Preterm babies are newborns born at a gestational age of less than 37 weeks. Preterm infants are further categorized as early preterm, midterm and late preterm. Gestational age greater than or equal to 28 weeks and less than 32 weeks are classified as early preterm; greater than or equal to 32 weeks and less than 35 weeks are classified as intermediate preterm; and greater than or equal to 35 weeks and less than 37 weeks are classified as late preterm. According to the weight of less than 2.5 kg is considered as low birth weight; between 1.5-2.5 kg is considered as very low birth weight; less than 1 kg is considered as ultra-low birth weight. The physiological characteristics of preterm babies are different from those of full-term babies, and the development of various organs and systems is not perfect, so in principle, preterm babies need to be hospitalized.