Whose clock makes yours tick? How maternal cardiorespiratory physiology influences newborns’ heart rate variability

•Longitudinal mother–infant paced-breathing experiment (age 1, 2, 4, 8, 12 weeks).•Infants adjusted their RSA-levels to their mothers’ in week (W) 1–8 but not in W12.•The result in W1–8 may be interpreted as the continuation of intra-uterine effects.•We propose a delayed shift from intra to extra-uterine conditions between W8 and W12.•Possible parallel timing with 2-month developmental shift of social orientation. This study examined the existence of direct maternal–infant physiological relatedness in respiratory sinus arrhythmia (RSA) when the infant was age 1, 2, 4, 8, and 12 weeks. We instructed mothers to breathe at 6, 12, 15, 20, and 6 cycles per minute while their infants lay on their body. The mother–infant ECG and respiration were registered and video recordings were made. RR-interval (RRI), respiration rate (fR) and RSA were calculated and mother–infant RSA response-patterns were analyzed. The results revealed that infants adjusted their RSA levels to their mothers’ levels during the first 2 months of life, but not at 3 months of age, which could be interpreted as a continuing intra-uterine effect. The attenuation between 2 and 3 months could be a reflection of the 2-month developmental shift of social orientation.