The aims of this randomized controlled trial are investigate the effects of breast milk odor on pain response and saliva cortisol level duration heel stick blood sampling in preterm infants. Partipants will randomly assigned into the experimental group (breast milk odor) or control group (distilled water odor). The Premature Infant Pain Profile-Revised (PIPP-R) and Salimetrics® Cortisol Enzyme Immunoassay Kit are used to measuring the pain response and saliva cortisol level.

Preterm infants need to undergo more invasive medical interventions than full-term healthy newborns to maintain their lives. Physiological functions changes brought about by pain and stress stimulations may complicate nerve development in preterm infants. Therefore, providing positive sensory supportive interventions to reduce pain and stress is imperative for preterm infants. Studies have confirmed that the use of painkillers, breastfeeding, swaddling, kangaroo care, giving glucose or sucrose can help reduce pain. The sense of smell matures at 28 weeks of gestation age, and newborns can recognize the mother's smell after birth (Nishitani et al., 2009).

In recent years, several randomized studies have explored the effects of smell on reducing pain and stress in preterm infants, but the results are still inconclusive due to the lack of randomization blinding and a variety of smell interventions. It is desirable to plan a high-quality study. Although premature infants need to be separated from their mothers in nurseries due to medical needs, breast milk is still the best source of nutrition. The aims of this study are to investigate the effect of breast milk odor on pain response and saliva cortisol level duration heel stick blood sampling in preterm infants with gestation age at 30 to 37 weeks. A Randomized Controlled Trial will be designed. Preterm infants, who will receive heel sticks, are randomly assigned into the experimental group (breast milk odor) or control group (distilled water odor). The Premature Infant Pain Profile-Revised (PIPP-R) and Salimetrics® Cortisol Enzyme Immunoassay Kit are used to measuring the pain response and saliva cortisol level. Data will be analyzed by SPSS 22.0 (SPSS Inc., Chicago, IL, USA) using descriptive statistics, independent t-test, ANOVA, Generalized Estimating Equation linear multiple regression analysis… etc.

Inclusion Criteria:

1. Premature babies whose gestational age is between 30 weeks to 36 weeks and six days, with a birth weight > 1000 grams.
2. Apgar Score > 6 at 5 minutes of birth.
3. Within 10 days of birth, the vital signs are stable.
4. Those who have medical needs, such as blood monitoring of neonatal serum bilirubin, blood sugar, electrolyte, or neonatal screening, etc., need heel puncture blood sampling.

Exclusion Criteria:

1. Those who have been diagnosed by a physician with severe congenital malformations, chromosomal abnormalities, respiratory distress, epilepsy, necrotizing enterocolitis, intracranial hemorrhage, periventricular leukomalacia, sepsis, meningitis, or hyperbilirubinemia.
2. Those who have bronchopulmonary dysplasia or other chronic lung diseases that require the use of ventilator or non-invasive positive pressure respiratory support.
3. Have used analgesics or anesthetics within 48 hours before giving the intervention of the study, or are participating in other studies that provide intervention for pain reduction.