M Merialdi and M de Onis World Health

Organization, Geneva, Switzerland

© 2005 Elsevier Ltd. All rights reserved.

It is widely accepted that weight at birth is a key indicator of fetal and neonatal health, both for individuals and for populations. The strong association between low birthweight and perinatal mortality and morbidity is now well recognized by health care providers, as are the different determinants and health consequences of low birthweight. These epidemiological associations became progressively evident during the past century. In the United States, the practice of weighing infants at birth was introduced at the end of the nineteenth century when low birthweight infants were categorized as 'premature' and usually left unattended with minimal or no intervention attempted to prevent their deaths.

When information on birth weight and gestational age was introduced in the birth certificate in mid-twentieth century, it became apparent that prematurity was the most important cause of infant deaths at the national level.

With progressive awareness of the importance of low birthweight as a predictor of infant mortality, it appeared that being born small could be due either to a restriction of the normal process of fetal growth or to delivery before the term of gestation. Thus, the World Health Organization (WHO) made a distinction between the condition of low birthweight (birth weight less than 2500 g) and prematurity (delivery at less than 37 completed weeks, i.e., 259 days). A further development was the introduction of the concept of small for gestational age (SGA) that better describes infants affected by intrauterine growth restriction (IUGR). According to this classification, infants with birth weight below the 10th percentile of a reference population are considered SGA. Although these distinctions and definitions are commonly applied in developed countries, their use is more difficult in developing countries where information on gestational age is often nonexistent or unreliable. This is unfortunate because low birthweight conditions due to growth restriction or preterm birth have different determinants and prognosis, as well as different epidemiological distributions that vary by country and socioeconomic status. Thus, before discussing the causes, prevalence, and prevention of low birthweight, it is important to understand how its two components (gestational age and fetal growth) can be correctly identified and quantified for epidemiological purposes and what are the major limitations in doing so.

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