The primary function of early caregiver recognition and attachment is ensuring newborn infant closeness and survival [10]. The newly born infants experience a burst of norepinephrine during the birth process that facilitates learning their mother’s odor and approaching their mothers [10,11]. In addition, many hormonal and neurotransmitter mechanisms have been developed to draw parents and their babies into close proximity and emotional closeness with each other [11,12]. Therefore, disturbances in these regulations may have various effects on the infant’s cognitive and emotional development [10]. Nurtured attachment from parents plays an important role in the infant’s stress regulation [13].

Oxytocin is a well-known hormone that plays an important role in the formation of a close mother-infant relationship. A great amount of oxytocin is secreted in pulsatile releases at birth [14]. The mother’s closeness and caring actions after birth increase this pulsatile secretion of oxytocin [14]. Oxytocin is known to enhance hypothalamus-pituitary-adrenal axis function to regulate the infant’s fear and stress behaviors [15]. Early sensory functions affect which part of the brain undergoes enhancement of the oxytocin “pathway” and how the brain reacts during human interactions later in life [15].

Dopamine is another well-known neurotransmitter involved in fundamental bonding. Dopamine-producing neurons are close to oxytocin receptor neurons in the striatum and ventral tegmentum, enabling oxytocin bursts produced by closeness and caretaking actions to activate the brain’s reward processes [16]. Through complicated neural networks and integration mechanisms, the oxytocin and dopamine systems participate in and perform many different actions and influences, including the neuromodulation of social behavior, stress regulation, and associative learning [14-16].

During the third trimester, the fetal brain grows fourfold, accompanied by a marked increase in brain surface area, resulting in the emergence of sulci and gyri [17]. While significant changes occur in a very short period of time, many neuronal connections develop by the mid-gestation, for which environmental factors are essential [17,18]. The most important neural pathways, the thalamocortical and corticothalamic, are involved in sensorimotor information [18]. Thalamocortical pathways relay sensory and motor information from sensory receptors in the body through the thalamus to the sensorimotor area of the neocortex; the corticothalamic pathway transmits information generated in the cortex back to the thalamus [17,18]. Because these essential pathways develop from 26 weeks of gestation [18], preterm infants are at high risk for motor, cognitive, and behavioral deficits that become evident in childhood [19].

The NICU environment exposed to preterm infants, including bright lights, high levels of noise, and frequent noxious interventions, may have detrimental effects on the immature brain that alter its subsequent development [20,21]. Imaging studies reported that preterm infants have smaller cortical surface areas and lower volumes of gray matter, white matter, basal ganglia, and cerebellum than term infants [22–24]. A functional magnetic resonance imaging study showed that the most extensively affected brain areas in premature infants were short-range connections in the frontal, parietal, and occipital regions, which are linked to cognitive efficiency, social cognition, emotion, and language development [19].

The earliest noted fetal sensory response involves the movement of the extremities, head, or mouth or changes in heart rate in response to external stimulation by 21–23 weeks of gestation [25]. This early development of tactile sense suggests the importance of skin-to-skin contact (SSC) in parent-infant closeness among preterm infants during NICU hospitalization. Moreover, the sense of taste develops by 16–18 weeks of gestation, while the olfactory sense develops by 28–32 weeks [26,27]. SSC is now a proven primary recommended care practice that has clear positive effects for both parents and infants [3,28]. Moreover, a parent’s presence is a prerequisite for SSC and any other forms of physical parent-infant closeness.

The first primitive model of FCC and its effectiveness in the care of preterm infants was introduced in the mid-1980s in response to medical staff and equipment shortages [29,30]. As increasing numbers of reports on the positive effects of physical closeness between parents and infants, many Western countries started loosening the visitation restrictions for parents from the late 1990s to the early 2000s [31]. The parent’s presence can be supported by the creation of a meaningful role for them in the care of their infant during hospitalization [32]. Nowadays, in most Western NICUs, parents actively care for their infants by providing SCC, taking parts in their infant’s pain management, managing the infant’s daily care routines, and participating in medical decision-making, thus being an important part of the care team [33,34].

The term FCC has been used to represent various family-focused and -partnered care models. The components of a neonatal FCC model are imprecisely defined, presenting a challenge when evaluating research on FCC [35]. A fundamental aspect of FCC in the NICU context is that the infant should never be cared for without considering the whole family and partnership with the parents. Families are acknowledged as integral members of the care team and considered the primary decision-makers for their infants [36]. In 2005, the European Science Foundation suggested 8 principles for neonatal FCC (Table 2), and a more recent study announced that the implementation of these principles does not require additional research due to the concrete body of evidence [37].

Beyond understanding the principles of care, implementing FCC knowledge in everyday practice and guiding parents to participate in neonatal care is challenging. Many different FCC intervention models have been developed and studied that benefit infant development (Table 3). In particular, the Infant Behavioral Assessment and Intervention Program, Mother Infant Transaction Program, and Parent Baby Interaction Program aimed to help parents interpret their infants’ behavioral cues and sensitize them to how their infants handle environmental information to enhance confident and coregulatory interactions [4,38,39]. The Guided Participation, Creating Opportunities for Parent Empowerment, and Positive Parenting Program are supporting parents to interpret their infants’ cues and to enhance their parenting ability [40-42]. By using standardized information material, the parents are prepared by learning about infant behavior and reactions, and become informed about how they can support their infants in such situations. The Family Nurture Intervention and Family Integrated Care focus on integrating parents and family members as partners in the infant’s care, including daily calming, nurturing, and interaction activities in NICUs [43,44]. Most recently, a Swedish group developed the Early Collaborative Intervention program to facilitate early parent-infant interactions that start within the first days of the infant’s life [45].

While most of these FCC models have focused on educating or supporting parents to enhance their coping skills and parenting capacity, the Close Collaboration with Parents (CCP) program developed in Finland aims to train NICU staff to improve their skills in collaboration with parents and promote FCC [46]. Historically, many NICUs have failed to recognize parents as partners, and parents have reported that they are insufficiently included in their infants’ care [46]. To ensure FCC success, it is necessary to develop a new relationship with parents, including mutual respect, cooperation, and support for parents in every aspect of daily care among NICU staff. The implementation of FCC in NICUs requires new role definitions among healthcare professionals to involve parents more in their infants’ care. In addition, informational support from doctors as well as nurses must be incorporated to make FCC successful. This educational intervention for neonatal staff succeeded in improving the quality of FCC and changed the care culture in NICUs throughout Finland [47,48]. In this regard, the CCP program has the potential to offer a breakthrough model in Korea for training healthcare teams to facilitate communication and collaborative skills and support parental participation [48].

The concept of individual rooms for NICU patients was first proposed in the early 1990s [30]. The rationale for building single-family rooms (SFRs) was to provide a calmer and more private environment for parents and their infants [49]. In the SFR, the infant is less frequently exposed to noise and artificial light from the NICU setting [50]. A more homelike and peaceful atmosphere for the family supports the 24-hour presence of parents, privacy for pumping and breastfeeding, and SSC.

SFRs have been shown to enhance not only enteral nutrition, breastfeeding, and growth, but also neurobehavioral function [51]. They also reduce infections, apnea, the risk of bronchopulmonary dysplasia, and length of hospitalization compared to open-bay units [51-53]. However, the greatest benefit is the development of a close parent-infant relationship and nurturing parental psychological well-being, which can improve preterm infants’ long-term outcomes. Although concerns have been raised regarding infant safety and staff stress, studies have revealed that SFRs decrease adverse physiological events such as apnea, bradycardia, and desaturation [54]. Neonatal care inSFRs is also reportedly associated with lower stress and improved work satisfaction among nurses [55]. This might be related to the fact that they can focus more on their own professional role while letting parents perform infant care routines. Although the cost per incubator is reportedly greater in the SFR than openbay model, the total cost of care was significantly reduced in an adjusted analysis that included admission variables, treatment variables, respiratory variables, and length of hospitalization [56].

Beyond providing separate private spaces, the SFR model means that the infant and family should be major considerations in the design of the NICU architecture [57]. The SFR is generally equipped with an appropriate headwall for power, gas, and suction; an area for the incubator; and family accommodations such as a fold-out couch, bed, recliner, closet, a refrigerator for storing breastmilk or formula, a sink, and storage cabinets [50]. In addition, materials used for facilities should be selected to control the environment, such as sound-dampening walls and ceiling tiles that reduce ambient noise, as well as indirect overhead lights that can be fully regulated to meet the infant’s needs [50]. Along with SFRs, space for families to congregate and interact with their own and other infants’ family members also should be incorporated [57]. These may include a lounge, dining area, waiting area, and facilities for bathing, laundry, computer, and internet access.