Is early childhood development impeded by the birth timing of the younger sibling?

BACKGROUND: This study investigated whether the timing of birth of the younger siblings was associated with the risk of the older siblings' developmental vulnerability in early childhood. METHODS: Linkage of population-level birth registration, hospital, and perinatal datasets to Australian Early Development Census (AEDC) records (2009-2015), enabled follow-up of a cohort of 32,324 Western Australia born singletons. Children with scores <10th percentile on an individual AEDC domain (Physical Health and Wellbeing; Social Competence; Emotional Maturity; Language and Cognitive Skills (school-based); and Communication Skills and General Knowledge) were classified as developmentally vulnerable. Modified Poisson Regression was used to estimate relative risks (RR) for associations between post-birth interpregnancy intervals (IPIs) and developmental vulnerability. RESULTS: Relative to post-birth IPIs of 18-23 months, post-birth IPIs of <6 and 6-11 months were associated with an increased risk of children being classified as DV1 (aRR 1.21, 95% CI: 1.11-1.31) and DV2 (aRR 1.31, 95% CI: 1.15-1.49); and DV1 (aRR 1.10, 95% CI: 1.03-1.17) and DV2 (aRR 1.21, 95% CI: 1.09-1.34), respectively. Post-birth IPIs of <6 months were associated with an increased risk on four of the five AEDC domains. Post-birth IPIs of 48-60 months were associated with an increased risk of developmental vulnerability; however, the risk was statistically significant for DV1, DV2 and the domains of Emotional Maturity and Language and Cognitive Skills (school-based). CONCLUSIONS: Developmental vulnerability was associated with having a closely spaced younger sibling (<12 months post-birth IPIs). Optimising birth spacing should be further investigated as a potential means for improving child development outcomes.