Development in childhood is dynamic in nature and a complex process accompanying biological and psychological changes along with physical growth. Early childhood, which usually refers to the first 5 years of life, is the fastest period of growth. During that period, the developing brain is most sensitive to stimulation and nurturing [1]. Therefore, in such sensitive periods, various factors can positively or negatively affect to development in early childhood [2].

Developmental delay (DD) in childhood has been adversely related to academic achievement in adolescence. The term of “developmental disorder” or “developmental disabilities” (DDs) refers to a heterogeneous group of sharing mental or physical impairments that result in substantial functional limitations in overall life activities [3,4]. They are chronic conditions that give rise to physical, psychological, and economic burdens for families and societies resulting in high medical expenses and low labor productivity. Increasing the prevalence of DDs is the common phenomenon occurring worldwide, although there are some variations of rate of prevalence depending on countries (Table 1). Therefore, the increasing trend of DDs has aroused attention of the societies, and is relevant with an increasing number of epidemiological studies for DDs [5,6].

Early detection and proper intervention can reduce the chances of future developmental disorders and prevent secondary sequelae [7]. A first necessary step in order to plan for early intervention should start with knowing the estimated prevalence of DD and features of various types of delays [8]. Up-to-date reporting of DDs prevalence is important for understanding populations with disabilities, efficiently targeting such populations, administering interventions, making relevant policies, and finally monitoring their effectiveness [6].

DD can be developed within various domains, which are not mutually exclusive. In addition, it is not unusual for one of these areas to be unnoticed often [9]. Furthermore, the wide normal variation among children often makes parents or primary care pediatricians easily miss subtle findings which can be the clue of DD [10]. A strategy of developmental surveillance and periodic developmental screening test (DST) may help pediatricians more efficiently find such subtle clinical issues.

Early detection of DD requires comprehensive judgements of medical history, developmental evaluation, physical and neurological examination along with a DST. Most of them have been described in many literatures. Therefore, this paper will limit the scope to review the estimated prevalence of DDs in the recent literature worldwide and update the results of the Korean DST, finally addressing the pediatrician’s role in early detecting developmental issues and timely intervention.

Although direct comparison of the prevalence of DDs among worldwide countries is not possible due to many reasons such as methods, diagnostic criteria, ethnicity, age differences of population, and socioeconomic status, the increasing tendency of the prevalence of DDs is a globally common phenomenon (Table 1). For instances, the World Health Organization estimated that 8% of all children under 5 years of age have some types of developmental deficit [11]. In the United States, the prevalence of DDs among children between ages 3 and 17 increased by more than 2%, from 12.84% to 15.04% over a period of 12 years ranging from 1997 to 2008, and then 1.5% more increased during next 9 years till 2017 [5,6]. The same phenomenon is found in Asian and other countries (Table 1).

Recently, Rah et al. [12] reported a population-based epidemiological study of the prevalence and incidence of DDs in South Korea. That study shows an evident increase over the 15-year period in both the incidence rate and prevalence of 8 types of DDs among the younger population in South Korea as follows: attention deficit hyperactivity disorder (ADHD), autism spectrum disorder (ASD), cerebral palsy (CP), DD, intellectual disability (ID), language disorder, learning disorder, and special sensory disorder [12]. The prevalence of DDs steadily increased by more than 4 times (from 0.6 to 2.5) from 2003 to 2017 (Table 1). Boys had higher incidence than girls throughout the period, during which the gap increased from 19.1% to 31.4%.

The increase in medical cost or other expenses in children with DDs makes societies or countries focus on the importance of early detection and timely intervention. Therefore, many countries have implemented health screening programs for younger populations to early identify affected children to secure healthy national population and to lessen future medical expenses or other socioeconomic costs, starting from early 2000s.

DDs typically persist throughout a child's life and therefore require significant resources and planning for support services over the life span for optimal health, education, and functioning [21]. As compared to children without disabilities, children with at least one developmental disability need higher rates of hospital visit, personal carers, special education, and increased prescribed medication use [22]. Therefore, timely surveillance of DDs is crucial for developing effective programs and policies, and providing early intervention to maximize health and well-being.

Developmental surveillance is a systematic approach aiming to early detect children who may be at risk of future DD and plan next steps such as making referral to subspecialty or scheduling next appointment [23]. Dworkin defined it as “a flexible, continuous process whereby knowledgeable professionals perform skilled observations of children during the provision of health care.” [24] Its components include hearing or asking about parents’ concerns of their child development, taking developmental history, skilled observations of children, identifying risk and protective factors, and offering parents anticipatory guidance on health and developmental issues with consultation with other relevant professionals [3,25].

Surveillance begins by eliciting and addressing parents’ concerns at every well-child visit [23]. It is a longitudinal process designed to help clinicians focus on identification and intervention into the early stage of problems in order to potentially prevent them [26].

When pediatricians are heard of parents’ concern about developmental issue, most of it are found to be within normal variation. Therefore, clinicians may answer to parents simply for reassurance [10]. However, pediatricians should cautiously use the phrase “The child will grow out of it,” a kind of “wait and see” approach [10]. Just waiting without objective evidence and only relying on clinical judgement may cause late recognition, finally resulting in parental dissatisfaction, anxiety, and loss of the benefits from early identification and intervention [3].

Along with eliciting parents’ developmental concerns, systematic history taking is followed. It includes pre-/peri-/postnatal histories, milestones, 3-generation pedigree in family history relating to DD or other neurologic disorders, and socioeconomical backgrounds (Fig. 1). When taken history, it should be made based on the presence of identifiable risk factors, which are as followings; biologic risk (prenatal or perinatal insult), environmental risk (familial circumstances such as financial issue, alcohol, and abuse), and established risk (previously diagnosed neurological disorders) [10,27,28].

In addition, clinical judgement may lead to miss mild phenotype of DD depending on variable causes such as the degree of pediatrician’s experience, time constraints, and attitude attempting to focus on developmental issues. DST can help physicians identify potential issues more efficiently, which are not yet progressing to be overt.

Developmental screening is one of several strategies in the prevention and amelioration of DDs and complications [2]. DST is “a brief assessment procedure designed to identify children who should receive more intensive diagnosis or assessment.” [29]

Speech and language delays are the most common developmental problem among preschool children. Its prevalence is estimated to be 5%–12% in children 2 to 5 years of age [37]. Among them, expressive language delay is one of the most common reasons that young children are referred for further evaluation [38]. Language is linked to both cognitive and emotional development and is served to guide cognition and behavior [39]. Children with receptive language delay are fairly likely to have continuing language delay, as well as being at risk for later learning, behavioral, and emotional problems [40]. Therefore, impaired language learning is correlated with poor outcomes in academic achievement, reading, comprehension, socio-behavioral development, and self-esteem [41]. So, language delay may be the 1st indication of ID, ASD, and child neglect [39].

Children with having future ID usually show different pattern of DD in early childhood according to their age. In early infancy, a lack of visual or auditory responsiveness, abnormal muscle tone (hypo- or hypertonia), and feeding difficulties are observed. Between 6 and 18months of age, gross motor delay is the most common complaint. Language delay and behavioral problems are common concerns after 18months [39]. In infants and toddlers, motor and language milestones are often the best substitute for cognitive assessments [10]. Accordingly, ID can be mostly manifested as language or behavioral problems since 18 months of age before definite diagnosis of ID after 6 years of age.

The term “late talkers” (LT) refers to young children aged 18–35 months who have limited expressive vocabulary and/or receptive language in the absence of neurodevelopmental disabilities [40,42]. Several studies reported that 9% to 20% of 2- to 3-year-old children are late talkers [41,43]. Longitudinal studies revealed that many of LT catch up to their normal peers within 4 years of age [44,45]. However, 6% to 44% of late talkers have persistent language impairments [46].

The large and prospective study reported that 3 protective factors associated with late talking were identified: book reading, play opportunities, and formal childcare experiences at child care centers [41]. They are all examples of stimulation activities for language-based social interaction with various communication partners [41]. Especially, reading aloud to children is a powerful strategy to promote language development, vocabulary, and parent-child relationship [39].

If there are family histories of LT of parents, no other accompanying neurologic deficits, and no social/behavioral problem along with the result of “follow-up recommendation,” pediatricians should recommend the retest of K-DST in 2 months or 3 months following the initial K-DST according to their ages with stimulation activities and perform close observation of children’s language development over time (Fig. 1).

Sensory development such as auditory, visual, tactile, and proprioception is the crucial neurodevelopmental process helping children experience and control their environments. Through sensory experiences, children’s brains mature as new neuronal pathways are reorganized interplaying with environmental exposure along with strengthening of existing pathways [39]. Therefore, sensory deprivation at the stage of reorganization of synapses such as preservation and pruning has led to adverse effects in early childhood development. About 30% of hearing-impaired children have at least one other neurological disability such as ID, CP, and craniofacial anomalies [39]. Therefore, vision and hearing evaluation are essential to detect and treat for sensory impairments.

Based on the report regarding global trend of prevalence of DDs between 1990 and 2016, vision loss and hearing loss were most common among 6 types of DDs such as ID, ADHD, epilepsy, ASD, hearing loss, and vision loss [11]. Pediatricians can find 2 questions asking about hearing and visual function on the first page of K-DST sheets. It is essential to check the status of hearing and vision at the checkup [11].

Maximum brain development occurs within the first 3 years of a child’s life [15]. Each neuron develops on average 15,000 synapses by that age [39]. During early childhood, through preservation of frequently used synapses pathways and pruning of less-used ones, reorganization of neuronal circuits occurs. It acts important roles in brain plasticity, which is the ability of the brain to be shaped by experience, continuing into adolescence [37]. Neuronal plasticity makes the central nervous system reorganize its networks depending on features of environmental stimulants in positive or negative ways [39]. Therefore, the critical period of treatment for DD is before the child reaches 3 years of age [47]. There is a high chance of catch-up growth by early intervention performed during the periods of plasticity. However, if its periods close without early intervention, more permanent deficits may occur [39].

Previous literature reported that early identification of DD and intervention can positively alter a child’s long-term trajectory through brain plasticity [1,48-50]. For example, for children who have communication problems, more than 65% can be improved when intervention happens before 3 years of age [51]. Del Tufo et al. [52] reported that early intervention decreased risk of poorer comprehension by almost 40%. In addition, they reported a positive association between receiving early intervention and expressive language development [52].

Children can have appropriate developmental progress despite childhood trauma through resilience. Resilience is the ability to withstand, adapt to, and recover from adversities. It is possible through strong connections within supportive families. Even children with biologic risk factors may do well developmentally in supportive family environment [39]. Pediatricians should be aware of that fact and attempt to level up resilience in their patients and families [3,39].

Biological factors are influenced by environmental factors. By the time children start talking, language learning abilities depend on how well biological variables and their linguistic environment are interplayed [40]. Therefore, through providing language enrich environment, early intervention can help improve their language abilities. Further, when related with significant environmental neglect or abuse, it is important to intervene in order to provide more supportive and stimulating environments to children [40].

In primary care settings, children with DD are normally identified through 3 major channels: during routine developmental surveillance or screening; parental concern; and third parties’ reports by such as school teachers or nursery carers [35]. Pediatricians should play an important role not only in the first channel, but also should be alert on parental or third parties’ concerning reports.

Early recognition of delays requires in-depth knowledge of child development [53]. The efficacy of surveillance and screening test depends on the clinician’s experience in being capable of taking adequate physical and neurologic examination, right reading of developmental screening results, and knowledge of developmental milestones [54]. Adequate knowledge of developmental milestones is essential for the primary care physician to be able to provide anticipatory guidance and suggest appropriate activities to the parents or caregivers so that they can facilitate the next stage of development (Fig. 1).

In relevant with this, the Red flag table can be used a criterion for making referral to subspecialists to make sure that children are on the right path of their own development (Table 3). If your child shows 2 or more these signs, it is recommended making referral to subspecialty (Table 3). In addition, pediatricians also can see some questions suggestive of red flags in K-DST. The additional question section is regarding the possibility of CP, language delay, and ASD. These questions can be the clue of early signs of future DDs. Pediatricians need to pay attention to the answers of the questions [56].

As shown in Fig. 1, if showing normal developmental status through surveillance and screening test, it indicate that the child is at low risk of a developmental disorder, reassurance can be offered to parents with recommendation of next follow-up visit [3]. In reassuring parents, the pediatrician should emphasize the importance of serial surveillance and screening [3]. Physicians should also be aware of other reversible factors affecting development, such as sleep, diet, and caregiver’s stress of childcare [15].

Mothers are the primary environment for their children. They have close relationships both in good or bad ways. Some authors reported mother's depression and stress as significant factors for children's development [57,58]. Major depression that arises during pregnancy or in the postpartum period threatens the mother–child relationship. It is a risk factor for later cognitive and behavioral problems. Persistent maternal depression has been linked to decreases in child intelligence quotient scores at school entry [39]. Therefore, pediatricians need to check familial environment, especially parental emotional stress such as depression and anxiety, which easily can be evaluated by self-reporting test [59]. The information about parental emotion and stress level facilitates referral for therapy, which may provide long-term benefits to the child [39].

Knowledge regarding development and quality of training varies among pediatricians. Various educational programs should be regularly held for general pediatricians to conduct surveillance and screening test properly. Pediatricians should maintain and update their knowledge about developmental issues, risk factors, and screening techniques through varying educational program or other relevant educating materials

During sensitive periods for brain development, early detection and intervention for DD in children are important because a child’s development has lifelong implications for health, learning, and well-being. For successful early identification, it requires pediatricians to be skillful of screening techniques, actively to address parental concerns about development, and make connections with available community resources.

Based on these efforts, primary care pediatricians should conduct systematic approach to detect children with suspicious DD through developmental surveillance, K-DST, along with acknowledgement of red flags.

Repeated developmental assessments over time are more informative than one time assessment in planning investigations and management. A continuing, on-going relationship between pediatricians, parents, and preschool teachers will guarantee better good neurodevelopmental outcomes.