The fragile X premutation (FXpm) is caused by a CGG repeat expansion on the FMR1 gene. In adults, FXpm is linked with autonomic nervous system (ANS) dysfunction and impairment is associated with CGG repeat length. Given scant infancy research, we examined ANS functioning, via respiratory sinus arrhythmia (RSA) and interbeat interval (IBI), in 82 FXpm and neurotypical infants and their associations with CGG repeats. FXpm infants exhibited lower RSA but no IBI differences. There were no associations between ANS functioning and CGG repeat length. These findings identify an ANS biomarker consistent with the emerging pediatric phenotype in FXpm.

The fragile X premutation (FXpm) is a relatively common condition caused by an expansion of 55–200 cytosine–guanine–guanine (CGG) repeats in the fragile X messenger ribonucleoprotein 1 (FMR1) gene. Previous studies have identified sensory processing challenges in children with the FXpm and reduced autonomic regulation in FXpm infants; no research has examined the relationship between autonomic nervous system (ANS) functioning or molecular variables and sensory responsiveness during infancy. This study examined parent-reported sensory responsiveness and its association with baseline respiratory sinus arrhythmia (RSA), interbeat interval (IBI), and CGG repeat length in 12-month-old infants with the FXpm (n = 35) and neurotypical (NT) controls (n = 55). Results indicated no significant differences in hyporesponsive or hyperresponsive sensory behaviors and no significant associations between baseline RSA or IBI and sensory responsiveness in either group. Within the FXpm group, however, greater CGG repeat length was associated with lower hyperresponsive sensory scores. These findings suggest that sensory processing differences may not be behaviorally evident at 12 months of age despite the presence of biological variability associated with the premutation. The study contributes to the emerging literature on early FXpm development and highlights the importance of examining genetic and physiological factors that may precede later-emerging behavioral phenotypes.