A Short History of Birth Weight

For most of the previous century, birth weight has been treated as a dichotomy. "Low birth weight" is the category of babies weighing less than 2500 grams at birth, and "normal birth weight" is all the rest. For many years, the presumed reason for babies to be born at low birth weight (LBW) was their preterm delivery. Indeed, the terms "LBW" and "premature" were used interchangeably in the scientific literature from the 1920s to the 1960s.

However, not all small babies are premature, and not all premature babies are small. An accumulation of epidemiologic data during the 1950s and 1960s finally made this distinction clear. In 1961, the World Health Organization recommended that LBW no longer be used as the official definition of prematurity. By the 1970s, most researchers were complying, although as late as 1977 a book on LBW was titled The Epidemiology of Prematurity. Perinatal epidemiologists now avoid the word "premature" altogether, preferring the label "preterm" for a baby born too early.

As researchers began to recognize that LBW and preterm are not synonymous, they faced an uncomfortable new problem. Term babies born at less than 2500 grams nonetheless have a high risk of mortality. What accounts for this risk, if not preterm delivery?

This gap was filled by the invention of a new disease - intrauterine growth retardation (IUGR). The usual definition of IUGR is "small for gestational age" (SGA), the lightest 10% in each gestational age stratum. Under the percentile definition, the vast majority of IUGR babies are born at term. (This is simply a function of definition: under a percentile formula, the category of IUGR contains the same small percent of preterm births as is present in the general population.) Taken as a whole, IUGR babies correspond closely with the set of LBW babies at term, and provides these LBW babies with a "diagnosis". Thus, the creation of an entity called IUGR effectively preserved LBW as a group of babies with "preventable" ailments. Small babies who are not preterm are "growth retarded".

This convenient solution to the problem of term LBW infants led to rapid acceptance of the concept of IUGR during the 1970s. According to PubMed, the number of papers about IUGR swelled between 1970 and 1979 from a handful to more than 200 a year. In fact, this was not a new research area but a shift within LBW research from one label ("prematurity") to two ("preterm" and "IUGR").

Popular assumptions about LBW. The dichotomization of birth weight is deeply entrenched in public health research. Why have researchers been so determined to cling to this strategy? This practice rests on several assumptions about LBW.

1. "LBW causes infant mortality."
   
   In the first year of life, LBW babies are typically 20 or more times more likely to die than heavier babies. The sheer strength of this association with mortality is regarded as evidence of its causality.
   
   
2. "The percent LBW in a population is an indicator of infant risk."
   
   Infant death is rare (at least in developed countries), so researchers need a more prevalent surrogate indicator of perinatal risk. LBW serves this purpose nicely. Furthermore, under this assumption, the causes of LBW themselves become topics of investigation.
   
   
3. "LBW is preventable."
   
   If LBW is caused by either preterm delivery or fetal growth retardation, then LBW is presumably completely preventable. Thus, LBW provides a target for interventions to improve infant survival. The prevention of LBW is an explicit part of US public health policy to decrease infant mortality.

While these assumptions about LBW are generally accepted, not all aspects of LBW neatly fit into them. For example, groups with a larger percent of LBW babies do not invariably have the greater risk. A well-known example is the comparison of female and male babies. But the most telling contradiction is described in the next section, The Low Birth Weight Paradox.

Why study birth weight?

A short history of low birth weight

The low birth weight paradox

Frequency distribution of birth weight

Birth weight specific mortality

The Wilcox-Russell hypothesis

The analysis of infant mortality

Beyond low birth weight

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