Preterm infants are exposed to multiple stressors each day, posing a risk of toxic stress that can impact their developing brains during a critical period of sensitivity. Elevated levels of the stress hormone cortisol can impede neuronal connectivity and communication, thereby increasing the likelihood of cognitive impairment and behavioural problems. Synchronized social-emotional mother-infant interaction holds promise in buffering stress reactivity and mitigating long-term stress effects.

Our previous research has shown that preterm infants exhibit higher baseline saliva cortisol levels than full-term infants, along with blunted cortisol reactivity to stressors, irregular cortisol circadian rhythms, and delayed cortisol co-regulation between mother and infant. Another potential stress marker is saliva alpha-amylase (α-amylase), which has garnered increasing interest in adult research. However, there remains a significant gap in the literature concerning saliva α-amylase as a stress marker in preterm infants, warranting further investigation. The overall aim is to study development and relationships between three systems of parent-infant synchrony in preterm infants and their parents and elucidate potential confounding factors for a synchronous correlation. This will be done in relation to standardised care procedures commonly performed in the neonatal intensive care unit.

This observational study will involve 35 families undergoing three video-recorded procedures in the NICU. Saliva will be collected from infants and both parents before and after each procedure so we can analyse co-regulation of cortisol and alpha-amylase. Parent-infant interaction will be analysed from the videos using validated scales.

This study will be the first to document biological co-regulation and social-emotional parent-infant interaction simultaneously involving preterm infants and both parents in the NICU setting. Such insights are pivotal for the future design and implementation of tailored nursing interventions aimed at early stress mitigation, thereby reducing the risk of stress-related consequences.

This observational study adopts a correlational approach to investigate biological co-regulation and parent-infant interaction within families of preterm infants, utilizing a within-family design. Data collection occurs at multiple time points in the NICU, and at 6 and 12 months corrected age. To study parent-infant synchrony and co-regulation with both parents, both should be present and perform three diaper changes in three days. Two will be perforemd by the mother and one by the father. All occassions will be video recorded to study interaction and saliva will be collected before and after the procedure to study co-regulation in cortisol and alpha-amylase during baseline, reactivity and recocvery. At 6 and 12 months parents will respond to questionnaires regarding the child's behavior and development.

Sample size: A power calculation shows sample size of n=28 is sufficient, considering α (two-tailed) = 0.05, β = 0.20, and ICC = 0.5.To consider attrition, we paln to include 35 families.

Inclusion Criteria:

• Swedish- or English-speaking parents aged over 18, born in Europe, and their infants born between gestational weeks 30+0 and 34+6.

Exclusion Criteria:

• Parents with conditions or taking medications affecting the hormone system. Additionally, infants with major malformations or complex care needs that impede interaction possibilities are excluded.

Infants born preterm between 30+0 and 34+6 weeks gestational age and their parents > 18 years old