Introduction

In medical terminology, a newborn is defined as an infant during the first 28 days after birth, regardless of whether the infant is born preterm, full-term, or post- term [ 1 ]. Infants born before the completion of 37 weeks of gestation are classified as preterm [ 2 ]. A full- term infant is born between 39 and 40 weeks of gestation, a late-term infant is born at 41 weeks of gestation, and an infant born at or beyond 42 weeks of gestation is classified as post-term [ 3 ].

Research indicates that approximately 10%-15% of annual births in the United States require admission to the Neonatal Intensive Care Unit (NICU) [ 4 ], and approximately 50% of these admissions involve preterm infants [ 5 ]. The NICU is a specialized hospital unit that provides intensive medical care for preterm infants, low-birth-weight infants, infants with congenital anomalies, and those with other complex medical needs. It is staffed by specially trained healthcare professionals and equipped with advanced medical technology [ 4 ].

Admission to the NICU is a highly stressful experience for both infants and their parents. Infants hospitalized in the NICU are exposed to numerous unfamiliar and potentially distressing environmental stimuli, including excessive light and noise [ 5 ]. In a study by Grunberg et al. (2018), parents described their infant's hospitalization in the NICU as an emotionally fluctuating experience characterized by stress, fatigue, anger, happiness, and helplessness. They also reported that these emotional experiences varied depending on their infant's clinical condition [ 6 ].

According to Pineda et al. (2017), establishing a secure attachment between parents and their infants is essential for all newborns, particularly those admitted to the Neonatal Intensive Care Unit (NICU). Therefore, the presence and active involvement of parents in the care of preterm infants and other hospitalized newborns with medical complications are of paramount importance. Positive sensory experiences, such as skin- to-skin contact (SSC) between the parent and infant, including kangaroo care, can facilitate the development of a secure parent-infant attachment relationship[ 7 ].

When an infant is born prematurely, the developing bond between the mother and the infant may be disrupted unexpectedly, making it more difficult for parents to recognize and respond appropriately to their infant's cues for closeness. This disruption, combined with the stressful environment of the Neonatal Intensive Care Unit (NICU), may increase maternal distress and contribute to attachment-related difficulties. Interruptions in the bonding process may adversely affect neonatal neurological development and compromise the child's future resilience, emotional regulation, neurobehavioral functioning, and social and cognitive development [ 8 ].

Attachment theory emerged as a major framework for understanding child development in the 1950s. John Bowlby and Mary Ainsworth were the pioneers who established its fundamental principles. Bowlby described attachment as the enduring emotional bond that develops between a child and their primary caregiver [ 9 ].

Parent-infant attachment is considered a developmental process that begins at birth. The quality of maternal attachment influences the infant's attachment to the mother. Therefore, attachment is regarded as a reciprocal process and, because of its strong emotional component, plays a fundamental role in the child's cognitive, emotional, and social development [ 10 ].

According to Yazdanimehr et al. (2023), the terms attachment and bonding are often used interchangeably; however, some theoretical frameworks distinguish between the two concepts because of their differing characteristics [ 11 ]. Lee et al. (2003) define bonding as the emotional connection that a parent develops with their child. This bond encompasses feelings of intimacy, love, protection, self-sacrifice, and parental concern for the infant [ 12 ].

In contrast, Yazdanimehr et al. (2023) clearly differentiate between attachment and bonding and describe several important distinctions. Whereas attachment primarily refers to the infant's relationship with their primary caregiver, bonding relates to the parent's internal emotions, perceptions, and behaviors toward the infant. The developmental trajectories of these constructs also differ: bonding typically begins immediately after birth, whereas attachment develops progressively throughout the infant's first year of life. Conceptually, bonding is often regarded as the parent's emotional connection with the infant. In contrast, attachment refers to the infant's enduring emotional relationship with the caregiver, which is strongly influenced by the caregiver's responsiveness. Furthermore, these constructs are assessed using different methods. Bonding is generally measured using parental self-report instruments, whereas attachment patterns are commonly evaluated using the Strange Situation Procedure after the child reaches 12 months of age [ 11 ].

Given the highly specialized nature of the Neonatal Intensive Care Unit (NICU), delivering high-quality care requires effective teamwork and interdisciplinary collaboration. Occupational therapy is one of the disciplines that can play a significant role in this setting. Through therapeutic positioning, sensory, motor, and neurological assessments, parent education, environmental modifications, and support for infant feeding, occupational therapists can facilitate the interaction between the infant's abilities and the physical and social environment, thereby promoting optimal developmental outcomes.

As discussed previously, hospitalization in the NICU and the associated medical interventions can be stressful and distressing for both the infant and the mother. In the study by Tanta et al. (2010), NICU hospitalization was reported to disrupt the development of parent-infant bonding and attachment. In this context, occupational therapists can assess the needs of both the infant and the family and support the development of bonding and attachment by providing parent education, emotional and psychological support, interventions to facilitate infant feeding, and opportunities that promote positive parent-infant interactions [ 13 ].

Considering the ongoing debate among researchers regarding the use of the concepts of attachment and bonding in relation to infants, this systematic review aims to identify the factors that influence attachment and bonding in infants hospitalized in the Neonatal Intensive Care Unit (NICU). In accordance with the conceptual distinctions described above, attachment and bonding are considered and analyzed as separate constructs in this review.

Methods

Search Strategy

This study employed a systematic evidence-based review methodology [ 14 ] and was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines [ 15 ]. A comprehensive literature search was performed across Scopus, PsycNet, PubMed, and the Cochrane Library using the following keywords: bonding, attachment, NICU, infant, preterm infant, mother-infant, parents' attachment style, mother- infant relationship, stress, anxiety, and depression. In addition, Medical Subject Headings (MeSH) terms were used to identify relevant keywords and their synonyms, and the Boolean operators AND and OR were applied to combine the search terms. The literature search was conducted between February 21 and March 17, 2024.

Eligibility Criteria

Cross-sectional,longitudinal,descriptive- correlational, eye-tracking, and randomized controlled trial (RCT) studies published between 2006 and 2024 were considered eligible for inclusion. These study designs corresponded to the predefined levels of evidence considered appropriate for this review.

The inclusion criteria were as follows: (1) articles published after 2006 in peer-reviewed English- language journals; (2) studies involving preterm infants with a history of Neonatal Intensive Care Unit (NICU) admission; (3) studies investigating parent-infant attachment or bonding; (4) studies evaluating interventions designed to improve bonding or attachment, including psychological, physiological, and cognitive interventions; (5) studies examining the effects of skin-to-skin contact interventions in NICU infants; and (6) studies evaluating the effects of parental mentalization interventions on parent-infant bonding and attachment.

The exclusion criteria included conference presentations, conference abstracts, theses and dissertations, qualitative studies, expert opinion articles, and case reports.

Study Selection

All eligible records identified through the database search were imported into EndNote 20, where duplicates were identified and removed. The remaining studies were independently screened by two reviewers (N.M. and S.H.), who evaluated the titles and abstracts against the predefined inclusion criteria. Subsequently, the full texts of potentially eligible studies were independently assessed by two additional reviewers (L.R. and A.P.) to determine their eligibility for inclusion in the review.

Data Extraction

A total of 163 articles were initially identified; 19 met the inclusion criteria and were included in the final review. Among these, 15 studies were classified as Level III evidence, including six cross-sectional studies, seven longitudinal studies, one descriptive- correlational study, and one eye-tracking study. The remaining four studies were classified as Level IV evidence and consisted of randomized controlled trials (RCTs).

After the eligible studies had been identified, two reviewers (N.M. and A.P.) independently extracted the relevant data. The extracted information included the first author's name, year of publication, study title, country, maternal and infant age, study design, assessment tools, sample size, main findings, and the factors associated with or influencing bonding and attachment.

Data Synthesis

The findings were synthesized according to the characteristics summarized in the data extraction table and the factors associated with bonding and attachment. The extracted data were presented in tabular form and subsequently described, analyzed, and discussed.

Results

As the objective of this systematic review was to identify the determinants and associated factors influencing parent-infant bonding and attachment among premature infants and their parents, only studies classified as Level III and Level IV evidence according to the American Occupational Therapy Association (AOTA) evidence hierarchy were included.

A total of 163 records were identified through the database search. After duplicate records were removed, 80 articles remained for title and abstract screening. During the initial screening, 18 articles were excluded. Subsequently, 62 full-text articles were assessed for eligibility. Of these, 43 articles were excluded for the following reasons: 9 were published before 2006, 15 were qualitative studies or case reports, 16 did not meet the eligibility criteria after full-text review, and 3 were published in languages other than English. Ultimately, 19 studies met the inclusion criteria and were included in the final review (Figure 1).

Figure 1. PRISMA Flow Diagram of the Study Selection and Screening Process

In accordance with the predefined eligibility criteria, 19 studies were included in the final review. Of these, 6 were conducted in the United States, 2 in Taiwan, 2 in Norway, 3 in the Netherlands, and 1 each in Spain, Iran, Colombia, Scotland, Australia (Sydney), Germany, Argentina, Italy, Türkiye, Sweden, and Croatia.

The included studies were reviewed in accordance with the objective of this systematic review. The findings indicated that while some factors were associated with parent-infant bonding and attachment, others demonstrated a direct influence on these outcomes. Overall, the identified associated or influencing factors could be classified into six main categories: demographic factors (4 studies), psychological factors (7 studies), cognitive factors (2 studies), physiological factors (2 studies), skin-to-skin contact (SSC) (2 studies), and attachment styles (6 studies) (Table 1).

Affecting and Associative factors Demographic Characteristics

Based on the reviewed studies, the findings indicated that maternal age, maternal weight, infant birth weight, race, the number and sex of siblings, multiple births, parental educational level, socioeconomic status, and family income [ 26 ] influenced parent-infant attachment. In addition, maternal personality characteristics before childbirth [ 30 ] and infant age [ 17 ] were associated with attachment. Infant age [ 17 ] was also reported to be associated with parent-infant bonding.

Psychological Features

The reviewed studies showed that maternal stress during pregnancy, fear of childbirth, and depression [ 32 ] influenced mother-infant attachment. Furthermore, mindfulness-based interventions were associated with both mother-infant attachment and bonding [ 4 ]. Maternal stress and anxiety, as well as depressive symptoms during pregnancy and the postpartum period [ 30 , 32 ], were identified as factors influencing attachment. In addition, maternal stress and postpartum depression [ 18 ], as well as paternal anxiety, were found to influence parent-infant bonding. Conversely, maternal mental health problems [ 32 ], postpartum anxiety [ 28 ], and depression during pregnancy and the postpartum period [ 22 ] were associated with bonding.

Cognitive Factors

The reviewed studies indicated that mindfulness- based interventions were associated with both mother-infant attachment and bonding [ 4 ].

Physiological Factors

The findings suggested that parental sleep quality could influence parent-infant attachment and bonding [ 23 ]. In addition, infant sleep patterns, including frequent and prolonged nighttime awakenings, were associated with parent-infant attachment [ 24 ]. Feeding infants with breast milk was also found to be associated with both attachment [ 26 ] and bonding [ 24 ].

Skin to Skin Contact (SSC)

The reviewed studies showed that maternal skin-to- skin contact (SSC) positively influenced mother-infant bonding [ 24 ]. Similarly, paternal SSC interventions were found to promote father-infant attachment [ 25 ].

Attachment Style

According to the reviewed studies, parental attachment style was associated with both parent- infant attachment [ 21 ] and bonding [ 18 , 28 ], and was also reported to influence bonding [ 22 ] directly. In addition, the infant's attachment style was associated with parent-infant attachment [ 20 ].

Discussion

This systematic review was conducted to provide a better understanding of the factors associated with and influencing parent-infant bonding and attachment. To achieve this objective, the identified factors were examined and synthesized. The findings indicated that demographic, psychological, cognitive, physiological, skin-to-skin contact (SSC), and attachment style factors all play important roles in the development of parent-infant bonding and attachment.

Demographic Characteristics

Regarding demographic factors, infant birth weight may influence mother-infant bonding because mothers of very low birth weight (VLBW) infants often face medical and social challenges that can reduce their ability to produce an adequate milk supply (33). Furthermore, Gibbs et al. investigated the relationship between breastfeeding and attachment and reported that breastfed infants experienced greater maternal warmth and tenderness than infants who were breastfed less frequently. These findings suggest that breastfeeding practices may positively influence mother-infant bonding [ 26 ].

Premature infants are at an increased risk of disrupted mother-infant bonding [ 23 ]. Medina et al. reported that establishing a bond with a premature infant can be challenging because of the stressful NICU environment and adverse social and emotional circumstances, which may increase parental stress [ 34 ]. Consequently, premature infants show a higher prevalence of insecure attachment than full-term infants [ 30 ].

Female infants exhibit a greater hypothalamic- pituitary-adrenal (HPA) axis response than male infants [ 39 ]. Therefore, infant sex may influence the development of parent-infant attachment [ 26 ].

According to Ludwig et al., higher maternal body weight at the time of childbirth is associated with an increased risk of adverse health outcomes [ 38 ], which may, in turn, influence the development of mother- infant attachment.

Li et al. demonstrated that parental educational level plays an important role in strengthening parent-child attachment [ 39 ]. Consequently, parental education may also contribute to the quality of parent-infant bonding [ 11 ].

Ferenczi et al. (2013) reported that individuals living in countries with lower levels of security exhibited greater attachment avoidance than those living in more secure environments [ 40 ]. These findings suggest that attachment may be associated with ethnicity, cultural background, and population characteristics [ 26 ].

Finally, unstable employment, poverty, and insufficient family income may be associated with parent-child attachment. Poor socioeconomic conditions can contribute to more anxious attachment patterns, which may subsequently influence the quality of parent-infant bonding [ 26 , 22 , 20 ].

Cognitive Factors

Mindfulness-based interventions encourage parents to reflect on both their own behaviors and those of their infants across different contexts and mental states. Greater parental mentalizing capacity has been associated with higher-quality caregiving and, consequently, more secure parent-infant attachment [ 41 ]. Furthermore, mindfulness-based interventions can reduce parental anxiety and stress, thereby promoting higher-quality parent-infant bonding. However, in the study by Pan et al. (2023), the quality of parent-infant bonding was not significantly influenced by mentalization interventions [ 24 ].

Physiological Factors

According to the study by Perpétuo et al. (2023), sleep is a complex process influenced by familial, social, and biological factors. Because attachment contributes to the development of self-regulation (44), good sleep quality may positively affect parents' emotional well-being and their attachment to their infants [ 2 ]. In this regard, Witte et al. reported that improving infant sleep-wake regulation can positively influence parent-infant attachment [ 32 ].

Psychological Factors

The transition to motherhood, fear of childbirth, and adaptation to the maternal role during pregnancy and the postpartum period can contribute to depression, increased stress, and anxiety. These psychological challenges may lead mothers to perceive their relationship with their infant less positively, thereby negatively affecting mother-infant bonding [ 18 , 21 , 11 , 30 ].

Post-traumatic stress disorder (PTSD) may also develop following childbirth and is characterized by distressing psychological symptoms. The study by Radoš et al. (2020) demonstrated that postpartum PTSD has a significant impact on the quality of mother-infant bonding. Furthermore, PTSD, through its association with depressive symptoms, may also adversely affect mother-infant attachment [ 27 , 28 ]. Although no direct association has been reported between paternal anxiety and mother-infant bonding, an association has been observed between maternal postpartum depression and paternal anxiety.

According to the study by Modak et al. (2023), the close physical contact established during breastfeeding stimulates the release of oxytocin, often referred to as the "bonding hormone." Oxytocin promotes positive emotional responses, enhances trust, and facilitates the development of attachment and bonding between the mother and infant [ 44 ].

SSC

Research by Moore et al. (2012) demonstrated that, among mothers who underwent cesarean delivery, those who experienced skin-to-skin contact (SSC) reported significantly lower pain levels than mothers who were separated from their infants. Mother-infant SSC promotes the release of hormones, particularly oxytocin, which may strengthen mother-infant bonding. In addition, SSC has been associated with several beneficial physiological responses. For example, infants receiving SSC have been reported to exhibit body temperatures approximately 1°C higher, heart rates about three beats per minute higher, and increased respiratory rates compared with infants who do not receive SSC [ 24 ].

In the study conducted by Chen et al. (2017), the positive effects of paternal SSC were also investigated. The authors concluded that SSC positively influenced father-infant attachment. This effect may be attributable to oxytocin release stimulated by physical touch, warmth, and affectionate contact. Oxytocin promotes positive emotional responses, enhances feelings of calmness and satisfaction in parents, and contributes to stress reduction [ 25 ].

Attachment Style

The level of prenatal attachment is positively correlated with the level of postnatal attachment, such that stronger prenatal attachment is associated with stronger postnatal attachment [ 22 ]. Because parental attachment style influences the parent-infant relationship and provides a framework for regulating emotional states, it plays an important role in the development of parent-infant attachment and bonding [ 44 ]. According to the research conducted by Hairston et al., insecure parental attachment styles are associated with difficulties in parent-infant bonding [ 18 , 21 ].

An avoidant attachment style has been associated with greater anger, increased rejection, and weaker mother-infant bonding during the first eight weeks after childbirth [ 18 ]. Similarly, an anxious-ambivalent attachment style is characterized by heightened child- focused anxiety and has been associated with reduced bonding quality [ 45 ].

According to Sroufe et al. (2006), in addition to parental attachment style influencing bonding, the infant's attachment style may also affect the parent- infant relationship. A secure attachment style can mitigate the impact of later emotional and behavioral problems, whereas an insecure attachment style predicts subsequent developmental and psychological difficulties. Moreover, anxious attachment has been associated with numerous challenges during the preschool years. Compared with children who develop secure attachment, those with insecure attachment generally demonstrate poorer developmental outcomes and fewer adaptive skills. Consequently, the infant's attachment style may influence parent-infant interaction and bonding, as well as the child's overall development and self-regulation [ 20 ].

This review has several limitations. First, the risk of bias of the included studies was not formally assessed. Future systematic reviews should incorporate a standardized quality appraisal framework to evaluate the methodological quality of included studies and minimize potential bias. Second, the number of studies that explicitly distinguish attachment and bonding as two separate constructs is limited, which may have complicated the literature search and study selection process.

Conclusion

The findings of this systematic review indicate that parent-infant attachment and bonding are influenced by a wide range of factors, including demographic, psychological, cognitive, and physiological characteristics, as well as skin-to-skin contact (SSC) and parental and infant attachment styles. Therefore, healthcare professionals and families should consider these two important constructs to promote secure attachment and healthy bonding and to reduce the risk of subsequent emotional and behavioral disorders.

Acknowledgment

None.

Authors’ Contributions

Conceptualization: E.D.H., N.T., and M.P; Data acquisition: E.D.H. and N.T. Supervision: M.P and L.A; Formal analysis: E.D.H. and M.P.; Drafting of the manuscript: E.D.H.; Critical revision of the manuscript for important intellectual content: E.D.H. and N.T.

Funding

This research received no external funding.

Conflict of Interest

The authors declare no conflicts of interest.

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