Journal Issue: School Readiness: Closing Racial and Ethnic Gaps Volume 15 Number 1 Spring 2005
Low Birth Weight and School Readiness
Extensive research confirms that low birth weight children are at greater risk for cognitive and school performance problems than are their normal birth weight peers, and that the risk for adverse outcomes increases as birth weight decreases.36 A meta-analysis of case control studies reported from 1980 to November 2001 found that the mean IQ for school-aged children born very preterm is approximately two-thirds of a standard deviation below that of controls who were born at term.37 A population-based study using linked birth certificate and school records from Florida found that the risk of specific school-identified disabilities increases as birth weight decreases.38 Enrollment in special education also follows a birth weight gradient, with the lightest babies being most likely to be placed in such programs.39 While all of these findings are based on cohorts born before the major advances in neonatal intensive care of the 1990s, research on later cohorts yields similar results.40
Children born preterm have greater difficulty completing tasks involving reading, spelling, and math than their full-term peers, though math scores are more consistently related to preterm birth or very low birth weight than are reading achievement scores.41 Preterm children tend to have language difficulties related to grammar and abstraction.42 They also tend to be more inattentive, aggressive, and hyperactive, as well as less able to handle leadership roles than their full-term peers.43
Some cognitive deficits are the direct result of medical disorders.44 Compromised motor skills in many preterm infants, for example, may lead to learning disabilities and handicaps.45 Studies of the brains of preterm and full-term children have identified areas that correspond to the cognitive deficits observed. Brain volume tends to be reduced, resulting in larger ventricles containing more cerebrospinal fluid, thinning of the corpus callosum (which indicates less white matter), and a reduction in gray matter. The sensorimotor cortex, amygdala, and hippocampus are also often reduced.46 These anatomical deficiencies are most likely a result of immaturity, physiological instabilities, or stressful experiences as neonates.47
Birth weight may also have indirect effects on cognitive development through parenting. The medical, developmental, and behavioral problems of a very light infant may heighten parental stress, which may in turn impair the child's learning. Research in this area is in its infancy. According to one recent study, mothers of very low birth weight infants suffered more psychological distress than mothers of term infants at one month, at two years, and at three years, with the severity of stress positively related to the child's developmental outcomes.48
Collectively, past studies based on hospital or regional cohorts have found that among extremely low birth weight infants, 8 to 18 percent have IQ scores under 70 (a cutoff often used to define mental retardation), and 25 to 29 percent have IQs in the 70–84 (borderline) range at school age (generally ages six or eight to ten). The corresponding figures for very low birth weight infants (here, 1,000–1,499 grams) are 5 percent and 19 percent; for moderately low birth weight infants, 5 percent and 17 percent; and for normal birth weight infants, 0 to 4 percent and 4 to 14 percent (the figures for very low and moderately low birth weight infants are based on only one study).49
Birth Weight and Socioeconomic Status
Birth weight is but one of many risk factors for cognitive impairment. One of the most salient risk factors is low socioeconomic status. Disentangling the effects of birth weight from those of the many socioeconomic disadvantages linked with low birth weight is difficult. Research to date indicates that very low birth weight (and—much less so— moderately low birth weight) does have independent deleterious effects on early cognitive outcomes, such as IQ and PIAT scores.50 But while it might be interesting from a variety of vantage points to disentangle the effects of birth weight and socioeconomic status, the two are so highly correlated that it may not be relevant for policy purposes to do so.
Low Birth Weight and Aggregate Educational Outcomes
Clearly, individual children born low birth weight can be seriously disadvantaged with respect to schooling. But because most serious birth weight–related disabilities tend to occur at the lowest weight ranges and therefore affect a very small proportion of children, low birth weight may not explain much of the observed variation in educational attainment at the aggregate level. A recent study of children born in the 1958 British birth cohort, for example, found that children born at low birth weight passed significantly fewer O-level exams. But being born at low birth weight, or being born to a mother who smoked during pregnancy (also a predictor of poor educational outcomes), explained only 2.5 percent of the variation in O-level results.51
Low Birth Weight and the Black-White Gap in Test Scores
Only two studies of which I am aware have presented data indicating the potential effect of low birth weight on racial test score gaps. Yolanda Padilla and her coauthors, in a study using National Longitudinal Survey of Youth (NLSY) child data and focusing on the effects of the Mexican-American birth weight advantage on early childhood development, found that low birth weight explains less than 1 percent of the (unadjusted) black-white gap in scores on the Peabody Picture Vocabulary Test-Revised (PPVT-R) among three- and four-year-olds in the late 1980s and early 1990s.52 Jeanne Brooks-Gunn and her coauthors presented a similar estimate in a recent analysis of the contributions of family and test characteristics to the black-white test score gap.53 Also using NLSY child data, they found that low birth weight and gender together explain less than 2 percent of the unadjusted racial gap in PPVT-R scores at age five.
My own estimate of the potential impact of birth weight on the racial gap in one test of cognitive ability—full-scale IQ score—is similar, though somewhat higher. My subject is all black and white infant survivors born in 2000, including multiples. In contrast to Padilla and Brooks-Gunn I do not use the NLSY data, because although that data set has actual test scores, it may under-represent the very lightest babies. Instead I use vital statistics data, which provide exact race-specific birth weight distributions for surviving infants in the United States, though test scores must be imputed. I assigned an IQ score to each survivor, based on the infant's birth weight. I then computed the racial gap in imputed IQ scores and divided this figure by the total observed racial gap in IQ scores, to compute the maximum proportion of the overall gap that can be explained by birth weight.54 Using various distributions of IQ scores based on past research and a range of assumptions, I found that birth weight explains a maximum of 3 to 4 percent of the racial gap in IQ scores, or one-half a point in IQ.