Psychological Medicine

Original Articles

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

M. Losha1 c1, D. Essermana2a3, H. Anckarsätera4a5, P. F. Sullivana6 and P. Lichtensteina7

a1 Roxelyn and Richard Pepper Department of Communication Sciences and Disorders, Northwestern University, Evanston, IL, USA

a2 Department of Medicine, Division of General Medicine and Clinical Epidemiology, UNC Chapel Hill, NC, USA

a3 Department of Biostatistics, Gillings School of Global Public Health, UNC Chapel Hill, NC, USA

a4 Department of Clinical Sciences, Lund University, Sweden

a5 Institute of Neuroscience and Physiology, University of Gothenburg, Sweden

a6 Department of Genetics, UNC Chapel Hill, NC, USA

a7 Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Sweden


Background 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.

Method 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.

Results 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.

Conclusions 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.

(Received November 18 2010)

(Revised September 09 2011)

(Accepted September 12 2011)

(Online publication December 02 2011)


c1 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: