Epidemiology of late and moderate preterm birth

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Summary

Preterm birth affects 12.5% of all births in the USA. Infants of Black mothers are disproportionately affected, with 1.5 times the risk of preterm birth and 3.4 times the risk of preterm-related mortality. The preterm birth rate has increased by 33% in the last 25 years, almost entirely due to the rise in late preterm births (34–36 weeks’ gestation). Recently attention has been given to uncovering the often subtle morbidity and mortality risks associated with moderate (32–33 weeks’ gestation) and late preterm delivery, including respiratory, infectious, and neurocognitive complications and infant mortality. This section summarizes the epidemiology of moderate and late preterm birth, case definitions, risk factors, recent trends, and the emerging body of knowledge of morbidity and mortality associated with moderate and late preterm birth.

Introduction

Preterm birth (<37 weeks gestation) affects approximately one in eight (12.5% in 2008) of all births in the USA each year.1 Preterm birth is the most frequent cause of infant mortality, as well as the leading cause of long term neurologic disabilities in children, including cerebral palsy and developmental delays. It is estimated that preterm birth costs the US healthcare system more than $26 billion each year.2 Black infants are disproportionately affected; infants of non-Hispanic Black women have >1.5 times the risk of preterm birth and 3.4 times the risk of preterm-related mortality compared with infants of White mothers.3 Reducing the high burden of preterm birth, and its associated morbidity, mortality, and racial disparities, has thus been identified as a public health priority, as reflected by the 2006 PREEMIE Act,4 the 2007 Institute of Medicine Report on preterm birth,2 the 2006 Surgeon General’s conference,5 the 2007 US Department of Health and Human Services’ public awareness campaign,6 and Healthy People 2010 and 2020 objectives.7

The survival of preterm infants improved greatly in recent decades, primarily due to advances in clinical management including neonatal intensive care units (NICUs), paediatric ventilators, and use of surfactant and antenatal steroids. Concomitant with improvements in preterm survival, the preterm birth rate increased by 33% from 1981 to 2006, almost exclusively due to a rise in late preterm births (34–36 weeks’ gestation).1, 8 Currently, ∼72% of all preterm births are due to infants born late preterm and 84% are due to late and moderate (32–33 weeks) preterm combined (Fig. 1). Although the majority of preterm-related deaths occur among very preterm infants (<32 weeks’ gestation), increased attention has recently been given to better understanding the reasons for the high rate of late and moderate preterm birth, its causes, short and long term sequelae, and opportunities for prevention. Research is uncovering significant, though often subtle, increased risks for late preterm infants compared with those born at term (i.e. 39–41 weeks’ gestation) for complications at birth and long term neurodevelopmental problems. Increased knowledge about the epidemiology of these moderate and late preterm births is critical for informing practices and guidelines related to the prevention of preterm-related morbidity and mortality.

Section snippets

Preterm birth definitions and subcategories

Fetal growth and maturation occur along a continuum throughout pregnancy. As such, case definitions based on discrete categories of gestational age may appear somewhat arbitrary. However, standard categorization of preterm infants based on gestational age is valuable for assessments of morbidity and mortality risk, comparisons across populations and research studies, generating health policy guidelines, and guiding patient care.9 Generally, preterm birth is defined as birth of an infant at <37

Estimating gestational age

Ensuring accurate and standardized estimation and reporting of gestational age may often be challenging. Gestational age is routinely estimated according to the number of weeks’ gestation after the onset of the mother’s LMP. However, this estimate may be unreliable and prone to error in maternal recall. The gold standard for accurate determination of gestational age is first trimester ultrasound.11 Nevertheless, early ultrasound is currently not recommended for routine gestational age dating

Causes of late and moderate preterm birth

Preterm birth is not a single entity, but a common final outcome of a heterogeneous collection of underlying maternal and fetal factors. Approximately two-thirds of all singleton preterm births are spontaneous, often with no known cause, and approximately one-third are the result of medical intervention (i.e. medically indicated) to protect the health of the mother or infant.14 Complications of pregnancy that lead to both spontaneous and indicated preterm birth are multiple, complex, and vary

Epidemiology of preterm birth

Surveillance of preterm birth is essential for informing and evaluating clinical practices, research, programs, and policies aimed at reducing infant morbidity and mortality. Surveillance and epidemiologic analyses can measure the contribution of preterm birth to infant morbidity and mortality, identify populations at highest risk, detect changes in obstetric practices, and guide the development, implementation, and evaluation of programs.17

Increases in the singleton preterm birth rate since

Conflict of interest statement

None declared. The findings and conclusions in this paper are those of the authors and do not necessarily represent the official position of the Centers for Disease Control and Prevention.

Funding sources

None.

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