Although the mortality rate for preterm infants and the gestational age-specific mortality rate have dramatically improved over the last three to four decades, infants born preterm remain vulnerable to many complications, including respiratory distress syndrome, chronic lung disease, injury to the intestines, a compromised immune system, cardiovascular disorders, hearing and vision problems, and neurological insult. Infants born at the lower limit of viability have the highest mortality rates and the highest rates of all complications. Few studies have reported mortality and morbidity rates in gestational age-specific categories, which limits the information available for counseling parents before a preterm delivery and for making important decisions on the timing and the mode of preterm delivery. Although much progress in the treatment of infants born preterm has been made, many of the medications and treatment strategies used in the neonatal intensive care unit have not been adequately evaluated for their efficacies and safety. The high rates of neurological injury in preterm infants highlight the need for better neuropro-tective strategies and postnatal interventions that support extrauterine neuromaturation and the neurodevelopment of infants born preterm.

The significance of preterm birth lies in the complications of prematurity sustained by the infant and the impacts of these complications on the infant's survival and subsequent development. Many clinical research studies of infants born preterm limit their outcomes to neonatal mortality and morbidity. Complications and the disturbance of normal development may result from factors that influence prenatal development and the etiology of preterm birth, but the extent to which this happens is often unknown. Although this chapter is by no means a complete catalog of complications of preterm birth, it discusses how these various complications reflect immaturity; the impact that they have on survival, organ maturation, and health; and the efficacies of a number of intervention strategies designed to prevent and mitigate the effects of these complications. As outlined in Chapter 2, information based on gestational age is preferred over information based on birth weight because of the value of knowledge of gestation age in making decisions regarding preterm delivery and prenatal counseling of the parent.

The complications of preterm birth arise from immature organ systems that are not yet prepared to support life in the extrauterine environment. The risk of acute neonatal illness decreases with gestational age, reflecting the fragility and immaturity of the brain, lungs, immune system, kidneys, skin, eyes, and gastrointestinal system. In general, more immature preterm infants require more life support. There is controversy about how infants at the border of viability should be managed (see also Chapter 2 for discussion of Perinatal Mortality of Infants Born at the Limit of Viability). Neonatologists may vary in terms of how conservative they are with regard to treatment of these infants and some may regard treatment of infants at these very early gestational ages as experimental. The reader is referred to Appendix C for further discussion of ethical aspects of decision-making at the threshold of fetal viability.

The response of the infant's organ systems to the demands of the extrauterine environment and the life support provided have an important impact on the infant's short and long-term health and neurodevelopmental outcomes (Chapter 11). These outcomes are also influenced by the etiology of the preterm birth; maternal and family risk factors; and the extrauterine environment, including the neonatal intensive care unit (NICU), home, and the community.

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