Caregivers of children with autism spectrum disorder (ASD) report greater stress due to unique parenting demands (e.g.; Estes et al. in Brain Dev 35(2):133–138, 2013). Stress is often studied through self-report and has not been extensively studied using physiological measures. This study compared parenting stress in mothers of children with and without ASD traits. Twenty-seven mother–child dyads participated in an interaction task while measuring mother’s heart rate variability (HRV) and mothers self-reported stress levels. Results demonstrated that while self-report and physiological stress measures were not correlated, ASD symptomology did account for HRV change score (i.e., more severe ASD symptoms were positively related to HRV change). This may reflect an atypical coping response. Implications for using physiological indicators for studying parenting stress are explored.