Many innovative measures to relieve pain in preterm infants are considered by various neonatal intensive care units (NICU) worldwide [
1,
2]. It is assumed that neonatal pain in preterm infants can adversely affect their development in such multiple domains as nociceptive changes, altered brain development, stress systems, and functional abilities. Prolonged exposure to pain has also been associated with impaired brain development while preterm infants are in the NICU [
3]. Pain assessment methods are currently performed through physiological (heart rate and respiratory rate) and behavioral criteria (crying time, changes in facial expression and limb movements) [
4].
The premature infant pain profile (PIPP) is a set of measurable behavioral and physiological responses such as facial expression changes (squeezing eyes, raising eyebrows, wrinkling nasolabial groove) as well as changes in heart rate, SaO2, intrauterine age, and behavioral status of the infants, which are all definite reasons demonstrating pain in premature infants [
5].
There is a strong tendency to use non-pharmacological interventions, as simple and secure techniques, for relieving pains in infants. Several methods have already been applied to relieve pain based on five senses [
5]. Among them, the sense of smell is fully developed at birth [
6] which can affect the neonate’s emotional relationship with his/her mother [
7]. Familiar odors, maternal odor for instance, supposedly have soothing effects on newborn infants. It is widely known that infants have the ability to detect their mother’s breast odor even without experiencing breastfeeding at birth [
8]. The breast milk odor (BMO) can enhance infants’ sucking through the facial and trigeminal motor nerves in the brain, which, in turn, stabilizes the physiological state in infants [
9]. In some cases, research findings has demonstrated that breastfeeding in human newborn infants can completely eliminates pain responses, and animal models have also depicted that the pain modulating effect of breastfeeding is likely mediated by opioid and non-opioid mechanisms [
10]. Some studies have shown that Some other studies have also shown that fetal-maternal odors (mother’s breast milk, body and amniotic fluid odors) can decrease stress responses including crying and motor activities in infants, especially those separated from breast milk or the ones under painful interventions [
11]. In a relevant study, it was suggested that the maternal breast milk odor (MBMO) had a soothing effect on preterm infants, and that their pain score was lower than that of those exposed to formula odor [
12]. Nevertheless, the results of Küçük Alemdar et al. (2017) demonstrated that the BMO made no statistically significant difference in the physiological and behavioral responses of MBMO group compared to other groups (amniotic fluid odor, maternal body odor and control groups) [
13].
Given all the contradictory results on the effect of MBMO and another mother’s BMO on preterm infants and the importance of pain relief for preterm infants, this study strove to investigate the effect of inhaling human milk on the behavioral responses of pain caused by HB vaccine in preterm infants.