Developmental issues during infancy or early childhood can lead to learning difficulties or behavioral issues in school-age children, subsequently impacting the quality of life during adolescence and later [1]. Developmental delay (DD) is defined as a slower rate of milestone acquisition than what is normally expected. Global DD (GDD) is a significant DD (performance of 2 standard deviations [SDs] or more below the mean on age-appropriate, standardized norm-referenced testing) in 2 or more domains affecting children under the age of 5 years [2]. DD is one of the most important health issues among children; although the exact prevalence of DD is unknown, GDD are reported to occur in 1% to 3% in children under 5 years old [2]. The World Health Organization estimated that 8% of all children under 5 years of age have some types of developmental deficit [3].

Developmental domains include gross or fine motor skills, speech and language, cognition, personal-social skills, and activities of daily living. DD can develop within various domains, which are not mutually exclusive. It is not unusual for delays in one of these domains to go unnoticed [4]. During early childhood, the brain exhibits plasticity, therefore, early detection of DD and proper interventions can reduce the chances of future developmental disorders and prevent secondary sequelae [1,5,6].

The Korean Developmental Screening Test (K-DST) is conducted as part of the national health screening program for Infants and Children (NHSPIC) for the early detection of DD. K-DST is a screening tool completed by caregivers and is indicated for infants and children between the ages of 4 and 71 months. The first edition of K-DST was revised in 2017, which showed high sensitivity and specificity [6,7]. In the NHSPIC, K-DST is first performed from 9 months of corrected age. Therefore, there is limited KDST data from infants younger than 9 months.

During the first years of life, children attain basic neurodevelopmental achievements at an impressive speed. Although neurodevelopment follows a predictable course, the developmental pathways are influenced by continuously interacting biomedical and sociocultural factors [8]. Screening for DD is based on the assumption that early delays in development predict later delays. However, early development is characterized by considerable variability [9]. Even when DD is suspected, many develop normally. Appropriate follow-up is essential based on the accurate experts analysis [10].

Therefore, we investigated the stability of the DD classification obtained from the revised K-DST score from 4–6 to 10–12 months age in healthy term infants.

At the first visit at 4–6 months of age, 55 infants were assessed by the K-DST 4- to 5-month questionnaire, while the remaining 14 infants were assessed by the K-DST 6- to 7-month questionnaire. At the second visit at 10–12 months of age, 63 infants’ development were assessed by the K-DST 10- to 11-month questionnaire, while the remaining 6 infants were assessed by the K-DST 12- to 13-month questionnaire (Table 1).

In this study, at 4–5 months, over 50% of infants were categorized into the ≥-1 SD group, with the lowest prevalence (52.7%) in the gross motor domain. Seven infants (10.1%) scored below -2 SD in at least 1 domain. The prevalence of scores below -2 SD was 7.3% in the gross motor and fine motor domains. At 10–12 months, more than 70% were categorized into the ≥-1 SD group, except for language. Six infants (9.5%) scored below -2 SD in at least 1 domain. The prevalence of scores below -2SD was 4.8%, 3.2%, 3.2% in the cognition, language, and gross motor domains, respectively. The serial follow-up results from 4 to 12 months showed that a significant number of infants improved to the peer and high-level group (≥-1 SD), especially in the gross motor domain. Among 7 infants who scored below -2 SD in at least one domain at 4–5 months, only 2 infants remained in the -2 SD score at 10–12 months.

Previous studies showed variable prevalences of DD in young infants [14,15]. In a United States (US) longitudinal study, the rate of score below <-2 SD was 2.3% on the Bayley Short Form-Research Edition (BSF-R) mental scale and 2.5% on the BSFR motor scale in 9 month infants [14]. A Norwegian longitudinal study of 1.244 infants in well baby clinics showed that the overall prevalence of suspected DD in one or more domains was 7.0% at 4 months, based on the Norwegian version of Ages and Stages Questionnaires cutoff points (10.3%, US cutoff points), 5.7% (12.3% US cutoff) at 6 months and 6.1 % (10.3 % US cutoff) at 12 months. In addition, during the first year of life, DD is most frequent screened within the gross motor area [15]. It is difficult to compare the prevalence rates of DD in early infancy, due to variable test measures, variable study populations with or without high risk, and variations in age at the time of testing.

According to Darrah et al. [16], normally developing infants are not stable in the rate of emergence of gross motor skills throughout the first 13 months. Fluctuations in the percentile rankings of motor abilities of an infant are not necessarily indicative of motor dysfunction. A low gross motor percentile could be attributable to a period in which few or no new motor skills were developed, compared with infants who mastered many new skills. In addition, typically developing infants showed a nonlinear rate of development, rather than a constant development rate [17]. The study of Valla et al. [18] showed the variability in infants’ development patterns. They reported that all classes of 1,555 infants showed a decrease in gross motor developmental pathways from 4 to 9 months; the ‘high stable’ class (80%) and ‘the late bloomers’ classes (10%) improved at 16 months and remained stable at 24 months. The U-shaped class (10%) started with high scores at 4 months and subsequently showed a decrease, followed by a relatively stable period from 9 to 16 months before an increase in scores at 24 months [18]. The WHO Multicentre Growth Reference Study (2006) showed a great variability in the attainment of the milestones of infant gross motor development. For example, walking alone is achieved between the ages of 8.2 and 17.6 months. Ninety percent of infants achieved gross motor milestones in a particular sequence, starting from crawling and progressing to sitting and, finally, walking alone, whereas 4.3% of the infants did not exhibit hands-and-knees crawling [9].

There are many involved factors; both intrinsic (physical characteristics, temperament, child’s overall state of wellness) and extrinsic factors (parent and sibling personalities, nurturing methods, the cultural environment, socioeconomic status) are responsible for individual variation [19]. A number of risk factors have been associated with an increased risk of DD. Low birth weight and short gestational age have a persisting negative association with infant gross motor development. There is inconsistent evidence for an association of breast feeding, supine sleeping, and baby-walker use with infant gross motor development [20].

In the Korean Pediatric textbook, the descriptions of gross motor development states that rolling from prone to supine occurs prior (at 4 months) to rolling from supine to prone (at 5 months) [21]. However, in this study, many 4–5 months old infants were able to roll over from supine to prone but were unable to roll over from prone to supine. The study of 72 infants in Hong Kong also reported that they rolled from supine to prone (mean age, 5.1±1.5 months) prior to rolling from prone to supine (mean age, 5.7±1.3 months). Different childcare practices, e.g., reduced emphasis of ‘tummy-time’ and routine use of the supine sleep position, may influence the age at which infants learn to roll from prone to supine [22]. K-DST question items are arranged in order from easy to difficult, However, the question 5 is ‘rolling from prone to supine,’ and question 6 is ‘rolling supine to prone.’ Further large-scale studies are needed to determine if changing the question order is warranted.

In this study, among 7 infants who had scored below -2 SD at 4–5 month, 2 had a persistent DD. Wang et al. [23] reported that early motor skills predicted later communication skills, and Piek et al. [24] suggested a strong relationship between early motor skills and later school-aged cognitive development. Infants with delayed gross motor development during early life should be continuously monitored by periodic developmental screening tests.

This study had several limitations. First, the sample included a limited number of infants. Further large-scale, and extended longitudinal studies are needed. Second, K-DST is a parent-reported screening test. Although we reviewed the parents’ reports, parents may not be able to accurately report their child development status. This may have biased our results to some extent. Third, we did not investigate the factors responsible for the variability in the attainment of developmental milestones.

In conclusion, a single abnormal of the revised K-DST result at 4–5 months should be interpreted cautiously in terms of the DD risk. Early detection of DD requires a through history taking, comprehensive physical and neurological examination, as well as developmental screening testing. For infants presenting with suspected DD on the revised K-DST 4–5 months questionnaire, especially in the gross motor domain, in the absence of any abnormal findings on neurological examinations or any developmental red flags, close monitoring with appropriate stimulation activities can be suggested, and repeated assessments after short term period should be performed. However, early diagnosis of DDs is crucial, so if there is any uncertainty, it is recommended to refer to a pediatric neurologist. Early developmental assessments should be an ongoing process involving multiple time points.