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Preterm birth is a substantial global health issue with significant consequences to the newborn, family, and society. Preterm delivery affects nearly 15 million births worldwide, is the leading cause of death in children younger than 5 years, and remains a prominent issue in obstetrics (Chawanpaiboon, 2019). Although the burden of preterm birth is clear, defining the biology of human parturition and identifying strategies to reduce preterm birth rates remain elusive.


Preterm birth is delivery before 37 completed weeks, that is, before 366/7 weeks. Subdivisions vary among organizations. According to the American College of Obstetricians and Gynecologists (2021b), births occurring between 34 and 36 completed weeks are considered late preterm. The Centers for Disease Control and Prevention recognizes this late preterm definition but also labels births before 336/7 weeks as early preterm (Martin, 2021). Instead, the World Health Organization (2018) defines births before 28 completed weeks as extremely preterm, those from 28 to 32 weeks as very preterm, and from 32 to 37 weeks as moderate to late preterm.

These definitions lack a functional basis and should be distinguished from the concept of prematurity, which represents incomplete development of various organ systems at birth. For example, the lungs are particularly affected and may be susceptible to the respiratory distress syndrome (Chap. 34, p. 615). Similarly, neonates born before term can be small or large for gestational age but are still preterm by definition. Low birthweight refers to neonates weighing 1500 to 2500 g; very low birthweight describes those between 1000 and 1500 g; and extremely low birthweight refers to those <1000 g (World Health Organization, 2019).


In the United States, the preterm birth rate rose from 10.02 percent in 2018 to 10.23 percent for 2019. When interpreting these data, important factors and trends merit discussion. First, during the past two decades, the percentage of preterm neonates fell from 2007 to 2014 but has since risen (Martin, 2021). Some argue that the rate drop reflected changes in obstetrical dating criteria rather than true declines (Frey, 2016). Specifically, beginning in 2014, the National Center for Health Statistics transitioned to a new standard for estimating newborn gestational age for birth certificate completion (Martin, 2015). The new measure, which is the obstetrical estimate of gestational age at delivery, replaced calculations based on the date of the last menstrual period. These measures differ and do not provide equivalent absolute numerical comparisons of preterm birth rates. Thus, current national data are now not directly comparable to previously reported preterm birth rates.

Although the percentage of neonates born early preterm rose slightly, the rise in the overall preterm rate from 2018 to 2019 was due primarily to the increase in late-preterm births. ...

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