Abstract
Post-newborn infants refer to infants from >28 days to <100 days after birth. During this period, infants are still completely dependent on breast milk or/and formula milk for feeding. Up to now, the concept of post-newborn has not been mentioned in classic textbooks. With the development of perinatal medicine, mortality rate of diseases in neonates such as premature infants, asphyxia, infectious diseases have decreased significantly, and consequently, issues of the quality of life for these survivors have aroused widespread concerns. The post-newborn infants have some important characteristics differing from both newborn infants and infants after the period: (1) different fatal diseases and mortality rate; (2) the diseases inherited from newborn period requiring early and prompt treatments; (3) some peculiar diseases during this period requiring much attention; (4) either similar or different immune function; (5) rapid growth and uneven development of organ systems. Establishment of the new concept of post-newborn will further reveal the nature of life, reduce the mortality rate of infants, and improve the quality of life.