The earliest signs and symptoms occur in newborns and consist of hypotonia, but show up in youth as developmental delays, low muscle tone, learning disabilities, being overweight, autism-like symptoms, seizures, eczema, asthma, chest and ear infections, and abnormalities in face, hands, and feet. Autism-like symptoms consist of odd obsessions, repetitive behavior, poor use of eye contact, impaired speech, poor understanding of others’ emotions, idiosyncratic use of words or phrases. People with this disorder also tend to have a characteristic appearance, including prominent forehead, thin, highly arched eyebrows, depressed nasal bridge, full cheeks, deficient nasal alae and prominent columella, thin upper lip, and various minor anomalies of the pinnae. Heart, brain, gastrointestinal, and kidney problems such as Wilms tumor, hernias, spinal curvatures, Osteopenia, hearing and sight difficulties can also occur.

It has been associated with Wilms' tumor and brachydactyly.

Below are the common clinical features of those diagnosed with Floating–Harbor syndrome. Patients will show varying degrees of some or all FHS symptoms. Facial abnormalities are the most defining aspects of those diagnosed with this disease.

The differential diagnosis of broad thumbs includes Rubinstein-taybi syndrome, where they are a cardinal feature. FHS is also in the differential, which logically agrees with the thought that the disease is a result of a mutation in SRCAP, as this gene interacts with CBP.

The severity of symptoms of idic(15) vary greatly between individuals. Individuals with idic(15) usually have delays in language development and motor skills such as walking or sitting up. Other traits may include low muscle tone (hypotonia), seizures (>50%), short stature, and intellectual disability. Distinctive facial features associated with idic(15), where present, are usually very subtle but may include epicanthal folds (skin folds at the inner corners of one or both eyes), broad forehead, a flattened nasal bridge, button nose, and a high arched palate (roof of the mouth). Many individuals with idic(15) display features of autism, such as problems with communication and social interactions, obsessional interests (often with interactive mechanisms like wheels, doors or switches), unpredictable sleep cycles (and a reduced need for sleep), and repetitive and stereotyped behaviors (e.g., lining up toys, playing with a toy in the same manner over and over again, hand flapping, rocking back and forth). A high pain threshold is often observed. If speech develops, it is often echolalic but some individuals do grasp some language. With a severely affected person there may be an inability to walk or talk.

2q37 deletion syndrome is a disorder caused by the deletion of a small piece of chromosome 2.2q37 deletion syndrome describes a disorder in which one or multiple of 3 sub-bands, 2q37.1, 2q37.2, and 2q37.3, of the last band of one of the chromosome 2’s long arms are deleted. The first report of this disorder was in 1989.

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.

Children with fragile X have very short attention spans, are hyperactive, and show hypersensitivity to visual, auditory, tactile, and olfactory stimuli. These children have difficulty in large crowds due to the loud noises and this can lead to tantrums due to hyperarousal. Children with FXS pull away from light touch and can find textures of materials to be irritating. Transitions from one location to another can be difficult for children with FXS. Behavioral therapy can be used to decrease the child’s sensitivity in some cases.

Perseveration is a common communicative and behavioral characteristic in FXS. Children with FXS may repeat a certain ordinary activity over and over. In speech, the trend is not only in repeating the same phrase but also talking about the same subject continually. Cluttered speech and self-talk are commonly seen. Self-talk includes talking with oneself using different tones and pitches.

FG syndrome's major clinical features include intellectual disability, usually severe; hyperactive behavior, often with an outgoing personality; severe constipation, with or without structural anomalies in the anus such as imperforate anus; macrocephaly; severe hypotonia; a characteristic facial appearance due to hypotonia, giving a droopy, "open-mouthed" expression, a thin upper lip, a full or pouting lower lip, and partial or complete loss of the corpus callosum. About a third of reported cases of individuals with FG syndrome die in infancy, usually due to respiratory infection; premature death is rare after infancy.

AHC patients exhibit a wide range of symptoms in addition to hemiplegic attacks. These can be further characterized as paroxysmal and non-paroxysmal symptoms. Paroxysmal symptoms are generally associated with hemiplegic attacks and may occur suddenly with hemiplegia or on their own. Paroxysmal symptoms may last for variable amounts of time. Non-paroxysmal symptoms tend to be side effects of AHC which are present at all times, not just during episodes or attacks. Epilepsy, which is also considered a paroxysmal symptom, plays an important role in the progression and diagnosis of AHC.

Associated with agenesis (loss) of the corpus callosum, intellectual disabilities are common among individuals with FG syndrome. Motor ability is also impaired as a result of having FG syndrome and its effects on the development of neurons. During infancy, problems arise in the gastrointestinal and gastroesophageal systems of the body. The most common gastrointestinal problems include constipation from imperforated anuses and gastroesophageal reflux. Cardiopulmonary defects also contribute to roughly 60% of premature deaths in infants with FG syndrome. Of all of the congenital heart defects septal defects are the most common. After infancy, long term survival has been recorded to individuals surviving beyond the age of 50.

Neurological effects are believed to be more severe as the number of extra X chromosomes increases; a male with 48, XXXY is likely to have more severe symptoms than a male with Klinefelter syndrome. Developmental delays are common in infancy and childhood. Expected symptoms include speech delays, motor delays, and hypotonia (lack of muscle tone), also known as floppy baby syndrome.Individuals with XXXY syndrome exhibit cognitive and behavioral problems.

Patients typically show altered adaptive behavior, which is the ability of an individual to demonstrate essential living skills, including: social skills, community living, safety, functional use of academic skills and self-care. People with XXXY syndrome were found to score significantly less in the domains of daily living skills and communication compared to XXYY, and XXY individuals. This means that they typically demonstrate little ability in the domains of self-care, social skills, safety, application of academic skills, and responsibility.

Individuals with this syndrome also experience emotional symptoms such as anxiety symptoms, obsessive-compulsive behaviors, behavioral dysregulational and emotional immaturity. People with this syndrome typically have an IQ in the range of 40-60, where the average IQ range is 95-110. They also experience language-based learning disabilities that can affect their communication with others. Those with XXXY syndrome tend to display less externalizing and internalizing behaviors compared to those with 48, XXYY syndrome, which may have a positive effect on their social functioning. These individuals may also have increased vulnerability for autistic features. Changes in testosterone as well as androgen deficits may contribute to these individuals’ social behaviors that put them at increased risk for autistic features.

Isodicentric 15, also called idic(15), partial tetrasomy 15q, or inverted duplication 15 (inv dup 15), is a chromosome abnormality in which a child is born with extra genetic material from chromosome 15. People with idic(15) are typically born with 47 chromosomes in their body cells, instead of the normal 46. The extra chromosome is made up of a piece of chromosome 15 that has been duplicated end-to-end like a mirror image. It is the presence of this extra genetic material that is thought to account for the symptoms seen in some people with idic(15). Individuals with idic(15) have a total of four copies of this chromosome 15 region instead of the usual two copies (1 copy each on the maternal and paternal chromosomes).

The syndrome is also often referred to by the broader term "Chromosome 15q11.2-q13.1 Duplication Syndrome", shortened to Dup15q syndrome, a name that is supported and actively promoted by the US-based support organization Dup15q Alliance. Dup15q syndrome is a broader disease term, as it includes both idic(15) and interstitial 15q11.2-q13.1, another type of duplication that causes similar clinical traits.

The extra chromosome is occasionally found in the mosaic state, i.e. some of the cells carry the marker chromosome. However, mostly because of the marker's instability and tendency to be lost during cell division (mitosis), some cells are completely normal with 46 chromosomes. Occasionally, cells may have more than one idic(15), resulting in 48 or 49 chromosomes in all or some of their cells. A similar clinical picture albeit to a milder degree could be expected in individuals that have the extra chromosome 15 material as an interstitial duplication (when the extra piece of chromosome 15 is included "within" the long arm of one of the two copies of chromosome 15, rather than as a small extra 'marker' chromosome) - often abbreviated to int dup(15); the individual thus having 46 chromosomes.

Chronologically, hemiplegic attacks are not always the first symptom of AHC, but they are the most prominent symptom, as well as the symptom for which the disorder is named. Hemiplegic attacks may affect one or both sides of the body, and attacks which affect both sides of the body may be referred to as either or quadriplegic attacks. One of the unique characteristics of AHC is that hemiplegic attacks, as well as other symptoms which may co-occur with hemiplegia, cease immediately upon sleep. During strong attacks, the symptoms may reoccur upon waking. Hemiplegic attacks can occur suddenly or gradually, and the severity of an attack can vary over its duration. The attacks may alternate from one side of the body to another, though this is rare. The length of attacks may also vary from minutes to weeks, though length of attacks varies more greatly between people than between attacks for one person. Both bilateral and hemiplegic attacks are associated with pseudobulbar features such as dysphagia, dysarthria, and respiratory difficulty. Paralysis is also often accompanied by changes in skin color and temperature, sweating, restlessness, tremor, screaming, and the appearance of pain. Hemiplegic attacks happen irregularly and can occur with speech, eating, and swallowing impairment. Patients with AHC are frequently underweight due to these side effects. The average age of onset for hemiplegic episodes has been found to be 6–7 months of age. This early onset gives the name of this disorder the slightly misleading ending 'of childhood'. AHC is not exclusively limited to childhood – attacks become milder after the first ten years of life, but they never completely disappear.

Separation anxiety disorder

- excessive stress when separated from home or family

- fear of being alone

- refusal to sleep alone

- clinginess

- excessive worry about safety

- excessive worry about getting lost

- frequent medical complaints with no cause

- refusal to go to school

Selective mutism

- unable to speak in certain social situations, even though they are comfortable speaking at home or with friends

- difficulty maintaining eye contact

- may have blank facial expressions

- stiff body movements

- may have a worrisome personality

- may be incredibly sensitive to sound

- difficulty with verbal and non-verbal expression

- may appear shy, when in reality, they have a fear of people.

Reactive attachment disorder of infancy or early childhood

- withdrawing from others

- aggressive attitude towards peers

- awkwardness or discomfort

- watching others but not engaging in social interaction

Stereotypic movement disorder

- head banging

- nail biting

- hitting or biting oneself

- hand waving or shaking

- rocking back and forth

Males with 48, XXXY can have average or tall stature, which becomes more prominent in adulthood. Facial dysmorphism is common in males with 48, XXXY and can include increased distance between the eyes (hypertelorism), skin folds of the upper eyelid (epicanthal folds), up-slanting opening between the eyelids (palpebral fissures) and hooded eyelids. Other physical features include the fifth finger or "pinky" to be bent inwards towards the fourth finger (clinodactyly), short nail beds, flat feet, double jointedness (hyperextensibility) and prominent elbows with cubitus varus where the arm rests closer to the body. Musculoskeletal features may include congentical elbow dislocation and the limited ability of the feet to roll inwards while walking and upon landing. Micropenis is another common symptom of this syndrome.

Individuals affected with XXXY are also prone to developing Taurodontism, which often presents early in life, and can be an early indicator of XXY syndrome. Those with this syndrome are also prone to hip dysplasia, and other joint abnormalities. An individual’s symptoms vary due to differing androgen deficiencies, and also with alter with age. Prepubescent boys with XXXY syndrome may not differ in physical appearance from a child without the syndrome. This is likely because androgen levels do not differ among pre-pubescent boys, but a difference does arise as puberty progresses. Those with XXXY syndrome may also experience feminine distribution of adipose tissue, and gynecomastia may also be present. Tall stature is more likely to appear in adolescence, when androgen levels begin to differ between those with XXXY syndrome and those that do not have it.

CDD is a rare condition, with only 1.7 cases per 100,000.

A child affected with childhood disintegrative disorder shows normal development and he/she acquires "normal development of age-appropriate verbal and nonverbal communication, social relationships, motor, play and self-care skills" comparable to other children of the same age. However, between the ages of 2 and 10, skills acquired are lost almost completely in at least two of the following six functional areas:

- Expressive language skills (being able to produce speech and communicate a message)

- Receptive language skills (comprehension of language - listening and understanding what is communicated)

- Social skills and self care skills

- Control over bowel and bladder

- Play skills

- Motor skills

Lack of normal function or impairment also occurs in at least two of the following three areas:

- Social interaction

- Communication

- Repetitive behavior and interest patterns

In her book, "Thinking in Pictures", Temple Grandin argues that compared to "Kanner's classic autism" and to Asperger syndrome, CDD is characterized with more severe sensory processing disorder but less severe cognitive problems. She also argues that compared to most autistic people, persons with CDD have more severe speech pathology and they usually do not respond well to stimulants.

Children with a tic disorder may exhibit the following symptoms:

- overwhelming urge to make movement

- jerking of arms

- clenching of fists

- excessive eye blinking

- shrugging of shoulders

- kicking

- raising eyebrows

- flaring of nostrils

- production of repetitive noises such as grunting, clicking, moaning, snorting, squealing, or throat clearing

Some syndromes that frequently include ACC are Aicardi syndrome, Andermann syndrome, Shapiro syndrome, acrocallosal syndrome, septo-optic dysplasia (optic nerve hypoplasia), Mowat–Wilson syndrome, John Sayden syndrome, Menkes syndrome, and L1CAM Syndrome. Some conditions that are sometimes associated with ACC include maternal nutritional deficiencies or infections, metabolic disorders, fetal alcohol syndrome, craniofacial abnormalities, and other oral and maxillofacial pathologies.

People with Down syndrome may have some or all of these physical characteristics: a small chin, slanted eyes, poor muscle tone, a flat nasal bridge, a single crease of the palm, and a protruding tongue due to a small mouth and relatively large tongue. These airway changes lead to obstructive sleep apnea in around half of those with Down syndrome. Other common features include: a flat and wide face, a short neck, excessive joint flexibility, extra space between big toe and second toe, abnormal patterns on the fingertips and short fingers. Instability of the atlantoaxial joint occurs in about 20% and may lead to spinal cord injury in 1–2%. Hip dislocations may occur without trauma in up to a third of people with Down syndrome.

Growth in height is slower, resulting in adults who tend to have short stature—the average height for men is 154 cm (5 ft 1 in) and for women is 142 cm (4 ft 8 in). Individuals with Down syndrome are at increased risk for obesity as they age. Growth charts have been developed specifically for children with Down syndrome.

The childhood disintegrative disorder (CDD), also known as Heller's syndrome and disintegrative psychosis, is a rare condition characterized by late onset of developmental delays—or stunning reversals—in language, social function, and motor skills. Researchers have not been successful in finding a cause for the disorder. CDD has some similarity to autism, and is sometimes considered a low-functioning form of it. In May 2013, the term CDD, along with other types of autism, was fused into a single diagnostic term called "autism spectrum disorder" under the new DSM-5 manual. Therefore, CDD is now also called "regressive autism", being that this term can now refer to any type of autism spectrum disorder that involves regression, including CDD.

CDD was originally described by Austrian educator Theodor Heller (1869–1938) in 1908, 35 years before Leo Kanner and Hans Asperger described autism. Heller had previously used the name "dementia infantilis" for the syndrome.

An apparent period of fairly normal development is often noted before a regression in skills or a series of regressions in skills. The age at which this regression can occur varies, but typically after 3 years of normal development. The regression can be so dramatic that the child may be aware of it, and may in its beginning even ask, vocally, what is happening to them. Some children describe or appear to be reacting to hallucinations, but the most obvious symptom is that skills apparently attained are lost.

Many children are already somewhat delayed when the disorder becomes apparent, but these delays are not always obvious in young children. This has been described by many writers as a devastating condition, affecting both the family and the individual's future. As is the case with all pervasive developmental disorder categories, there is considerable controversy about the right treatment for CDD.

Those with Down syndrome nearly always have physical and intellectual disabilities. As adults, their mental abilities are typically similar to those of an 8- or 9-year-old. They also typically have poor immune function and generally reach developmental milestones at a later age. They have an increased risk of a number of other health problems, including congenital heart defect, epilepsy, leukemia, thyroid diseases, and mental disorders.

Signs and symptoms of ACC and other callosal disorders vary greatly among individuals. However, some characteristics common in individuals with callosal disorders include vision impairments, low muscle tone (hypotonia), poor motor coordination, delays in motor milestones such as sitting and walking, low perception of pain, delayed toilet training, and chewing and swallowing difficulties. Laboratory research has demonstrated that individuals with ACC have difficulty transferring more complex information from one hemisphere to the other. They also have been shown to have some cognitive disabilities (difficulty in complex problem solving) and social difficulties (missing subtle social cues), even when their intelligence quotient is normal. Recent research suggests that specific social difficulties may be a result of impaired face processing. The unusual social behavior in childhood is often mistaken for or misdiagnosed as Asperger syndrome or other autism spectrum disorders. Other characteristics sometimes associated with callosal disorders include seizures, spasticity, early feeding difficulties and/or gastric reflux, hearing impairments, abnormal head and facial features, and a mental handicap.

MPS VII, Sly syndrome, one of the least common forms of the mucopolysaccharidoses, is estimated to occur in fewer than one in 250,000 births. The disorder is caused by deficiency of the enzyme beta-glucuronidase. In its rarest form, Sly syndrome causes children to be born with hydrops fetalis, in which extreme amounts of fluid are retained in the body. Survival is usually a few months or less. Most children with Sly syndrome are less severely affected. Neurological symptoms may include mild to moderate intellectual disability by age 3, communicating hydrocephalus, nerve entrapment, corneal clouding, and some loss of peripheral and night vision. Other symptoms include short stature, some skeletal irregularities, joint stiffness and restricted movement, and umbilical and/or inguinal hernias. Some patients may have repeated bouts of pneumonia during their first years of life. Most children with Sly syndrome live into the teenage or young adult years.

As of 2001, only one case of MPS IX () had been reported. The disorder results from hyaluronidase deficiency. Symptoms included nodular soft-tissue masses located around joints, with episodes of painful swelling of the masses and pain that ended spontaneously within 3 days. Pelvic radiography showed multiple soft-tissue masses and some bone erosion. Other traits included mild facial changes, acquired short stature as seen in other MPS disorders, and normal joint movement and intelligence.

Developmental disorders comprise a group of psychiatric conditions originating in childhood that involve serious impairment in different areas. There are several ways of using this term. The most narrow concept is used in the category "Specific Disorders of Psychological Development" in the ICD-10. These disorders comprise language disorders, learning disorders, motor disorders and autism spectrum disorders. In broader definitions ADHD is included, and the term used is neurodevelopmental disorders. Yet others include antisocial behavior and schizophrenia that begins in childhood and continues through life. However, these two latter conditions are not as stable as the other developmental disorders, and there is not the same evidence of a shared genetic liability.

Developmental disorders are present from early life. They usually improve as the child grows older, but they also entail impairments that continue through adult life. There is a strong genetic component, and more males are afflicted than females.