Department of Sociology, New York University, New York, NY 10012, USA.
Int J Epidemiol. 2011 Aug;40(4):1008-18. doi: 10.1093/ije/dyr030. Epub 2011 Feb 28.
Intra-uterine growth is a powerful predictor of infant mortality and of health, developmental and socioeconomic outcomes in adulthood. The question about whether this relationship is causal rather than driven by unobserved characteristics of low-weight infants is, however, still open. We use twin models to examine the hypothesis that in utero growth has a detrimental impact on cognitive development in childhood.
We merge birth registry information on birthweight with standardized Math and Spanish test scores for all fourth graders in Chile to create a prospective data set. Twin fixed-effects models are used to estimate the causal effect of intra-uterine growth on test scores. Fixed-effect estimates are compared with traditional regression results in a cross-section of births to gauge the omitted variable bias emerging from unobserved genetic, maternal and pregnancy-related factors in cross-sectional models.
Birthweight differences within twin pairs have a substantial effect on test scores. A 400-g increase in birthweight results in a 15% standard deviation increase in Math scores. The effect is larger among (estimated) monozygotic than dizygotic pairs, reaching >20% standard deviation. The effect varies across family socioeconomic status. It is strong among disadvantaged families but it nearly disappears among advantaged ones.
Scarcity of uterine resources resulting in intra-uterine growth restriction has a detrimental effect on cognitive development in childhood. This effect interacts with family socioeconomic status (SES), so that low-SES families reinforce the effect of low birthweight and high-SES families fully compensate for it. Findings are particularly relevant in the developing world, where intra-uterine growth restriction is the main determinant of low birthweight.
宫内生长是婴儿死亡率和成年后健康、发育和社会经济结果的有力预测指标。然而，关于这种关系是因果关系还是由低体重婴儿未被观察到的特征所驱动的问题仍未解决。我们使用双胞胎模型来检验宫内生长对儿童认知发展有不利影响的假设。
我们将出生登记处的出生体重信息与智利所有四年级学生的标准化数学和西班牙语考试成绩合并，创建了一个前瞻性数据集。使用双胞胎固定效应模型来估计宫内生长对考试成绩的因果影响。固定效应估计值与横断面出生的传统回归结果进行比较，以衡量横断面模型中未被观察到的遗传、母体和妊娠相关因素所产生的遗漏变量偏差。
双胞胎内的体重差异对考试成绩有很大影响。体重增加 400 克，数学成绩提高 15%的标准差。在（估计的）同卵双胞胎中比异卵双胞胎的效果更大，达到超过 20%的标准差。这种影响因家庭社会经济地位而异。它在贫困家庭中很强，但在富裕家庭中几乎消失了。
子宫资源匮乏导致宫内生长受限对儿童期的认知发展有不利影响。这种影响与家庭社会经济地位（SES）相互作用，因此低收入家庭强化了低出生体重的影响，高收入家庭则完全弥补了这一影响。这些发现在发展中国家尤为重要，因为宫内生长受限是低出生体重的主要决定因素。
