Firstborn Child with Autism Spectrum Disorder: Effect of Birth Order Relative to Unaffected Sibs

Background: In families with no child with an autism spectrum disorder (ASD), birth order may affect communication, social development and parents’ perception of development. Children with ASD may be first born or have younger, older, or both younger and older sibs without a diagnosis of ASD.  Parents complete the ADI-R (Lord, 1994) which probes social, communication, and repetitive stereotyped behaviors (RSB) to help confirm diagnosis.  However, little is written about whether birth order of the non-ASD sibs relative to the birth order of the first child diagnosed with ASD affects parent perception of that child’s social, communication and RSB behaviors. 
Objectives: The aims of this study are (1) to evaluate whether parent perceptions of symptoms as reported on the ADI-R of their first child diagnosed with ASD are affected by birth order and (2) to evaluate whether parent report of symptoms differs in families where the child with ASD was an only child or had no younger siblings (stoppage) from families with a younger child born within 3 years of the child with ASD or had a younger child born over 3 years later than the child with ASD.
Methods: Children and adolescents (n=275) were enrolled from a genetic study of ASDs. The sample was predominantly Caucasian (n=214) and male (n= 214). The child’s ADI-R social, communication and RSB domain scores were used to evaluate if there was a birth order effect on parent perception of the child’s symptoms. In multiplex families, the first affected proband was included in the analysis.  There were 150 firstborn children with ASD and 125 with older, younger, or both older and younger sibs.  The analysis to evaluate the effect of stoppage included 156 probands (STOP) who were only children or had no younger siblings, 44 ASD children (YS<3) with younger siblings born within 3 years of ASD child, and 75 ASD children (YS>3) with younger siblings born over 3 years after ASD child.
Results: The results of multivariate ANOVA showed there were no significant main effects for birth order and ADI-R Social (firstborn, X=19.819±SE.532; not firstborn, X=20.025±SE.628),  Communication (firstborn, X=13.959±SE.397; not firstborn, X=13.385SE.469), and RSB (firstborn, X=5.699±SE.213; not firstborn, X=5.945±SE.252)  domain scores. A second multivariate ANOVA indicated no significant main effects for stoppage and ADI-R Social (STOP: X=19.611±SE.43; YS<3, X=18.799±SE.809; YS>3, X=19.42±SE.62), Communication (STOP, X=13.958±SE.317; YS<3, X=13.714±SE.597; YS>3, X=13.535±SE..458), and RSB (STOP, X=5.922±SE.177; YS<3, X=5.843±SE..333; YS>3, X=5.627±SE.265) domain scores.
Conclusions: Neither birth order of the first child identified with ASD nor stoppage after the birth of a child with ASD appear to affect parent perception of social, communication and RSB symptomatology at the time the parent completes the ADI-R.  Research to evaluate whether birth order affects parent report of social and communication symptoms for the child with ASD 10 years later in families where there is an older or younger both unaffected sibs might better focus therapy. This data also highlights the fact that 43.3% of parents had at least one other child after the diagnosis of the first child with ASD.