The Effects of Mother-infant Skin-to-skin Contact on Stress Response of Preterm Infants

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.