The researcher in charge has left the affiliation.

This study will evaluate the effect of maternal-infant skin-to-skin contact (SSC) versus routine care on general stress in preterm infants in the neonatal intensive care unit (NICU).

Primary outcome of this trial to investigate the effects daily skin-to-skin contact on cumulative stress through measures salivary cortisol. The intervention group will receive mother-infant skin-to-skin contact at least 1 continuous hour everyday from infant 3 days of life until discharge. The control group will receive routine neonatal care during hospitalization except skin-to-skin contact. The concentration of salivary cortisol and other infant and parental outcomes will be measured in both groups.

Inclusion Criteria:

• 31 weeks≤gestational age<37 weeks;
• Birth weight ≥ 1500g;
• Admission age < 24h;
• No congenital malformations;
• Vital signs are stable and does not use ventilator and other auxiliary life support systems.

Exclusion Criteria:

• Severe periventricular / intraventricular hemorrhage (grade III);
• Receiving sedation or vasopressor or analgesics opiodis and corticosteroids;
• Mother unable to communicate and communicate normally; Mother has a history of depression.