The Effects of Birth Order and Birth Spacing on Autism Symptom Severity in Simplex Families

Background:

Previous research has demonstrated several interesting findings related to birth order and birth spacing and autism spectrum disorder (ASD). Regarding birth order, studies have found an increase in the severity of ASD between first and second born siblings. Other studies have reported an increased incidence of ASD in second born children compared to first born children. The increases in severity and incidence appear to be related to the spacing between pregnancies. To further explore this phenomenon, we looked at measures of ASD severity across birth orders in the Simons Simplex-Collection (SSC) dataset, created by the Simons Foundation Autism Research Initiative (SFARI).

Objectives:

The purpose of this study is to analyze how birth order and birth spacing affects autism symptom severity. To this end, comparisons were made between first and second born ASD cases. In addition, ASD cases that were born within 2 years of an older sibling were compared to those that were born greater than 2 years after a sibling.

Methods:

The SSC collection consists of data collected from 12 participating university-affiliated research clinical sites. The data is a comprehensive collection of approximately 4,000 to 6,000 phenotypic data points for each SSC family. As a starting point, we compared data from several measures including: Social Responsiveness Scale (SRS), Vineland Adaptive Behavior Scale-II (VABS-II), Child Behavior Checklist (CBCL), and verbal and non-verbal intelligence scores. Independent samples t-tests using Bonferroni corrected alpha levels were used to determine significant differences in clinical measures of ASD severity.

Results:

The group of second born ASD cases demonstrated significantly greater symptom severity than the group of first born ASD cases across multiple measures. Furthermore, the group of ASD cases born within 2 years of an older sibling was shown to exhibit significantly greater symptom severity than the group of ASD cases born more than 2 years after a sibling. This finding was also observed across multiple measures.

Conclusions:

The results of this study confirm and extend previous studies on the effects of birth order and birth spacing on ASD. ASD symptoms were more severe in probands who were born after an unaffected sibling, especially if their birth occurred within 2 years of their older sibling.