Some children develop more slowly than expected.
They may sit later.
Walk later.
Speak later.
Sometimes they even regress after an initial period of apparently normal development, losing skills they had already acquired.
Months become years.
Physiotherapy.
Speech therapy.
Neurology.
Brain MRI.
Blood tests.
Yet one question remains unanswered:
Why is this happening?
Developmental delay is a clinical presentation that may mark the beginning of a long and often complex diagnostic journey.
In many children, the underlying cause is genetic.
→ If this situation feels familiar, start the diagnostic pathway here.
Clinical Clues
DELAYED MILESTONES
Sitting, walking or speaking
later than expected.
REGRESSION
Loss of previously
acquired skills.
ASSOCIATED SIGNS
Hypotonia, seizures or
distinctive physical features.
NO DIAGNOSIS
Multiple investigations
without an explanation.
Understanding developmental delay
Developmental delay refers to a slower-than-expected acquisition of developmental milestones during infancy and early childhood. It may involve motor skills, language, cognition, social interaction or multiple developmental domains.
When delays affect two or more developmental domains in children under five years of age, the condition is generally referred to as Global Developmental Delay (GDD). Beyond early childhood, once standardized cognitive assessment becomes reliable, the term Intellectual Disability (ID) is usually preferred.
Developmental milestones that deserve attention
Children develop at different rates, and minor variations are often part of normal development. However, a consistent delay in reaching developmental milestones, particularly when it affects multiple domains or is associated with developmental regression, should prompt further clinical evaluation.
Motor milestones such as independent sitting, walking and fine motor skills are usually assessed together with language development, social interaction and cognitive abilities. Delays limited to a single domain may have different causes than a global developmental delay involving several areas of development.
Some clinical features deserve particular attention. Loss of previously acquired skills, persistent hypotonia, seizures, feeding difficulties, abnormal head growth or distinctive physical features may all suggest an underlying genetic disorder rather than an isolated developmental variation.
Although developmental milestones alone cannot establish a diagnosis, they often provide the first indication that a more comprehensive diagnostic approach, including genomic testing, may be appropriate.
Syndromic and non-syndromic developmental delay
One of the first questions during the evaluation of a child with developmental delay is whether the condition appears syndromic or non-syndromic.
Children with syndromic developmental delay present not only with delayed development but also with additional clinical features such as congenital malformations, distinctive facial characteristics, abnormal growth, hypotonia, epilepsy or involvement of other organ systems. These associated findings may suggest a specific genetic syndrome and help guide the diagnostic work-up.
In contrast, non-syndromic developmental delay refers to children in whom delayed cognitive, language or motor development is the predominant or only clinical finding. Although the absence of additional features may initially make diagnosis more challenging, many single-gene disorders can present in this way, particularly during early childhood.
It is important to recognize that the distinction is not always clear-cut. Some genetic conditions become progressively syndromic as children grow older, while subtle clinical features may only become apparent after careful examination or over time.
In addition, two fundamental concepts in medical genetics should always be considered: inter- and intrafamilial variability, whereby the same genetic disorder may present with different clinical manifestations even among affected members of the same family, and incomplete penetrance, whereby some individuals carrying a disease-causing variant remain clinically unaffected despite being able to transmit the condition to their offspring.
For this reason, the diagnostic approach increasingly combines careful clinical assessment with genomic testing, rather than relying solely on phenotype.
Causes of developmental delay
Developmental delay is not a diagnosis itself but a clinical manifestation that may result from many different conditions. Identifying the underlying cause is essential for establishing prognosis, guiding clinical management, estimating the recurrence risk in future pregnancies and providing accurate genetic counselling.
Possible causes include prenatal and perinatal complications, environmental factors, metabolic disorders and neurological diseases. However, a substantial proportion of children with unexplained developmental delay are ultimately found to have an underlying genetic disorder.
Genetic causes are highly heterogeneous and include chromosomal abnormalities, copy number variants (CNVs), single-gene (monogenic) disorders, mitochondrial diseases and, less frequently, epigenetic disorders or repeat expansion disorders. Together, these conditions account for a significant proportion of children presenting with global developmental delay or intellectual disability.
The rapid expansion of genomic technologies, particularly → Whole Exome Sequencing (WES) and → Whole Genome Sequencing (WGS), has profoundly changed the diagnostic approach. While conventional investigations remain important in selected clinical situations, genomic testing has substantially increased the ability to identify the molecular basis of developmental disorders, even in children with non-specific clinical findings.
The role of genetic testing
Genetic testing plays a central role in the diagnostic evaluation of children with unexplained developmental delay, particularly when developmental regression, epilepsy, hypotonia, congenital anomalies or a positive family history raise the suspicion of an underlying genetic disorder.
Traditionally, the diagnostic work-up often began with chromosomal analysis, Fragile X testing and chromosomal microarray analysis (CMA). Although these investigations remain appropriate in selected clinical situations, advances in genomic technologies have profoundly changed current diagnostic strategies.
However, considering that the estimated prevalence of Fragile X syndrome (FXS) has been substantially revised downward since the identification of the FMR1 gene, with recent estimates suggesting a prevalence of approximately 1 in 7,100 males, and that modern Whole Exome Sequencing (WES) combined with CNV analysis is able to investigate both single-gene disorders and a large proportion of clinically relevant copy number variants within a single assay, the diagnostic paradigm has progressively shifted towards comprehensive genomic testing.
For this reason, standard karyotyping, Fragile X testing and chromosomal microarray analysis (CMA) are increasingly performed as targeted complementary investigations when specifically indicated by the clinical presentation, rather than as mandatory sequential first-line tests in every child with unexplained developmental delay.
Indeed, → Whole Exome Sequencing (WES) has become one of the most informative first-line genomic investigations for children with unexplained developmental delay or intellectual disability. By simultaneously analysing thousands of disease-associated genes, WES substantially increases the likelihood of identifying the molecular cause compared with sequential single-gene testing.
Whenever possible, → trio exome sequencing further increases diagnostic yield by facilitating the interpretation of de novo variants and improving variant classification.
In selected situations, particularly after a non-diagnostic exome or when structural or non-coding variants are strongly suspected, → Whole Genome Sequencing (WGS) may provide additional diagnostic information.
Delayed acquisition of motor, language, cognitive or social skills
Motor, language, cognitive, adaptive or social development
Loss of previously acquired skills requires particular attention
Hypotonia, epilepsy, congenital anomalies, abnormal growth or distinctive physical features
Clinical assessment and targeted neurological, metabolic or imaging investigations when indicated
Whole Exome Sequencing, Trio WES or Whole Genome Sequencing within a structured diagnostic pathway
When no genetic diagnosis is found
Even after comprehensive genomic testing, not every child with developmental delay receives a molecular diagnosis.
A negative result does not necessarily exclude a genetic cause. It may reflect the current limits of scientific knowledge—for example, the fact that some disorders are clearly inherited within families but the responsible gene has not yet been identified—the technical limitations of the test performed, or the presence of genomic alterations that remain difficult to detect using standard diagnostic approaches.
These may include → deep intronic variants, which are generally detectable only by → Whole Genome Sequencing (WGS), certain repeat expansion disorders, epigenetic alterations requiring dedicated assays such as MS-MLPA or methylation arrays, low-level mosaicism, complex structural variants, or pathogenic changes located in genomic regions that are not adequately captured or interpreted by current technologies.
When the limitation lies in our current scientific knowledge rather than in the sequencing technology itself, reanalysis of WES or WGS data after an interval—typically 18 to 24 months following an initial negative result—may provide a diagnosis. As new disease genes and gene-disease associations are continuously reported in the scientific literature, periodic reanalysis increases the overall diagnostic yield by approximately 5% on average.
A negative genomic test should therefore not always be regarded as the end of the diagnostic journey. Rather, it may represent the point at which a different technology, additional functional studies or simply the passage of time becomes the key to reaching a diagnosis.
Our perspective: five observations from recent literature
1. Different types of developmental delay
Delays in different developmental domains may progress independently or in Parallel
Developmental delay is not a single condition but a clinical presentation involving one or more developmental domains. These include gross and fine motor skills, language, cognition, social interaction, adaptive behaviour and, later in life, pubertal and sexual development.
Some children present with an isolated delay, such as language or motor development, while others show impairment across multiple domains, a condition commonly referred to as global developmental delay (GDD).
Importantly, these developmental domains do not necessarily progress at the same pace. Some delays remain isolated, whereas others gradually involve additional functions over time. For this reason, evaluating the pattern of developmental delay is often more informative than considering each symptom in isolation.
2. Early diagnosis changes management, not only prognosis
For many years, establishing a genetic diagnosis was primarily considered a way to provide a name for the condition, estimate prognosis and calculate recurrence risk for future pregnancies.
Today, this perspective has profoundly changed. An early molecular diagnosis may directly influence clinical management by guiding surveillance programmes, avoiding unnecessary investigations, enabling disorder-specific treatments when available and facilitating access to appropriate rehabilitation and supportive care.
Perhaps even more importantly, early diagnosis allows intervention during the period of greatest brain plasticity. In children with neurodevelopmental disorders, timely speech therapy, physiotherapy, occupational therapy and educational support may significantly improve functional outcomes, even when the underlying genetic condition itself cannot yet be cured.
For this reason, reducing the diagnostic odyssey is no longer only a scientific objective but also an important opportunity to improve the quality of life of affected children and their families.
symptom in isolation.
3. Genetics is only one part of the picture
Although genetic disorders account for a substantial proportion of unexplained developmental delay, genetics alone does not determine neurodevelopment. Increasing evidence suggests that Adverse Childhood Experiences (ACEs) may also contribute to developmental delay.
These include, for example, physical or emotional abuse, neglect, exposure to domestic violence, parental substance abuse or severe mental illness, the loss of a parent or caregiver, and other forms of early-life adversity. The cumulative burden of these experiences has been associated with poorer developmental outcomes.
Considering both genetic and environmental factors is therefore essential to provide the most appropriate diagnostic evaluation and clinical care for each child.
4. Primary care plays a crucial role
Developmental delay is often recognized long before a definitive diagnosis is established. Routine health visits provide an opportunity to monitor developmental milestones, identify early warning signs and distinguish normal developmental variability from persistent delay.
Primary care physicians and pediatricians therefore play a key role in the early recognition of developmental disorders. Early referral for specialist evaluation and, when appropriate, genomic testing may substantially reduce the diagnostic odyssey and allow earlier intervention.
5. Developmental delay is a dynamic condition
Developmental delay should not be regarded as a static diagnosis. As children grow, new clinical features may emerge, while others become less evident. A child who initially appears to have isolated developmental delay may later develop epilepsy, movement disorders, autism spectrum features or other manifestations that help define a specific genetic syndrome.
Likewise, genomic interpretation is continuously evolving. Variants previously classified as uncertain may be reclassified, and newly discovered disease genes may explain cases that were previously considered unsolved. For this reason, periodic clinical reassessment and WES/WGS data reanalysis should be regarded as integral components of the diagnostic pathway rather than exceptional procedures.
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