Is Adverse Perinatal Outcome Associated with IQ in Children with Autism Spectrum Disorders?

Background:  Measured Intellectual Quotient (IQ) is associated with genetic, perinatal and postnatal factors. Studies suggest the impacts of genetic and perinatal environment on IQ are mediated by socio-economic status (SES) such that genetic and perinatal effects are most evident in mid-high SES populations without competing postnatal effects and are diminished or not observed in disadvantaged populations.

Having an autism spectrum disorder (ASD) is also highly associated with IQ deficits; 30-60% of children with an ASD have a co-occurring intellectual disability (ID).  Whether environmental factors independent of a child’s ASD pose additional risk for low IQ is not well studied.

Objectives:  To examine associations between adverse perinatal outcomes and IQ among children with an ASD.

Methods: Singleton children born in 1994 and identified through school and health record review as having an ASD by age 8 years were selected from a US population-based surveillance network. The sample included 1,129 children from10 sites that adequately captured IQ data who were born in the same state (and linked to their birth record). Mean IQ and dichotomous IQ outcomes, ≤borderline (<85), ≤mild ID (≤70), and ≤moderate ID (<50), were examined according to whether a child was preterm (<37 weeks’ gestation) or term small-for-gestational-age (tSGA, term delivery and birthweight <10^th percentile for gestational age of a US referent adjusted for sex). Results were stratified by race and maternal education at birth and adjusted for child sex and ASD classification (autism; ASD-NOS; behavioral description consistent with DSM-IV but no previous ASD diagnosis/classification).    

Results: Overall, mean IQ was significantly (p<0.05) lower in children delivered preterm (69.5) than term (74.5). Among those delivered term, mean IQ was significantly lower in those with tSGA (69.3 vs. 75.3). The preterm effect was evident across ID levels. The tSGA effect was limited to borderline-mild ID. After stratification and adjustment, the preterm-IQ association remained significant only among non-Hispanic white children with maternal education ≤high school. In this group, adjusted mean IQ was 8 points lower among those delivered preterm (65.4) than term (73.8). After stratification and adjustment, tSGA was associated with a statistically significant 8-point deficit in mean IQ (75.5 vs. 83.8) in non-Hispanic white children with maternal education >high school, and a 6-point IQ deficit that approached significance (68.4 vs. 74.5, p=0.10) in non-Hispanic white children with maternal education ≤high school. Non-Hispanic black children in both maternal education groups had significantly lower mean IQs than white children with little variation by preterm or tSGA. Results were similar using SES measures based on birth census tract rather than maternal education.  

Conclusions: The variation in IQ by socio-demographic and perinatal factors suggests that IQ deficits among children with ASD are partially independent of the ASD and possibly preventable. However, under-identification of minority children with higher functioning ASD might also contribute to the high proportion of non-Hispanic black children with IQ deficits. Studies of the level and effectiveness of early intervention services received by race, SES, and perinatal risk sub-groups are needed.