Approximately 1% of children are born with an autism spectrum disorder (ASD), which is a neurological condition that affects a range of behavioral and cognitive domains. Studies on ASD have focused on risk factors including maternal stress, parental age, environmental influences, and genetics. One potential risk factor that has also been looked at is birth weight. However, the relationship between birth weight and predisposition for ASD is still unclear. Some research involving twins has suggested a higher risk for ASD among monozygotic twins (MZ) or identical twins rather than dizygotic (DZ) or fraternal twins. This could be due to the totality of shared genetics among MZ versus DZ twins. In these studies and studies involving single births, birth weight has been linked to ASD.
To get a better picture of how birth weight affects risk and severity of ASD, Molly Losh of the Roxelyn and Richard Pepper Department of Communication Sciences and Disorders at Northwestern University in Illinois led a study on both MZ and DZ twins. She evaluated parent reports of ASD severity and compared the birth weights of over 3,700 same-sex twin sets that were part of the Child and Adolescent Twin Study of Sweden (CATSS).
Losh found that the low-birth-weight twins had a threefold increased risk for ASD than twins with heavier birth weights. She also discovered that every 100-gram weight increase resulted in a risk reduction of 13%. Additionally, the increased birth weights also correlated with decreases in symptom severity of spectrum disorders including attention-deficit hyperactivity disorder (ADHD). The results also revealed a clear link between birth weight and isolated deficits. Specifically, the twins with the lowest birth weights were more likely to have language and social impairments than problems with repetitive actions like tapping or rocking. Losh said that although genetic effects were very important, there might be a nongenetic influence associated with birth weight that contributes to the development of ASD.
Losh, M., Esserman, D., Anckarsater, H., Sullivan, P. F., Lichtenstein, P. (2012). Lower birth weight indicates higher risk of autistic traits in discordant twin pairs. Psychological Medicine, 42.5, 1091-1102.
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