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The global epidemiology of preterm birth

https://doi.org/10.1016/j.bpobgyn.2018.04.003Get rights and content

Highlights

  • Preterm birth affects approximately 11% of births worldwide. However, estimation is complicated by differences in measurement of gestational age, preterm definitions, as well as differences in data collection and reporting.

  • Although many factors have been shown to increase the risk of spontaneous preterm birth, the majority of preterm births occur in women without a clear risk factor.

  • Long-term studies have shown that the deleterious effects of preterm birth continue to affect health and welfare in adult life.

  • Addressing preterm birth is critical to addressing neonatal and child mortality and morbidity, particularly in resource-poor settings.

Abstract

This article is a part of a series that focuses on the current state of evidence and practice related to preterm birth prevention. We provide an overview of current knowledge (and limitations) on the global epidemiology of preterm birth, particularly around how preterm birth is defined, measured, and classified, and what is known regarding its risk factors, causes, and outcomes. Despite the reported associations between preterm birth and a wide range of socio-demographic, medical, obstetric, fetal, and environmental factors, approximately two-thirds of preterm births occur without an evident risk factor. Efforts to standardize definitions and compare preterm birth rates internationally have yielded important insights into the epidemiology of preterm birth and how it could be prevented.

Section snippets

How is preterm birth defined?

The World Health Organization (WHO) defines preterm birth as births before 37 completed weeks of gestation or fewer than 259 days from the first date of a woman's last menstrual period (LMP) [1]. It is a condition defined by the failure of a gestation to reach a certain length of time, rather than by the presence of specific signs or symptoms [2]. Depending on one's point of view, preterm birth may be considered as an adverse pregnancy outcome (where a fetus is unable to fulfill its in utero

How is gestational age measured?

The method by which a pregnant woman's gestational age (GA) is determined is an important factor. In general, in pregnancy, the later the GA is estimated, the greater is the uncertainty [10]. Early pregnancy ultrasound is considered the gold standard for GA assessment. Other methods such as calculation from the date of LMP, symphysis–fundal height measurement, postnatal examination of the newborn, or use of birthweight as a GA surrogate are often used in settings without access to ultrasound or

How is preterm birth monitored and reported?

WHO and other United Nations (UN) agencies have developed standardized global indicators to optimize collection, reporting, and international comparisons of data on conditions and diseases. WHO uses the indicator “live births before 37 completed weeks (whether singleton or multiple) per 100 live births” for estimates of preterm birth [17], [18]. In practice, countries often report preterm birth using different operational definitions—for example, preterm birth in singleton pregnancies only.

How is preterm birth classified?

Classification systems are developed through the systematic assembly, storage, and retrieval of basic information on a health event [22]. Multiple classification systems have been developed around preterm birth for different applications: to guide research on causes and determinants, to better identify at-risk populations, to implement and monitor prevention strategies, to facilitate preterm birth surveillance, and to allow standardized local and international data comparisons. It would be

The global burden of preterm birth

The first global and regional estimates of preterm birth were published in 2010 by Beck et al. (for the year 2005), followed by the global-, regional-, and national-level estimates in 2012 (for the year 2010) by Blencowe et al. [17], [28] Beck et al. identified data from 92 countries and estimated the global prevalence at 9.6% (95% confidence interval [CI]: 9.1%–10.1%) for the year 2005 [28]. Blencowe et al. identified data from 99 countries and estimated the 2010 global prevalence at 11.1%

Limitations in estimating the global burden of preterm birth

Estimates are helpful in understanding preterm birth epidemiology, developing and implementing health policies, raising awareness, and mobilizing resources. However, they are often developed in the absence of reliable, population-representative data, which can paradoxically obscure the need for additional investment to develop or improve data collection and monitoring systems, as well as data quality [29]. Even in countries with robust civil registration and vital statistics (CRVS) systems, GA

Trends in preterm birth rates

In some (mostly higher income) countries, preterm birth rates appear to be increasing. Blencowe et al. reported that for 65 countries with reliable preterm birth data, 62 countries had experienced increases between 2000 and 2010 [17]. A few methodological considerations should be remembered. Preterm birth definitions may change over time—a reduction in the lower threshold of fetal viability (i.e., when management of extremely preterm births changes over time) will create an apparent increase in

Why does preterm birth occur?

Although many socio-demographic, nutritional, medical, obstetric, and environmental factors have been shown to increase the risk of spontaneous preterm birth, its etiology remains imperfectly understood [23], [35]. When considering the literature on preterm risk factors, it is worthwhile keeping some general limitations in mind.

Despite the plethora of reported risk factors, the majority of preterm births have no clear risk factor. An individual participant data (IPD) meta-analysis of 4.1

Risk factors for preterm birth

A 2013 systematic review assessed the association between ethnic groups and preterm birth and reported an odds ratio (OR) of 2.0 (95% CI: 1.8–2.2) for black ethnicity; no significant associations were seen for Asian, Hispanic, or Caucasian women [40]. The risk of preterm birth appears higher in both adolescent pregnancies and advanced maternal age [41], [42], [43]. A meta-analysis of cohort studies found that nulliparous women below 18 years of age had the highest risk of preterm birth across

The consequences of preterm birth

Since 1990, the under-five mortality rate has dropped dramatically, from 93 deaths per 1000 live births in 1990 to 41 deaths per 1000 live births in 2016 [92]. Neonatal mortality (death in the first 28 days of life) has also steadily reduced, although at a disproportionately slower rate than under-five mortality. Consequently, the determinants of neonatal mortality (including preterm birth) have become a greater contributor to the mortality rate in children below five years of age over time.

Implications for practice, policy, and research

It is clear that further discovery research is needed on the mechanisms of spontaneous preterm birth, so that it can be better predicted and prevented. Several methodological issues related to international comparisons of preterm birth (such as definitional differences in indicators and differences in measurement of GA) could be addressed through better, standardized reporting of preterm data, as well as efforts to collect and report reliable GA data from resource-limited settings. Ideally,

Summary

Preterm birth remains an important public health priority worldwide. Evidence-based strategies to prevent prematurity from occurring, as well as mitigating its effects on preterm newborns, are needed, particularly in low-resource settings.

The quantity and quality of preterm birth data reported in most countries is inadequate, thus impeding accurate estimation at global, regional, and national levels; however, standardization of definitions, measurement, and reporting would allow international

Funding source

None.

Conflicts of interest

None.

Research agenda

  • Discovery research on mechanisms of spontaneous preterm birth, so that it can be better predicted and prevented.

  • Better tools for measuring gestational age and identifying preterm births.

  • Developing preventive interventions that are safe, effective, and scalable in low-resource settings, where the majority of preterm-associated child mortality is occurring.

Acknowledgments

None.

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