Global developmental delay is an umbrella term used when children are significantly delayed in their cognitive and physical development. There is usually a more specific condition which causes this delay, such as Fragile X syndrome or other chromosonal abnormalities. However, it is sometimes difficult to identify this underlying condition.

Other terms associated with this condition are failure to thrive (which focuses on lack of weight gain and physical development), intellectual disability (which focuses on intellectual deficits and the changes they cause to development) and developmental disability (which can refer to both intellectual and physical disability altering development).

Alopecia contractures dwarfism mental retardation syndrome or (ACD mental retardation syndrome) is a developmental disorder which causes mainly baldness and dwarfism in combination with intellectual disability; skeletal anomalies, caries and nearsightedness are also typical.

The ACD mental retardation syndrome was first described in 1980 by Albert Schinzel and only few cases have since been identified in the world. At the time Dr. Schinzel made no conclusion of the hereditary pattern of this syndrome but similarities between cases reported by year 2000 seem to suggest autosomal or x-linked recessive inheritance or possibly a dominant mutation caused by mosaicism as causes of this syndrome.

The most notable features of Wilson-Turner Syndrome are intellectual disability, obesity, hypogonadism, gynecomastia, and distinct facial features. All of the symptoms are chronic. Affected females are known to have less severe signs and symptoms than males. Female carriers of the disorder may have none or mild symptoms.

- Intellectual Disability is the limitation in an individual's mental functioning and skills. Patients of Wilson-Turner Syndrome have mental disability generally ranging from mild to severe, more frequently on the former. This symptom often coincides with delay in speech development and the occurrence of mood swings. Most males were noted to have a quiet and a cheerful disposition. However, individuals who displayed aggression and became easily upset were also seen. Children will display a delay in speech development often combined with excessive drooling and low voice tone. Some of the studied male patients had speech impairment ranging from little or no speech to minor stuttering.

- Obesity is the accumulation of excess fat on the body. Individuals with Wilson-Turner Syndrome are characterized to have truncal obesity, meaning the fat has accumulated in one's middle. Truncal obesity is often related to heart disease, kidney disease, and lowered blood immune system. Truncal obesity in this disorder becomes more apparent around the age of puberty.

- Tapered fingers is when one end of the finger is diminished in thickness, causing the ends of the fingers to appear pointed. This deformity is not debilitating in any particular manner. In addition to tapered fingers, both hands and feet tend to be small. Some males were observed to have pes planus, also known as flat feet.

- Hypogonadism is a condition in which the gonads have a decrease in function. This condition may result from the lack of sex hormone synthesis, such as androgen and estrogen. Hormones produced by the gonads may also decrease. Hypogonadism also influences the onset of other conditions of the Wilson-Turner Syndrome, such as gynecomastia and decreased testes size in males. It can also cause short stature in men and women. In addition to little genital development, pubic and body hair are scant.

- Some of the facial features that are associated with Wilson-Turner Syndrome include small head circumference, high forehead, prominent ears, and nose with a flattened bridge. There have been cases of a moderately high palate. Low muscle tone and subcutaneous swelling in facial tissue has also been noted. Thick eyebrows are also common. However, there has been reported cases where individuals had none of the mentioned facial features, which shows phenotypic abnormalities have possible environmental influences.

- Gynecomastia is a non-cancerous increase in male breast tissue. It is believed that disturbances in the endocrine system leads to an increase in estrogen and androgen hormones lead to the development of gynecomastia. A key feature of gynecomastia is a rubbery or firm glandular subcutaneous chest tissue that is palpated under the areola of the nipple, instead of the soft fatty tissue. There can also be in increase in diameter of the areola asymmetry in the chest tissue. The breast enlargement can occur in one or both side. Similar to truncal obesity, gynecomastia becomes apparent around the age of puberty.

Microcephaly is a disorder in which the circumference of the head is smaller than average for the person's age and gender. Most children with microcephaly also have a smaller than typical brain and intellectual disability. Some of the most common signs and symptoms associated with microcephaly are seizures, poor feeding, high pitched cry, intellectual disability, developmental delay, and increased movement of arms and legs.

Vision abnormalities in children with 1p36 have been wide-ranging, including:

Disorders considered in origin include:

1. Intellectual disability (ID) or intellectual and developmental disability (IDD), previously called mental retardation

2. Autism spectrum disorders, such as Asperger's syndrome or Kanner syndrome

3. Motor disorders including developmental coordination disorder and stereotypic movement disorder Tic disorders including Tourette's syndrome

4. Traumatic brain injury (including congenital injuries such as those that cause cerebral palsy)

5. Communication, speech and language disorders

6. genetic disorders, such as fragile-X syndrome, Down syndrome, attention deficit hyperactivity disorder, schizophrenia, schizotypal disorder, hypogonadotropic hypogonadal syndromes

7. disorders due to neurotoxicants like fetal alcohol spectrum disorder, Minamata disease caused by mercury, behavioral disorders including conduct disorder etc caused by other heavy metals, such as lead, chromium, platinum etc, hydrocarbons like dioxin, PBDEs and PCBs, medications and illegal drugs, like cocaine and others.

There are many signs and symptoms of Prader–Willi syndrome. The symptoms can range from poor muscle tone during infancy to behavioral problems in early childhood. Some symptoms that are usually found in infants, besides poor muscle tone, would be a lack of eye coordination; some are born with almond-shaped eyes; and due to poor muscle tone the infant may not have a strong sucking reflex. Their cry is weak, and they have difficulty waking up. Another sign of this condition is a thin upper lip.

More aspects seen in a clinical overview include hypotonia and abnormal neurologic function, hypogonadism, developmental and cognitive delays, hyperphagia and obesity, short stature, and behavioral and psychiatric disturbances.

Holm "et al." (1993) describe the following features and signs as pretest indicators of PWS, although not all will be present.

Neurodevelopmental disorder is a mental disorder. A narrower use of the term refers to a disorder of brain function which affects emotion, learning ability, self-control and memory and which unfolds as the individual grows.

Psychopathology and related behavioral abnormalities are typically seen in LFS, and they may be considered in the diagnosis of the disorder. The most common of these in LFS is an autism-like spectrum disorder, and LFS is considered as one of a number of genetic disorders associated with autism. Additional alterations of psychopathology with behavioral manifestations that have been observed in LFS include: psychotic behavior, schizophrenia, hyperactivity and attention-deficit hyperactivity disorder, aggression, oppositional defiant disorder, obsessive compulsive disorder, extreme shyness, learning disability, cognitive impairment, short-term memory deficit, low frustration tolerance, social dysfunction, lack of impulse control, eating disorder and associated malnutrition, attributed to psychogenic loss of appetite; and pyromania.

While psychiatric conditions like these are to be expected with LFS, there have also been cases of the disorder with some preservation of mental and behavioral abilities, such as problem solving, reasoning and normal intelligence.

The psychopathology of LFS usually exhibits schizophrenia. When schizophrenia is diagnosed in an individual known to be affected by intellectual disability, LFS may be considered in the differential diagnosis of schizophrenia, with confirmation of cause through appropriate psychiatric and genetic evaluation methods.

Symptoms of M2DS include infantile hypotonia and failure to thrive, delayed psychomotor development, impaired speech, abnormal or absent gait, epilepsy, spasticity, gastrointestinal motility problems, recurrent infections, and genitourinary abnormalities. Many of those affected by M2DS also fit diagnostic criteria for autism. M2DS can be associated with syndromic facies, namely an abnormally flat back of the head, underdevelopment of the midface, ear anomalies, deep-set eyes, prominent chin, pointed nose, and a flat nasal bridge.

Individuals with PWS are at risk of learning and attention difficulties. Curfs and Fryns (1992) conducted research into the varying degrees of learning disability found in PWS. Their results, using a measure of IQ, were as follows:

- 5%: IQ above 85 (high to low average intelligence)

- 27%: IQ 70–85 (borderline intellectual functioning)

- 39%: IQ 50–70 (mild intellectual disability)

- 27%: IQ 35–50 (moderate intellectual disability)

- 1%: IQ 20–35 (severe intellectual disability)

- <1%: IQ <20 (profound intellectual disability)

Cassidy found that 40% of individuals with PWS have borderline/low average intelligence, a figure higher than the 32% found in Curfs and Fryns' study. However, both studies suggest that most individuals (50–65%) fall within the mild/borderline/low average intelligence range.

Children with PWS show an unusual cognitive profile. They are often strong in visual organization and perception, including reading and vocabulary, but their spoken language (sometimes affected by hypernasality) is generally poorer than their comprehension. A marked skill in completing jigsaw puzzles has been noted, but this may be an effect of increased practice.

Auditory information processing and sequential processing are relatively poor, as are arithmetic and writing skills, visual and auditory short-term memory and auditory attention span. These sometimes improve with age, but deficits in these areas remain throughout adulthood.

There may be an association with psychosis.

LFS is clinically distinguished from other X-linked forms of intellectual disability by the accompanying presence of marfanoid habitus. Marfanoid habitus describes a group of physical features common to Marfan syndrome. Including Marfan syndrome and LFS, marfanoid features of this type have also been observed with several other disorders, one of which is multiple endocrine neoplasia type 2.

In LFS, specific features identified as marfanoid include: a long, narrow face; tall, thin stature; long, slender limbs, fingers and toes (not unlike arachnodactyly) with joint hyperextensibility, shortened halluces (the big toes) and long second toes.

The diagnosis of marfanoid habitus in LFS is often delayed because many of the physical features and characteristics associated with it are usually not evident until adolescence.

Aneuploidy is often fatal, but in this case there is "X-inactivation" where the effect of the additional gene dosage due to the presence of extra X chromosomes is greatly reduced.

Much like Down syndrome, the mental effects of 49,XXXXY syndrome vary. Impaired speech and behavioral problems are typical. Those with 49,XXXXY syndrome tend to exhibit infantile secondary sex characteristics with sterility in adulthood and have some skeletal anomalies. Skeletal anomalies include:

- Genu valgum

- Pes cavus

- Fifth finger clinodactyly

The effects also include:

- Cleft palate

- Club feet

- Respiratory conditions

- Short or/and broad neck

- Low birth weight

- Hyperextensible joints

- Short stature

- Narrow shoulders

- Coarse features in older age

- Hypertelorism

- Epicanthal folds

- Prognathism

- Gynecomastia (rare)

- Muscular hypotonia

- Hypoplastic genitalia

- Cryptorchidism

- Congenital heart defects

- A very round face in infancy

Most children with Allan–Herndon–Dudley syndrome have weak muscle tone (hypotonia) and underdevelopment of many muscles (muscle hypoplasia). As they get older, they usually develop joint deformities called contractures, which restrict the movement of certain joints. Abnormal muscle stiffness (spasticity), muscle weakness, and involuntary movements of the arms and legs also limit mobility. As a result, many people with Allan–Herndon–Dudley syndrome are unable to walk independently and become wheelchair-bound by adulthood.

Several X-linked syndromes include intellectual disability as part of the presentation. These include:

- Coffin–Lowry syndrome

- MASA syndrome

- MECP2 duplication syndrome

- X-linked alpha thalassemia mental retardation syndrome

- mental retardation and microcephaly with pontine and cerebellar hypoplasia

Affected newborns generally have striking neurological defects and seizures. Severely impaired development is common, but disturbances in motor functions may not appear until later in life.

Infants with microcephaly are born with either a normal or reduced head size. Subsequently, the head fails to grow, while the face continues to develop at a normal rate, producing a child with a small head and a receding forehead, and a loose, often wrinkled scalp. As the child grows older, the smallness of the skull becomes more obvious, although the entire body also is often underweight and dwarfed. Development of motor functions and speech may be delayed. Hyperactivity and intellectual disability are common occurrences, although the degree of each varies. Convulsions may also occur. Motor ability varies, ranging from in some to spastic quadriplegia in others.

Ophthalmologic problems include strabismus. This requires early identification to avoid amblyopia. Surgery or patching are usually necessary to treat strabismus if diagnosed early. Refractive errors in patients with FXS are also common.

Wilson-Turner Syndrome (WTS), also known as Mental Retardation X Linked Syndromic 6 (MRXS6), or Mental Retardation X Linked with Gynecomastia and Obesity is a congenital condition characterized by intellectual disability and associated with childhood-onset obesity. It is found to be linked to the X chromosome and caused by a mutation in the HDAC8 gene, which is located on the q arm at locus 13.1. Individuals with Wilson–Turner syndrome have a spectrum of physical characteristics including dysmorphic facial feature, hypogonadism, and short stature. Females generally have milder phenotype than males. The study of X-linked mental retardation began in 1943 when Martin and Bell reported a family exhibiting sex-linked mental retardation. However, this syndrome was not recognized until 1991. Wilson studied 14 males from 3 successive generations that presented hypogonadism, mental retardation, gynecomastia, short stature, among other symptoms. Eventually, this disorder was ruled distinct from a syndrome presented by Prader and Willi (Prader-Willi syndrome) because of its mode of inheritance, gynecomastia, and presence of small hands and feet. However, there are some speculations that this syndrome is in the same spectrum as the Cornelia de Lange syndrome. This disorder affects all demographics equally and is seen in less than one in one million people.

Common signs of Say–Meyer syndrome are trigonocephaly as well as head and neck symptoms. The head and neck symptoms come in the form of craniosynostosis affecting the metopic suture (the dense connective tissue structure that divides the two halves of the skull in children which usually fuse together by the age of six). Symptoms of Say–Meyer syndrome other than developmental delay and short stature include

- Intellectual disability.

- Low-set ears/posteriorly rotated ears

- Intellectual deficit as well as learning disability

- Intrauterine growth retardation (poor growth of a baby while it is in the mother's womb)

- Posterior fontanel

- Premature synostosis of the lambdoid suture (the fusion of the bones to the joint is premature)

- Narrow forehead

- Trigonocephaly (a frontal bone anomaly that is characterized by a premature fusion of the bones which gives the forehead a triangular shape)

- Hypotelorism or hypertelorism (reduced or increased width between the eyes)

- Craniosynostosis (when one or more seam-like junctions between two bones fuses by turning into bone. This changes the growth pattern of the skull)

- Low birth weight and height

The affected patients sometimes show a highly arched palate, clinodactyly (a defect in which toes or fingers are positioned abnormally) and ventricular septal defect (a heart defect that allows blood to pass directly from left to the right ventricle which is caused by an opening in the septum). Overall, Say–Meyer syndrome impairs growth, motor function, and mental state.

Intellectual disability (ID) begins during childhood and involves deficits in mental abilities, social skills, and core activities of daily living (ADLs) when compared to same-aged peers. There often are no physical signs of mild forms of ID, although there may be characteristic physical traits when it is associated with a genetic disorder (e.g., Down syndrome).

The level of impairment ranges in severity for each person. Some of the early signs can include:

- Delays in reaching or failure to achieve milestones in motor skills development (sitting, crawling, walking)

- Slowness learning to talk or continued difficulties with speech and language skills after starting to talk

- Difficulty with self-help and self-care skills (e.g., getting dressed, washing, and feeding themselves)

- Poor planning or problem solving abilities

- Behavioral and social problems

- Failure to grow intellectually or continued infant-like behavior

- Problems keeping up in school

- Failure to adapt or adjust to new situations

- Difficulty understanding and following social rules

In early childhood, mild ID (IQ 50–69) may not be obvious or identified until children begin school. Even when poor academic performance is recognized, it may take expert assessment to distinguish mild intellectual disability from specific learning disability or emotional/behavioral disorders. People with mild ID are capable of learning reading and mathematics skills to approximately the level of a typical child aged nine to twelve. They can learn self-care and practical skills, such as cooking or using the local mass transit system. As individuals with intellectual disability reach adulthood, many learn to live independently and maintain gainful employment.

Moderate ID (IQ 35–49) is nearly always apparent within the first years of life. Speech delays are particularly common signs of moderate ID. People with moderate intellectual disability need considerable supports in school, at home, and in the community in order to fully participate. While their academic potential is limited, they can learn simple health and safety skills and to participate in simple activities. As adults, they may live with their parents, in a supportive group home, or even semi-independently with significant supportive services to help them, for example, manage their finances. As adults, they may work in a sheltered workshop.

People with severe or profound ID need more intensive support and supervision their entire lives. They may learn some ADLs, but an intellectual disability is considered severe or profound when individuals are unable to independently care for themselves without ongoing significant assistance from a caregiver throughout adulthood. Individuals with profound ID are completely dependent on others for all ADLs and to maintain their physical health and safety, although they may be able to learn to participate in some of these activities to limited degree.

X-linked intellectual disability (previously known as X-linked mental retardation) refers to forms of intellectual disability which are specifically associated with X-linked recessive inheritance.

As with most X-linked disorders, males are more heavily affected than females. Females with one affected X chromosome and one normal X chromosome tend to have milder symptoms.

Unlike many other types of intellectual disability, the genetics of these conditions are relatively well understood. It has been estimated there are ~200 genes involved in this syndrome; of these ~100 have been identified.

X-linked intellectual disability accounts for ~16% of all cases of intellectual disability in males.

Most young children do not show any physical signs of FXS. It is not until puberty that physical features of FXS begin to develop. Aside from intellectual disability, prominent characteristics of the syndrome may include an elongated face, large or protruding ears, flat feet, larger testes (macroorchidism), and low muscle tone. Recurrent otitis media (middle ear infection) and sinusitis is common during early childhood. Speech may be cluttered or nervous. Behavioral characteristics may include stereotypic movements (e.g., hand-flapping) and atypical social development, particularly shyness, limited eye contact, memory problems, and difficulty with face encoding. Some individuals with fragile X syndrome also meet the diagnostic criteria for autism.

Males with a full mutation display virtually complete penetrance and will therefore almost always display symptoms of FXS, while females with a full mutation generally display a penetrance of about 50% as a result of having a second, normal X chromosome. Females with FXS may have symptoms ranging from mild to severe, although they are generally less affected than males.

People with Aarskog-Scott syndrome often have distinctive facial features, such as widely spaced eyes (hypertelorism), a small nose, a long area between the nose and mouth (philtrum), and a widow's peak hairline. They frequently have mild to moderate short stature during childhood, but their growth usually catches up with that of their peers during puberty. Hand abnormalities are common in this syndrome and include short fingers (brachydactyly), curved pinky fingers (fifth finger clinodactyly), webbing of the skin between some fingers (cutaneous syndactyly), and a single crease across the palm. Other abnormalities in people with Aarskog-Scott syndrome include heart defects and a split in the upper lip (cleft lip) with or without an opening in the roof of the mouth (cleft palate).

Most males with Aarskog-Scott syndrome have a shawl scrotum, in which the scrotum surrounds the penis instead of hanging below. Less often, they have undescended testes (cryptorchidism) or a soft out-pouching around the belly-button (umbilical hernia) or in the lower abdomen (inguinal hernia).

The intellectual development of people with Aarskog-Scott syndrome varies widely. Some may have mild learning and behavior problems, while others have normal intelligence. In rare cases, severe intellectual disability has been reported.

ATR-16 syndrome affects the blood, development, and brain; symptoms vary based on the specific genes deleted on chromosome 16. Because it is so rare, it is difficult to determine the "core" symptoms of the disease. People with ATR-16 have alpha-thalassemia, a blood disorder where there is less normal hemoglobin in the blood than there should be, and the red blood cells are smaller than they should be (microcytic anemia). Affected children have various characteristic physical features, including clubfoot, "locked" little fingers, microcephaly (small head), hypertelorism (widely spaced eyes), broad, prominent nose bridge, downward-slanted palpebral fissures, small ears, retrognathia, and short neck. Children with ATR-16 syndrome also have mild to moderate intellectual disabilities, developmental delays/growth delays, and speech delays. Some children with ATR-16 have seizures, cryptorchidism (undescended testes), or hypospadias.

Allan–Herndon–Dudley syndrome is a rare X-linked inherited disorder of brain development that causes moderate to severe intellectual disability and problems with movement. This condition, which occurs almost exclusively in males, disrupts development from before birth.

Allan–Herndon–Dudley syndrome, which is named eponymously for William Allan, Florence C. Dudley, and C. Nash Herndon, results from a mutation of the thyroid hormone transporter MCT8 (also referred to as SLC16A2). Consecutively, thyroid hormones are unable to enter the nervous system, which depends on thyroid signaling for proper function and development.