology because the gestational age at which an infant is born significantly impacts their likelihood of survival and overall health outcomes. Preterm infants are at higher risk for various complications associated with underdeveloped organs and systems, particularly the lungs and brain.

The other options focus on different aspects of infant health. Birth weight is an important factor, but it is primarily used to classify low birth weight infants, which can occur in full-term babies as well. Birth after 42 weeks gestation defines a post-term infant, which is not related to preterm classification. Signs of hypothermia are indicative of various issues but do not define the gestational status of the infant. Thus, the definition of a preterm infant hinges solely on the timing of birth in relation to gestational weeks, making the first option the correct choice.