Abstract
Respiratory sinus arrhythmia (RSA) is a key index of parasympathetic function and environmental adaptability. Lower resting RSA has been linked to preterm (PT) birth in infancy and autism spectrum disorder (ASD) in childhood, yet RSA across the first 2 years in young infants born PT or later diagnosed with ASD remains unknown. This study examined resting RSA and mean interbeat interval (IBI) development from 1 to 24 months in infants at varying ASD likelihoods, including infant siblings of children with ASD and those born PT. A longitudinal design tracked resting RSA and mean IBI in 137 infants from 1 to 24 months. Infants were classified as elevated likelihood for ASD (EL), low likelihood for ASD (LL), or PT and later classified by developmental outcome as ASD, neurodivergent (ND), or typically developing (TD). Mixed-effects models examined developmental trajectories and group differences. Results indicated that both RSA and mean IBI increased across all groups from 1 to 24 months, with the most rapid growth observed in the first 6 months. PT infants exhibited lower RSA and mean IBI initially, but aligned with LL infants when age was corrected for prematurity. Infants later diagnosed with ASD showed no early RSA differences, but exhibited elevated RSA from 9 to 24 months, distinguishing them from TD and ND infants. Elevated resting RSA in ASD from 9 to 24 months may reflect reduced social monitoring, increased attentional regulation, or decreased stress during a resting period free of structured tasks. These findings contrast with lower RSA in older children with ASD, highlighting developmental shifts in autonomic function and the need for further research into RSA as an early biomarker for ASD.