In this article, we assess the relative importance of birthweight and weight gain between 0 and 24 and 24 and 48 mo for schooling outcomes. Other than an analysis of attained schooling in Pelotas (17
), we are not aware of other work addressing the topic. We found that weight gain from 0 to 24 mo had the strongest relationships with schooling outcomes followed by birthweight; weight gain from 24 to 48 mo had a weak or no relationship to schooling outcomes. The magnitudes of the relationships are of economic and public health importance. In fully adjusted models, stunting at 2 y, a widely used cumulative indicator of undernutrition during fetal and postnatal life, was associated with a reduction in schooling of 0.9 y, with a 16% increased risk of failing at least 1 grade in school and with older ages at enrollment in 3 of the sites. Given the estimate of 0.9 y of schooling lost, we would expect stunting to decrease lifetime income by ~10% in the countries included in our analyses (6
One SD increase in birthweight, equivalent to ~0.5 kg, is associated with 0.21 y of additional schooling and 8% decreased risk of ever failing a grade. One SD increase in CW gain between birth and 24 mo, equivalent to ~0.7 kg, was associated with higher estimates of 0.43 y more schooling and 12% decreased risk of failing. One SD increase of CW gain from 24 to 48 mo, equivalent to ~0.9 kg, was associated with only 0.07 y more schooling and was not associated with school failure.
In addition, CW gain from 0 to 24 mo was more important for schooling in children born small. In children born in the lowest tertile of birthweight, 1 SD increase of weight gain from 0 to 24 mo was associated with 0.50 y of schooling compared with 0.33 y in those in the upper tertile. This suggests a beneficial effect on schooling of catch-up growth in smaller babies.
Our study has numerous strengths. It used prospective data from 5 well-described cohort studies. Schooling outcomes included 3 important variables: highest grade attained, ever failed a grade, and age at school entry. Pooled analyses were possible for all but age at school entry, thus increasing sample sizes and power and providing assurance that most of the relationships we explored were consistent across countries. An additional strength was the use of weight information at birth and 24 and 48 mo to assess the relative importance of growth during specific preschool periods. Further, we used standardized, CW gain variables to appropriately assess relative importance. Our growth measures were, by design, uncorrelated with each other, and the use of SD scores permitted direct comparison of estimates across weight variables. For example, had we used initial birthweight and postnatal weight gain, or initial Z-score at any age and subsequent change in Z-score, as has been done in studies of mental health (16
) and cognitive development (12
), respectively, we would not have removed the phenomenon of regression to the mean or controlled-for common error terms (e.g. measurement error will generate a negative correlation between initial and change values, because larger-than-true measurements at baseline will lead to smaller change values and smaller-than-true initial values will lead to larger change values). Finally, our analyses also controlled for confounding associated with children's early environment.
Our study also has weaknesses. Our schooling information was limited; only highest grade attained was available for New Delhi and most Bt20 participants were enrolled in school, so that analyses of highest grade attained could not be carried out for this site. We lacked birth length for Pelotas and Bt20 and therefore relied on analyses using weight. However, we conducted analyses that used growth in height from 0 to 24 and 24 to 48 mo that was conditioned on birthweight, available for all 5 sites, and these analyses gave similar results to those presented here.
We controlled for maternal schooling and SES. Our analyses showed that these variables attenuated coefficients substantially but that relationships with birthweight and weight gain from 0 to 24 mo remained significant. In studies with extensive control for confounding, the relationship between stunting or weight faltering with schooling or cognitive development also remained significant (30
). Economists have been concerned with confounding in relating child size to schooling, particularly regarding unobserved characteristics (31
). Use of econometric techniques that rely upon instrumental variables to control for confounding show that relationships between birthweight or child height for age and educational outcomes remain significant and can actually increase in magnitude after controlling for unobserved characteristics (32
). Although economists often attribute causality in analyses of nonexperimental data with instrumental variables, they recognize that experimental data offer the best possibility for doing so. The nutrition intervention to which the Guatemalan cohort was exposed () improved diets and reduced stunting at 3 y of age (34
) and also had long-term effects on schooling (women), cognitive development (men and women), and wages (men) (35
), thus providing support for a causal effect. In addition, the specificity of our finding of a much larger role for growth in the first 2 y of life than for later growth is in agreement with the physiology of brain growth, also supporting a causal association.
Few would argue that it is child size or growth per se that causes impaired cognitive development and low educational achievement. Rather, growth failure in early childhood should be viewed as a marker of lack of nutrients at the cellular level that has systemic effects on growth and development in general, including the brain and neurological development.
From a public health perspective, it is important to place these findings in the context of what is known about the timing of weight gain and chronic disease outcomes. Other analyses from the 5 cohorts showed that rapid weight gain in every age range from birth to mid-childhood is associated with increased blood pressure (38
), but studies of body composition suggest that weight gains after 2 y of age are more closely associated with adult fat mass than earlier weight gains (39
). Further analyses are needed for other chronic disease outcomes, but, to date, there is a good case for promoting weight gain in early life in low- and middle-income populations in light of its positive association with human capital outcomes such as schooling.
We showed important associations of prenatal and postnatal growth to 2 y with schooling outcomes. There are many other influences, such as school quality, that also determine schooling outcomes and achieving the millennium development goal of universal primary education by 2015 will require action along a broad front. Our results suggest that designing effective nutrition interventions and targeting them to mothers and children <2 y of age should be among these actions.