Epidemiology
DOI: 10.1111/1471-0528.12071 www.bjog.org
Obstetric factors and different causes of special educational need: retrospective cohort study of 407 503 schoolchildren DF Mackay,a GCS Smith,b R Dobbie,c S-A Cooper,a JP Pella a
Institute of Health and Wellbeing, University of Glasgow, Glasgow, UK b Department of Obstetrics and Gynaecology, Cambridge University, Cambridge, UK c Information and Statistics Division, NHS Scotland, Edinburgh, UK Correspondence: Prof J Pell, Henry Mechan Professor of Public Health, Institute of Health and Wellbeing, University of Glasgow, 1 Lilybank Gardens, Glasgow, G12 8RZ, UK. Email
[email protected] Accepted 2 July 2012. Published Online 27 November 2012.
Objective To determine whether relationships with gestational age
and birthweight centile vary between specific causes of special educational need (SEN). Design Retrospective cohort study. Setting Scotland. Population A cohort of 407 503 schoolchildren. Methods Polytomous logistic regression was used to examine the risk of each cause of SEN across the spectrum of gestation at delivery and birthweight centile, adjusting for potential confounding factors. Main outcome measures Crude and adjusted odds ratios and
confidence intervals. Results Of the 19 821 children with SEN, 557 (2.8%) had sensory
impairments, 812 (4.1%) had physical or motor disabilities, 876 (4.4%) had language impairments, 2823 (14.2%) had social, emotional, or behavioural problems, 7018 (35.4%) had intellectual disabilities, 4404 (22.2%) had specific learning difficulties, and 1684 (8.5%) autistic spectrum disorder (ASD). Extreme preterm delivery (at 24–27 weeks of gestation) was a strong predictor of sensory
(adjusted OR 23.64, 95% CI 12.03–46.45), physical or motor (adjusted OR 29.69, 95% CI 17.49–50.40), and intellectual (adjusted OR 11.67, 95% CI 8.46–16.10) impairments, with dose relationships across the range of gestation. Similarly, birthweight below the third centile was associated with sensory (adjusted OR 2.85, 95% CI 2.04–3.99), physical or motor (adjusted OR 2.47, 95% CI 1.82–3.37), and intellectual (adjusted OR 2.67, 95% CI 2.41–2.96) impairments. Together, gestation and birthweight centile accounted for 24.0% of SEN arising from sensory impairment, 34.3% arising from physical or motor disabilities, and 26.6% arising from intellectual disabilities. Obstetric factors were less strongly associated with specific learning difficulties and social or emotional problems, and there were no significant associations with ASD. Conclusions The association between gestation and birthweight
centile and overall risk of SEN is largely driven by very strong associations with sensory, physical or motor impairments, and intellectual impairments. Keywords Achievement, educational, learning disabilities, low birthweight, fetal growth retardation, pregnancy, premature birth.
Please cite this paper as: Mackay D, Smith G, Dobbie R, Cooper S, Pell J. Obstetric factors and different causes of special educational need: retrospective cohort study of 407 503 school children. BJOG 2013;120:297–308.
Introduction Special educational needs (SEN) place a large burden on the educational sector, and have significant impact on both the child and family. Attempts to treat or prevent the causes of SEN require a good understanding of their underlying aetiology. Preterm delivery and intrauterine growth restriction are associated with long-term neurodevelopmental problems, such as learning and language difficulties, impaired cognitive function and behavioural problems,1–4 which in turn can
result in SEN.5–7 Most previous studies have tended to treat gestation at delivery and birthweight as binary variables. Whereas this approach has been useful in establishing an association, it loses important information on the true nature of the relationship. In relation to overall SEN, we previously showed that there is no threshold effect: rather, there is a dose relationship across the whole spectrum of gestational age and intrauterine growth.8 This finding is of clinical significance in light of the increasing numbers of elective, early term deliveries (at 37–39 weeks of gestation).9
ª 2012 The Authors BJOG An International Journal of Obstetrics and Gynaecology ª 2012 RCOG
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The relative risk among children born early term is much lower than for preterm children. However, because they account for a much higher percentage of deliveries, their contribution at a population level is much greater. It is not clear to what extent the relationships that we observed with gestational age and intrauterine growth are common to all causes of SEN, or whether they are specific to some rather than all. In particular, studies have produced conflicting results on whether obstetric factors have a true association with the risk of autistic spectrum disorder (ASD).10,11 The aim of our study was to determine whether gestation at delivery and intrauterine growth are associated with specific causes of SEN, including ASD, and to calculate the proportion of cases caused by each condition that can be attributed to these obstetric factors.
Methods Data sources Under the Special Educational Needs and Disability Act of 2001, both schools and local education authorities in the UK have a statutory duty to identify, assess, and make provision for children with SEN. The Department of Education defines SEN as a learning difficulty that requires special educational provision. In turn, a learning difficulty is defined as either significantly greater difficulty in learning than the majority of children of the same age or a disability that prevents or hinders a child from making use of educational facilities of the kind generally provided for children of the same age (http://www.education.gov.uk/aboutdfe/statutory/g00203393/ lda/definition). The term SEN includes: language impairments; specific learning difficulties (such as dyslexia or dyscalculia); intellectual disabilities; other developmental disorders that impair learning (including autism, Asperger’s syndrome, and attention deficit hyperactivity disorder); social, emotional, or behavioural problems that impair learning; and physical disabilities that impact on learning (including some sensory impairments, or physical or motor disabilities). In the database, the groups are mutually exclusive. Children with more than one cause of SEN are classified on the basis of their main impairment. For the purposes of this study, the intellectual disability groups (moderate, severe, and profound intellectual disabilities, with or without additional complex needs) were aggregated into one group. The school census covers all schools in Scotland (mainstream schools, special schools, and special classes and units within mainstream schools) irrespective of their funding source (local authority, grant-aided, independent, and self-governing schools). The census includes all primary and secondary school children. Children on long-term illness absence are included, but adults (>19 years of age) who were attending courses located in schools are excluded.
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The information is collected at the level of individual pupils and includes a record of need for all children with an SEN and the underlying condition. The data are collected by the head teacher of each school and are submitted to the relevant local education authority. The response rate among schools is 99.8%. Nineteen of the 32 local education authorities agreed to provide us with data from their 2005 school census. The participating authorities covered a total population of 3.8 million. The Scottish Morbidity Record (SMR2) collects information on all women discharged from Scottish maternity hospitals, including maternal and infant characteristics, clinical management, and obstetric complications. The SMR2 is subjected to regular quality assurance checks and has been more than 99% complete since the late 1970s. A quality assurance exercise performed in 2010 compared a 4.4% sample of SMR2 returns (n = 2531) with case records, and demonstrated high quality for all of the data fields used in our study: in particular, infant sex was 100% complete and accurate, birthweight was 99% complete and accurate, and estimated gestation was 92% complete and accurate.12 In the SMR2, gestational age at birth is defined as the number of completed weeks of gestation on the basis of the estimated date of delivery recorded in each woman’s clinical record. Gestational age has been confirmed by ultrasound in the first half of pregnancy in more than 95% of women in the UK since the early 1990s. Previous miscarriage was defined as a previous delivery of a conceptus showing no signs of life before 24 weeks of gestation, and excluded therapeutic abortions. Previous therapeutic abortion was defined as a previous therapeutic termination of pregnancy, by any means, prior to 24 weeks of gestation. Children’s postcodes of residence were used to determine their level of socioeconomic deprivation using the Scottish Index of Multiple Deprivation (SIMD). The SIMD is derived from 38 indicators across seven domains (income; employment; health; education, skills and training; housing; geographic access; and crime) using information collected from the population census applied at the data zone level (median population 769) (http://www.scotland.gov.uk/Topics/ Statistics/SIMD). As part of a previous study examining overall SEN,8 82% of children in the 2005 school census data had already been linked, via birth certificate data, to the relevant SMR2 record to provide individual-level obstetric data. We excluded individuals who were aged 19 years at the time of the school census, and births where the maternal height was recorded as less than 100 cm or more than 200 cm, the birthweight was recorded as less than 400 g or more than 5000 g, or the gestation at delivery was recorded as less than 24 weeks of gestation or more than 43 weeks of getsation. We also excluded multiple births because the
ª 2012 The Authors BJOG An International Journal of Obstetrics and Gynaecology ª 2012 RCOG
Obstetric factors and special educational need
SMR2 record does not record infant name. Therefore, in the case of multiple births, we could not ensure that the school census record was linked to the correct infant. Permission to access, link, and analyse these data was granted by both the South-East of Scotland Multi-Centre Research Ethics Committee and the Scottish Privacy Advisory Committee.
Statistical analyses Continuous variables were summarised by the median and the interquartile range. Univariate comparisons between groups were performed using Kruskal–Wallis, chi-square, and Cuzick’s tests for trends for continuous, categorical, and ordinal data, respectively. The P values for all hypothesis tests were two-sided and statistical significance was assumed at P < 0.05. The associations between obstetric factors and the risk of each cause of SEN were analysed using a single univariate, then multivariable, polytomous logistic regression model using no SEN as the common referent category. Separate models were constructed for each cause of SEN using children with no SEN as the common referent category. The covariates included in the multivariable analysis were infant sex, maternal age and height, marital status, parity, induction of labour, mode of delivery, year of delivery, previous spontaneous and therapeutic deliveries, and the 5–minute Apgar score. Gestation- and sex-specific birthweight centiles were calculated and included as a covariate. The year of birth ranged from 1986 to 2001. However, at the extremes of age, not all children were attending school. Therefore, there were fewer children in the extreme years of birth. Hence, years 1986 and 1987, and years 2000 and 2001, were combined because of the smaller numbers of children born in these years. The analyses were initially run including only cases with complete information, and were then re-run using imputation of missing values for maternal height (the most commonly missing variable). The ‘ice’ module available in STATA was used to calculate values for maternal height using multiple imputation by chained equations.13,14 Variables included in the imputation included all covariates in the final model and the outcome variable. Five imputed data sets were created. The goodness of fit was assessed using a plot of observed versus expected values as well as Hosmer–Lemeshow tests. These tests showed that the model was an adequate fit for the data. We tested the independence of the irrelevant alternatives (IIA) assumption using a Hausman test.15 The test showed that this assumption was satisfied, implying that the various SEN outcomes are independent of each other. Adjusted population attributable fractions were estimated using the ‘aflogit’ command in STATA to determine what proportion of SEN cases were potentially explained by gestational age at delivery.16 All statistical analyses were undertaken using STATA 11.2 (Stata Corporation, Texas, USA).
Results Of the 514 118 children included in the 2005 school census, 93 340 (18.2%) could not be linked to their SMR2 obstetric data. Of the 420 778 (81.8%) who were successfully linked, 13 275 were ineligible for inclusion because they fulfilled one or more exclusion criteria. Of the remaining 407 503 children, 362 688 (89%) had complete data on all variables. Of these, 19 821 (5.5%) had a record of an SEN. Among these, 557 (2.8%) had sensory impairment, 812 (4.1%) had physical or motor disabilities, and 876 (4.4%) had language impairment. A total of 2823 (14.2%) children had social, emotional, or behavioural problems, 4404 (22.2%) had specific learning difficulties, 7018 (35.4%) had intellectual disabilities, 1684 (8.5%) had ASD, and in 1647 (8.3%) children the underlying condition was unspecified (Table 1). Overall, 21 959 (5.4%) children were born preterm (
