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Aims--To examine the relation between birth weight and cognitive function at age 11 years, and to examine whether this relation is independent of social class.
Methods--Retrospective cohort study based on birth records from 1921 and cognitive function measured while at school at age 11 in 1932. Subjects were 985 live singletons born in the Edinburgh Royal Maternity and Simpson Memorial Hospital in 1921. Moray House Test scores from the Scottish Mental Survey 1932 were traced on 449 of these children.
Results--Mean score on Moray House Test increased from 30.6 at a birth weight of [less than]2500 g to 44.7 at 4001-4500 g, after correcting for gestational age, maternal age, parity, social class, and legitimacy of birth. Multiple regression showed that 15.6% of the variance in Moray House Test score is contributed by a combination of social class (6.6%), birth weight (3.8%), child's exact age (2.4%), maternal parity (2.0%), and illegitimacy (1.5%). Structural equation modelling confirmed the independent contribution from each of these variables in predicting cognitive ability. A model in which birth weight acted as a mediator of social class had poor fit statistics.
Conclusion--In this 1921 birth cohort, social class and birth weight have independent effects on cognitive function at age 11. Future research will relate these childhood data to health and cognition in old age.
(Arch Dis Child 2001;85:189-197)
Keywords: birth weight; Barker hypothesis; social class; intelligence
Intelligence is determined by a combination of genetic and environmental influences, the relative contributions of which are not yet established, and may vary over the lifespan.  Environmental influences originate while the fetus is developing in utero. The "fetal origins" or "programming" hypothesis [2,3] proposes that these influences cause permanent changes in the developing child, resulting in low birth weight, and a predisposition to chronic disease in adult life. The mechanism of this relation is suggested to be fetal undernutrition, with even brief periods of undernutrition during critical periods of rapid cell division causing permanent changes in various organs.  Malnutrition in utero affects brain development,  and the relation between birth weight and cognitive function has therefore been studied.
It has been known for many years that "low birth weight" or intrauterine growth restricted babies fare less well on various measures of mental development in later life.  Many studies have compared low birth weight babies ([less than]2500 g) with controls, showing impairment in various neurodevelopmental tests up to age 11. [5-7] Recent large longitudinal cohorts have allowed assessment of the relation between birth weight differences within the normal range and later differences in cognitive function. [8-11] These show that lower birth weight is associated with lower scores on tests of cognitive function at age 8 in the general population,  and at age 17-18 in army recruits. [9,10] A relation between birth weight and cognitive function was also seen through childhood to middle life,  but was largely explained by the influence of birth weight on cognition at 8 years. A study of older adults (mean age 60.9), which estimated early life mental ability, found the association between birth weight and cog nitive function to be not significant  (corrected for age and social class). Martyn et al therefore suggest that fetal growth is less important than genetic factors and postnatal environmental influences in determining adult cognitive performance. 
A recent review concluded that intrauterine growth restriction had little clinically significant effect on mental performance in childhood or adolescence, but was a useful surrogate for social deprivation. 
Much of the criticism surrounding the programming hypothesis concerns the confounding influence of factors other than fetal under-nutrition operating perinatally and throughout life.  In particular, the socioeconomic environment in which a child is conceived and develops will have an effect on both their physical  and mental  development. Another important potential confounding factor between birth weight and mental ability is gestational age: without this information, many studies have been unable to distinguish low birth weight caused by prematurity from "small for gestational age" or "intrauterine growth restriction". [7,15] When investigating early life influences on cognitive development, it is therefore important to consider the combination of birth weight and gestational age.  The relation between birth weight and placental weight  might also be relevant. There is also a suggestion of a non-linear relation between birth weight and intelligence, with relatively low cognitive perfor mance at the highest birth weights. [9,10]
There is therefore a need for further studies of birth weight and childhood intelligence to address these issues. Furthermore, if studies from different historical time periods find a consistent relation, this will increase the generalisability of their conclusions. Here we report on a well characterised sample from a distinct historical period. The sample's cognitive function at age 11 may be compared with that of all 11 year old children in Scotland as a result of the Scottish Mental Survey 1932; gestational age can be calculated; and there is information on socioeconomic status. We tested the competing hypotheses that birth weight: (1) is related to cognitive function at age 11 independent of socioeconomic status; and (2) acts as a mediator of the effect of socioeconomic status on cognitive function at age 11. We also assessed the contribution of other features of the child (for example, gestational age, placental weight, age at cognitive test) and mother (age, parity) to later cognitive function.
Subjects and methods
Detailed records of all admissions to the Edinburgh Royal Maternity and Simpson Memorial Hospital in Scotland have been retained in the Lothian Health Services Archive at the University of Edinburgh. The records for 1921 include date of birth, last menstrual period (from which gestational age can be calculated), previous pregnancies, maternal age and address, paternal occupation (if father known), birth weight and length, and placental weight. The records for admissions not relating to a live delivery were excluded, as were records for twins. This left 985 live singleton births.
MENTAL ABILITY DATA AGE 11
The Scottish Mental Survey was administered under the auspices of the Scottish Council for Research in Education (SCRE) to all children in Scotland at school on 1 June 1932, and born in 1921 (n = 87 498; 44 210 boys, 43 288 girls).  This test was closely related to the Moray House Test Number 12 used in the "11-plus" in England, and will be referred to hereafter as the Moray House Test (MHT). Only a …