One disturbing aspect of preterm birth rate trends in the United States is persistent racial and ethnic disparities. Rates of pre term birth among black women are markedly elevated above those for white and Hispanic women in every year recorded (Martin, 2017). Moreover, rates of births before 32 completed weeks in black women are higher than those in white and Hispanic women combined. Some investigators attribute this disparity to socioeconomic circumstances (Collins, 2007; Leveno, 2009). Internationally, the rates of preterm birth in the United States are also higher compared with those in other industrialized countries (Ananth, 2009; Delnord, 2017; Martin, 2017).
Newborns born before 37 weeks sufer various morbidities, largely due to organ system immaturity (Table 42-2). hat said, remarkable strides have been made in neonatal survival for those born preterm. his is especially true for neonates born after 28 weeks. In a study of more than 18,000 newborns weighing between 400 and 1500 g or aged between 22 and 32 weeks' gestation, survival rates were analyzed as a function of both birthweight and gestational age (Fanarof, 2007). Mter achieving a birthweight of : 1000 g or a gestational age of 28 weeks for females, or 30 weeks for males, survival rates reach 95 percent.
• Threshold of Viability
Births once considered to be "abortuses" because the fetus weighed <500 g are now classified as live births. In the United
States in 2014, 5863 live births <500 g were recorded (Martin, 2017). For those newborns delivered before 33 weeks' gesta tion, perinatal and neonatal care has advanced tremendously.
As a result, the threshold of viability, which is the lower limit of fetal maturation compatible with extrauterine survival, has been reassessed. Currently, the threshold of viability lies between 20 and 26 weeks' gestation.
Neonates born in this periviable period have been described as fragile and vulnerable because of their immature organ systems. Many of these are described in Chapter 34 (p. 639) and include brain injury from hypoxic-ischemic injury and sepsis. In this setting, hypoxia and sepsis start a cascade of events that lead to brain hemorrhage, to white-matter injury that causes periventricular leukomalacia, and to poor subsequent brain growth eventuating in neurodevelopmental impairment. Associated morbidities include intellectual disability, cerebral palsy, blindness, seizures, and spastic quadriparesis that can result in the need for a lifetime of medical care (Annas, 2004). Because active brain development normally occurs throughout the second and third trimesters, those born <25 weeks are believed to be especially vulnerable to brain injury.
To clariY obstetrical care of these fetuses, the Society for Maternal-Fetal Medicine, the Eunice Kennedy Shriver National Institute for Child Health and Human Development (NICHD), the American Academy of Pediatrics, and the American College of Obstetricians and Gynecologists convened a joint workshop in 2013 (Raju, 2014). he executive summary statement from this meeting served as the underpinnings for an Obstetric Care Consensus document from the American College of Obstetricians and Gynecologists (2017 e).
TABLE 42-2. Major Short-and Long-Term Problems in Very-Low-Birthweight Infants
Respiratory distress syndrome, air leak, bronchopulmonary dysplasia, apnea of prematurity Hyperbilirubinemia, feeding intolerance, necrotizing enterocolitis, growth failure
Hospital-acquired infection, immune deficiency, perinatal infection
Intraventricular hemorrhage, periventricular leukomalacia, hydrocephalus Retinopathy of prematurity