Table 1.

Features of the studies included.

Study	Scope of the Study	Participants	Findings	Physiological Outcomes
Butruille, 2017 [36]	Influence of skin-to-skin contact (SSC) on the parasympathetic activity evaluated by heart rate variability (HRV)	Twenty-two infants and their mothers	SSC had a favourable impact on maternal and premature infant parasympathetic activities with a pronounced neonate response when baseline HRV values were lower	HRV
Carozza, 2020 [37]	Brief review of how contact influences the development of infant somatosensory, autonomic, and immune systems		Contact is an essential pathway for establishing and maintaining behavioural, physiological, and neural levels of parent–child interaction synchrony	Stress
Cho, 2016 [38]	Explore the effects of kangaroo care (KC) on physiological functions of preterm infants and maternal stress.	Twenty infants were assigned to the experimental group and 20 to the control group	KC had positive effects on stabilizing infant physiological functions such as respiration rate and reducing maternal stress.	HRV and maternal stress
Cleveland, 2017 [39]	Explain the effect of SSC on full-term newborns		Recommendations of SSC for healthy newborns
Cong, 2015 [40]	Investigate how oxytocin modulates parental stress and anxiety during maternal and paternal SSC	Twenty-eight stable preterm infants and their parents	Both maternal and paternal oxytocin levels were significantly increased from baseline during the SSC. Both maternal and paternal cortisol levels decreased significantly from baseline during SSC.	Oxytocin and cortisol
Coskun, 2019 [41]	Investigate the effects of KC on the mother’s stress and amount of milk production	Eighty-four preterm newborns and their mothers	KC is effective in stimulating breast milk production and lowering maternal stress levels.	Maternal stress
El Farrash, 2019 [42]	Assess the effect of KC and its duration on the neurobehavioral system and salivary cortisol	One hundred and twenty stable preterm neonates	KC improved higher scores for regulation, non-optimal reflexes, and movement quality and lower scores for handling, excitement, and cortisol compared with the control group	Cortisol
Hardin, 2020 [43]	Examine EEG patterns along with basal oxytocin and cortisol reactivity in infants related to KC	Thirty-three mother–infant dyads at neonatal and three-month periods	KC increased oxytocin levels and decreased cortisol reactivity.	Oxytocin and cortisol
Harrison, 2017 [44]	Examine SSC’s effects on autonomic nervous system (ANS) functions	Eighteen infants and their mothers	HRV measures suggested improvements to the ANS functions after SSC	HRV
Harrison, 2019 [45]	Investigate the effects of SSC on learning and autonomic functions in three-month-old infants	Ten infants with congenital heart disease (CHD) who received neonatal SSC, 16 typically developing (TD) infants, and 10 infants with CCHD without SSC	The findings suggest improvements in cognitive and autonomic development in three-month-old CCHD infants who received neonatal SSC.	HRV
Jones et al. 2017 [46]	Explore the effect of SSC between parents and their neonates on parents’ heart rate (HR)	Twenty-six parents and their babies	A statistically significant difference between the parents’ initial HR and HR after SSC	HRV
Kommers, 2017 [47]	Explore whether HRV could be a measure to track regulatory changes during KC	Eleven preterm infants	A statistically significant difference in HRV between periods of KC and pre-KC	HRV
Kommers, 2018 [48]	Explore if a mattress that mimics breathing motion and heartbeat sounds can have the same effects as KC in preterm infants, as measured by HRV	Twenty preterm infants	HRV decreased during KC and after KC. No non-mattress effects were reported.	HRV
Kommers, 2018 [49]	Explore whether KC influences the salivary oxytocin concentration in preterm infants	Eleven twin pairs	Preterm infant twins’ oxytocin concentrations decreased during KC	Oxytocin
Kommers et al., 2019 [50]	Vital signs and HRV were analysed during KC with and without the use of a swaddling device to identify any potential changes	Twenty preterm infants	KC decreased heart rate, respiratory rate, and HRV. No changes were found regarding the swaddling device	HRV
Lisanti, 2020 [51]	Estimate SSC’s effect on mothers’ bio-behavioural stress measures (anxiety and salivary cortisol) before and after neonatal cardiac surgery	Thirty women and their infants	Significant reductions in self-reported anxiety and salivary cortisol scores were identified as a result of SSC.	Cortisol, HRV
Mirnia et al., 2017 [52]	Investigate the effect of SSC by fathers on salivary cortisol of infants	Forty-five premature infants and their fathers	SSC decreased levels of cortisol in babies.	Cortisol
Moberg, 2020 [53]	Describe the oxytocinergic system and the cutaneous sensory pathways activated by SSC		Decreased stress levels could be considered due to oxytocin’s ability to reduce the amygdala’s activity, the HPA-axis, and the sympathetic nervous system.	Oxytocin
Morelius, 2015 [54]	Investigate the effects of SSC on salivary cortisol, parental stress, depression, and breastfeeding	Thirty-seven families	SSC reduces infant cortisol reactivity in response to treatment and improves concordance between mother and infant salivary cortisol levels.	Cortisol and parental stress
Pados, 2019 [26]	Describe the physiological stress mechanisms that contribute to infant mortality and morbidity in the NICU and the physiological mechanisms by which SSC acts on the stress response system		Importance of SSC to mother–infant stress regulation	Stress
Pados et al., 2020 [55]	Investigate if SSC is an intervention used to reduce stress in the NICU		Research supports that SSC improves short-term cardiorespiratory stress outcomes compared with incubator care.	Stress, cortisol HRV, oxytocin
Varela, 2017 [56]	Explore paternal physiological stress during SSC with their babies	Forty-nine fathers	Fathers who practiced SSC showed a significant reduction in physiological stress outcomes.	Cortisol
Vitnner, 2018 [57]	Examine the relationship between parental involvement and salivary oxytocin and cortisol levels for parents participating in SSC intervention	Thirty-two stable preterm infants and their mothers and fathers	Significant relationships exist between parental engagement and salivary oxytocin and cortisol levels.	Oxytocin and cortisol