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Lower birth weight indicates higher risk of autistic traits in discordant twin pairs

  • M. Losh (a1), D. Esserman (a2) (a3), H. Anckarsäter (a4) (a5), P. F. Sullivan (a6) and P. Lichtenstein (a7)...



Autism spectrum disorder (ASD) is a neurodevelopmental disorder of complex etiology. Although strong evidence supports the causal role of genetic factors, environmental risk factors have also been implicated. This study used a co-twin–control design to investigate low birth weight as a risk factor for ASD.


We studied a population-based sample of 3715 same-sex twin pairs participating in the Child and Adolescent Twin Study of Sweden (CATSS). ASD was assessed using a structured parent interview for screening of ASD and related developmental disorders, based on DSM-IV criteria. Birth weight was obtained from medical birth records maintained by the Swedish Medical Birth Registry.


Twins lower in birth weight in ASD-discordant twin pairs (n=34) were more than three times more likely to meet criteria for ASD than heavier twins [odds ratio (OR) 3.25]. Analyses of birth weight as a continuous risk factor showed a 13% reduction in risk of ASD for every 100 g increase in birth weight (n=78). Analysis of the effect of birth weight on ASD symptoms in the entire population (most of whom did not have ASD) showed a modest association. That is, for every 100 g increase in birth weight, a 2% decrease in severity of ASD indexed by scores on the Autism – Tics, attention-deficit hyperactivity disorder (AD/HD), and other Comorbidities (A-TAC) inventory would be expected in the sample as a whole.


The data were consistent with the hypothesis that low birth weight confers risk to ASD. Thus, although genetic effects are of major importance, a non-genetic influence associated with birth weight may contribute to the development of ASD.


Corresponding author

*Address for correspondence: M. Losh, Ph.D., Roxelyn and Richard Pepper Department of Communication Sciences and Disorders, 2240 Campus Drive, 2-340 Frances Searle, Northwestern University, Evanston, IL 60208, USA. (Email:


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Lower birth weight indicates higher risk of autistic traits in discordant twin pairs

  • M. Losh (a1), D. Esserman (a2) (a3), H. Anckarsäter (a4) (a5), P. F. Sullivan (a6) and P. Lichtenstein (a7)...


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