OBJECTIVE: The goal of this study was to examine the degree to which children born within the “normal term” range of 37 to 41 weeks’ gestation vary in terms of school achievement.
METHODS: This study analyzed data from 128 050 singleton births born between 37 and 41 weeks’ gestation in a large US city. Data were extracted from city birth records to assess a number of obstetric, social, and economic variables, at both the individual and community levels. Birth data were then matched with public school records of standardized city-wide third-grade reading and math tests. Specifically, we assessed (1) whether children born within the normal term range of 37 to 41 weeks’ gestation show differences in reading and/or math ability 8 years later as a function of gestational age, and (2) the degree to which a wide range of individual- and community-level social and biological factors mediate this effect.
RESULTS: Analyses revealed that gestational age within the normal term range was significantly and positively related to reading and math scores in third grade, with achievement scores for children born at 37 and 38 weeks significantly lower than those for children born at 39, 40, or 41 weeks. This effect was independent of birth weight, as well as a number of other obstetric, social, and economic factors.
CONCLUSIONS: Earlier normal term birth may be a characteristic considered by researchers, clinicians, and parents to help identify children who may be at risk for poorer school performance.
- BOE —
- Board of Education
- CTB —
- California Testing Bureau
- DOHMH —
- Department of Health and Mental Hygiene
- NYC —
- New York City
What’s Known on This Subject:
Late preterm infants are at risk for a variety of developmental impairments; however, little is known about developmental differences among children born within the term range of 37 to 41 weeks’ gestation.
What This Study Adds:
This study links comprehensive birth record data from 128 050 term births to children’s school records 8 years later. Analyses establish that, even among the “normal term” range, gestational age is an important independent predictor of academic achievement.
The developmental risks of early preterm birth are well established. Recently it has been recognized that even late preterm infants, born from 34 to 36 weeks’ gestation, are at risk for adverse developmental outcomes.1–9 Less clear, however, is the degree to which developmental risk varies with gestational age among infants born “at term,” between 37 and 41 weeks’ gestation. Brain development continues throughout gestation, including rapid growth in the final month of pregnancy.10,11 Yet children born from 37 to 41 weeks’ gestation are frequently combined into a single reference group in studies investigating cognitive outcomes.1,3–6,8,9,12–14 It is unclear whether this cut point of 37 weeks is appropriate,12 and the degree of heterogeneity in academic achievement across this 5-week period of “normal” gestation remains largely uninvestigated.
This study uses a retrospective cohort design to examine a large sample of urban American children born across the range of term gestation, to investigate whether earlier gestational age at birth confers a continuum of risk for poor academic achievement. Access to a large data set linking comprehensive birth records to school records affords the unique opportunity to explore a wide range of individual- and community-level social and biological factors that may mediate this effect.
The study sample consists of all singleton births born to mothers residing in New York City (NYC) from 1988 to 1992 who (1) were between 37 and 41 weeks’ gestation, (2) subsequently enrolled in third grade in NYC public schools from 1996 to 2000, and (3) for whom third-grade standardized reading or math test scores were available. Birth records from the NYC Department of Health and Mental Hygiene (DOHMH) were matched to NYC Board of Education (BOE) records, as described previously.15,16 Briefly, a data file from the DOHMH, containing the full name, birth date, and gender of each child was sent to the BOE to be matched against a file containing information on all children who have attended NYC public schools. To be considered a match, the DOHMH and BOE records were required to be identical with respect to the first 6 characters of both the first and last name; the month, day, and year of birth; and gender. There were 150 589 children whose data were successfully matched in this manner. Additional criteria for inclusion in the study included having valid data for all 20 demographic and risk variables described in the next section, membership in 1 of 4 major ethnic groups (Asian American, non-Hispanic African American, non-Hispanic white, Hispanic), and delivery within 1 of the 5 NYC boroughs. In all, 128 050 (85.0%) met these criteria and also had available reading test data. A small number of these children did not have available math test data, and thus analyses involving math scores included a slightly smaller sample of 127 532 children (84.7% of the full matched sample). After matching, the data were de-identified by the BOE and made available for analysis, as part of a protocol approved by the DOHMH, BOE, and the Columbia University Institutional Review Board.
Specification of Variables
The outcomes of interest were the child’s scores on the California Testing Bureau (CTB) Achievement Test, a city-wide proprietary standardized test adapted from the Terra Nova test series specifically for the NYC BOE by CTB/McGraw-Hill. The standardized reading test measured students’ ability to understand continuous prose, focusing on evaluating meaning of written text.17 The standardized math test measured basic mathematical skills, such as computation and estimation.18 The CTB was administered to all NYC public school third-graders from 1996 to 2000. Because the scale of the CTB changed between 1996 and 1998, scores for each year were converted to T scores (M = 50; SD = 10) based on city-wide means and SDs provided by the BOE.15
NYC hospitals abstract information from the medical records of all deliveries, which is reported to the DOHMH. We derived 20 variables representing obstetric, individual-level, and community-level characteristics, many of which confer risk for poor school performance, as suggested by previous studies.15,16,19
Obstetric characteristics included gestational week at birth (defined by the start of the week, ie, from 37 weeks, 0 days, to 37 weeks, 6 days), birth weight, cesarean delivery, parity, low prenatal care (≤6 prenatal visits), and advanced maternal age (≥35 years).
Individual-level characteristics included years of maternal education, Medicaid status, teenage motherhood, marital status, mother’s nativity (foreign-born), history of maternal substance abuse (including alcohol), history of maternal smoking in pregnancy, child gender, and mother’s race/ethnicity (African American non-Hispanic, white non-Hispanic, Asian, and Hispanic).
Community-level characteristics were derived from US Census and NYC Department of Criminal Justice data, to characterize the neighborhood in which the mother resided at the time of delivery. The unit of analysis for community-level variables was the NYC Health Area, as defined by the DOHMH. Each Health Area contains ∼20 000 people and is an aggregate of 4 to 6 contiguous US census tracts. Community-level variables included percentage of residents living below the federal poverty level, percentage who immigrated within the previous 5 years, percentage of housing units with >1 person per room, percentage of residents with stable housing for ≥5 years, and neighborhood homicide rate.
We first assessed whether, within the “normal term” gestational range, there existed a significant relation between weeks of gestation at birth and third-grade reading and/or math scores. Relative risk of reading and math deficits were then calculated for infants born at each gestational week relative to the reference of 41 weeks. Next, as the relationship between gestational age and school achievement scores may be confounded by birth weight, the models were expanded to include birth weight. Last, we examined the effects of all individual, community, and obstetric characteristics described previously.
Table 1 describes the sample with respect to all obstetric, individual, and community-level factors. These summary statistics reflect the striking diversity of the NYC population, with a high proportion of mothers in various risk groups.
Reading and math scores were, unsurprisingly, highly correlated (R = 0.694; P < .001). Initial analyses consisted of 2 one-way analyses of variance, examining the effect of gestational week at birth on third-grade reading and math scores, respectively. Gestational age within the normal term range was significantly and positively related to both third-grade reading score (F [4, 128 045] = 21.635; P < 7.2 × 10−18) and third-grade math score (F [4, 127 527] = 27.904; P < 3.4 × 10−23), with scores improving with each week of gestation, as shown in Figs 1 and 2. Table 2 shows that both reading and math scores for children born at 37 and 38 weeks were significantly lower than those of children born at any other week, adjusting for multiple comparisons by using the Bonferroni method. Differences among children born at 39, 40, or 41 weeks’ gestation were not significant (although in all cases, the nonsignificant trend was for greater reading and math scores at later gestational ages).
We next assessed, for each week of gestation, the relative risk of mild, moderate, and severe reading and math impairments, defined as at least 1.0, 1.5, and 2.0 SDs below the population average, respectively. Table 3 shows that relative to children born at 41 weeks’ gestation, children born at 37 weeks have a 14% greater risk of having at least mild reading impairment, a 23% increased risk of having at least moderate reading impairment, and a 33% increased risk of having a severe reading impairment. Children born at 38 weeks’ gestation have an 8% increased risk of at least mild reading impairment and a 13% increased risk of at least moderate reading impairment. Table 4 shows that children born at 37 weeks have a 16% greater risk of having at least mild math impairment and a 19% increased risk of having at least moderate math impairment. Children born at 38 weeks have a 12% increased risk of having at least mild math impairment.
Because of the association between birth weight and gestational age,14,20 birth weight was introduced into the models. Both birth weight and gestational age were significant, independent predictors of reading score (birth weight: β = 0.062; P < .0001; gestational age: β = 0.011; P < .0001) and math score (birth weight: β = 0.092; P < .0001; gestational age: β = 0.007; P < .012).
To elucidate the possible mechanisms mediating the effect of gestational age on school achievement, we next developed 3 general linear models. In each model, both reading and math scores were included as dependent variables. The 3 models incorporated, respectively, the obstetric, individual-level, and community-level characteristics described previously. Most of these variables were highly significant predictors of third-grade reading and math scores. Table 5 shows that gestational age at birth continues to be a significant predictor of school achievement, even after adjusting for these obstetric, individual-level, and community-level characteristics.
The American Academy of Pediatrics and the National Institute of Child Health and Human Development recently classified infants born from 34 to 36 weeks’ gestation as “late preterm,”21 signaling an awareness that these infants are at increased risk for a number of developmental outcomes, including lower IQ,1,2 developmental delay,3,4 deficits in visuospatial and executive function skills,5 reading difficulties,6,7 behavioral disorders,8 attention-deficit/hyperactivity disorder,9 and even mental retardation3,8 and cerebral palsy.3,8 Surprisingly, far less is known about the degree to which earlier gestational age confers risk among infants born at term, from 37 to 41 weeks’ gestation.7,22–24 The brain continues to grow rapidly during this time, with a nearly 50% increase in cortical gray matter,10 a nearly threefold increase in myelinated white matter,10 and increasing neuronal and gyral differentiation.11 Further, among term infants born from 37 to 41 weeks, later gestational age is associated with greater gray matter density in middle childhood in bilateral superior and middle temporal gyri, and the left parietal lobe.25 These regions have been associated with reading26 and math performance,27 respectively.
Based on this reasoning, we hypothesized that the commonly held belief that children born between 37 and 41 weeks’ gestation will tend to develop “normally” without any difference as a function of the particular week of gestation may not be accurate.
This study provides support for this hypothesis. Among a sample of 128 050 children born at term, we found a significant, positive relationship between gestational age at birth and third-grade school achievement. Each week of increased gestation from 37 to 41 weeks showed an added benefit in both reading and math scores. Further, children born at 37 or 38 weeks performed significantly worse than children born at 39, 40, or 41 weeks, and have a significantly increased relative risk of impaired reading and math skills on standardized school achievement tests.
These findings have important implications in considering the definition of “term.” The intrauterine environment likely supports typical brain development, which may be more likely to be disrupted when children are born early, even within the commonly defined period of term gestation. This disruption may affect later academic achievement, as our findings suggest.
The mechanisms underlying the effect of gestational age at birth on school achievement are likely multifactorial. Because of the nature of this remarkable data set, in which birth records containing a large set of variables were matched with corresponding public school records 8 years later, we were able to examine the effects of a number of obstetric, economic, and social characteristics. Indeed, nearly all characteristics assessed were highly significant predictors of third-grade school achievement. Although a broad range of obstetric, individual-level, and community-level socioeconomic and demographic characteristics were considered, the effect of gestational age persisted even when controlling for these potential mediating factors. Future research is necessary to investigate the causal mechanism(s) explaining the relations described here. More specifically, we may ask: Is there a subtle yet meaningful risk of impaired development caused by birth at 37 to 38 weeks, relative to birth in the 39- to 41-week range? Or, is the apparent risk of what might be called “early term birth” (37 to 38 weeks’ gestation), seen in Figs 1 and 2, caused by the confounding effects of other unmeasured factors, risks that are themselves associated with both early term birth and school performance?
Regardless of the mechanism, the evidence presented previously suggests that it may be inappropriate to cluster children born between 37 and 41 weeks’ gestation together as a single category when considering developmental outcomes. This has several important implications.
From a scientific perspective, the inappropriate grouping of heterogeneous populations may lead to a loss of power when investigating developmental differences. From a clinical perspective, these data suggest that early term birth may be a characteristic by which pediatricians may identify children who may be at risk for poorer school performance. It should also be noted that there is an increasing trend for performing elective early deliveries for nonmedical reasons,28 contributing in part to the fact that the most common length of gestation for singleton births has shifted from 40 to 39 weeks.28 Although further research is needed, women or physicians seeking early delivery for social or logistical reasons may wish to consider this finding, particularly before 39 weeks.
This study has several limitations. Notably, although gestational age from 37 to 41 weeks showed a graded relationship with third-grade reading and math scores, the effect size was small. Many other social, economic, and obstetric factors predict academic achievement in elementary school; however, the goal of this study was not to provide a comprehensive model accounting for the largest possible amount of variance in school achievement. Rather, we asked specifically whether there would be a detectable difference in reading and math achievement among children born at different weeks of gestation within the commonly accepted normal range, and we have answered that question in the affirmative. Given that so many other powerful factors affect school performance in the years between birth and third grade, the fact that, 8 years later, we still observe statistically significant differences between children born only 1 week apart (eg, 38 vs 39 weeks), within the normal range of gestational ages, is noteworthy. Further, although the average difference in score by gestational week was small, the finding of a significantly increased relative risk for reading and math impairment renders these data clinically relevant. For example, children born at 37 weeks’ gestation were found to be 33% more likely to experience a severe reading deficit (defined here as 2 SDs below the mean) relative to children born at 41 weeks’ gestation. Of course, this study’s large sample size provided considerable statistical power. Thus, although smaller increases in relative risk were also detectable for more mild deficits, the degree to which some of these more moderate increases in risk should translate to effects on clinical decision-making is not yet clear.
Another limitation of the study may be restricted generalizability. The sample was obtained from birth records in a large American city, consisting of a relatively high proportion of minority and disadvantaged families. The causes underlying early term birth in this population (including whether early deliveries were performed on an elective basis) are unknown. However, other recent population-based studies also indicate a negative impact on cognition from birth at 37 to 38 weeks relative to later birth in Denmark,7 Belarus,22 Switzerland,24 and Scotland,23 suggesting that the effect reported here may indeed be robust.
This study involved secondary data in which the primary measures were not obtained by using rigorous research methods. Birth record data were abstracted by hospitals, and more precise information, such as whether gestational age was obtained by dates or ultrasound, is not available. Additionally, it is likely that some gestational ages may have been incorrectly assigned prenatally or in the delivery room. Factors that could further illuminate underlying mechanisms, such as the percentage of elective versus emergent deliveries, are unknown. The particular outcome measures used do not reflect all aspects of reading or math achievement, and effects on other skills (writing, mathematical reasoning) are unknown. Indeed, later editions of these standardized tests have subsequently been updated to reflect more current educational testing practices, focusing more, for instance, on abstract reasoning and solving real-life problems.18
Further, third-grade children taking tests in a classroom are distractible and not necessarily motivated to perform well, and differences between schools, classes, and testing environments may create variability in test scores unrelated to true reading and math ability. These factors, together with other unmeasured factors between birth and age 8, contribute to measurement error, and therefore reduce effect size. So although the error measurement inherent in the use of public records is a necessary limitation in research of this type,29–31 we would argue that because of the effect-attenuation of this error measurement, the negative impact of early term birth on academic achievement is likely greater than the small but significant effect we report here.
Increased gestational age at birth has a positive association with third-grade reading and math scores among children born in the 37- to 41-week range, commonly defined as term gestation. From a public health perspective, this may have important consequences, particularly in the realm of identifying children who may be at risk for poorer school achievement. Elucidating the mechanisms underlying this association will require further research; however, in light of the increasing trend for performing elective early deliveries for nonmedical reasons, researchers, clinicians, and parents are urged to consider this graded relationship between weeks of gestation and school performance.
- Accepted March 27, 2012.
- Address correspondence to Kimberly Noble, MD, PhD, Assistant Professor of Pediatrics, Columbia University, 630 W. 168th St, P&S Box 16, New York, NY 10033. E-mail:
FINANCIAL DISCLOSURE: The authors have indicated they have no financial relationships relevant to this article to disclose.
FUNDING: Funding for this research was provided by the John M. Driscoll, MD Scholars Program (K.G.N.), NIH grant P01ES009600 (H.F.A. and V.A.R.), ACYF Head Start Bureau grant 90-YD-0023 (H.F.A. and V.A.R.) and NIH grant R37 HD032773 (W.P.F.). Funded by the National Institutes of Health (NIH).
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- Copyright © 2012 by the American Academy of Pediatrics