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Rett syndrome is a rare genetic disease that causes developmental and nervous system problems, mostly in girls. It's related to autism spectrum disorder. Babies with Rett syndrome seem to grow and develop normally at first. Between 3 months and 3 years of age, though, they stop developing and even lose some skills. Symptoms include:

• Loss of speech
• Loss of hand movements such as grasping
• Compulsive movements such as hand wringing
• Balance problems
• Breathing problems
• Behavior problems
• Learning problems or intellectual disability

Rett syndrome has no cure. You can treat some of the symptoms with medicines, surgery, and physical and speech therapy. Most people with Rett syndrome live into middle age and beyond. They will usually need care throughout their lives.

NIH: National Institute of Child Health and Human Development

Developmental disabilities are conditions that are usually present at birth. They can affect a child's growth and development. These conditions can cause physical, learning, language, or behavior issues. They can include:

• Learning disabilities. These conditions affect the ability to learn.
• Autism spectrum disorder (ASD). This is a disorder that affects behavior, communication, and social skills.
• Cerebral palsy (CP). This is a condition that affects movement, coordination, and balance.
• Physical disabilities. These could include blindness or deafness.
• Conditions that can affect both physical and mental abilities. This could include Down syndrome.

These conditions can cause lifelong challenges in learning. Completing daily tasks may be difficult. Early treatment may help your child learn important skills and make the most of their strengths.

Developmental disabilities are often present at birth. They can occur due to various reasons. These can include:

• Genetic or chromosome abnormalities. These cause conditions such as Down syndrome, Fragile-X-Syndrome, and Rett syndrome.
• Use of substances while pregnant. For example, drinking alcohol when pregnant can cause fetal alcohol spectrum disorders.
• Certain infections during pregnancy.
• Preterm birth or low birth weight.

The way a developmental disability is diagnosed depends on the condition. Screening tests or a physical exam may be done to check for a medical problem. Other tests and questionnaires may help find out if your child has a developmental disability. These may include:

• Screening tests that are done during pregnancy or for newborns
• Developmental and behavioral screening tests or questionnaires to check if your child is developing on schedule
• Observing your child's behavior and how they interact with other people
• Testing your child's intellectual and school performance
• Asking about your family history

If your child has a developmental disability, these are usually lifelong conditions. Often, there is no cure, but treatment can help the symptoms. Treatments can include:

• Physical, speech, and occupational therapy
• Special teaching methods
• Psychological counseling
• To keep track of how your child grows and changes over time

NIH: National Institute of Child Health and Human Development

Genetic disorders are health conditions caused by changes (also called mutations or variants) in your genes. Genes are parts of DNA found in your cells that carry instructions for how your body grows, develops, and functions. Many genes tell your body how to make proteins, which are needed for your body to work properly.

A genetic disorder happens when a gene variant changes how a protein is made. The variant may cause the protein to work poorly or not be made at all. If genes don't make the right proteins, or don't make them correctly, it can lead to a genetic disorder.

Not all gene changes cause disease. Often, these changes have no effect. But sometimes, even a small change in DNA can affect how proteins are made.

Gene variants can be grouped into two main types:

• Inherited variants (also called germline variants) are passed down from your parents. They come from egg or sperm cells.
• Non-inherited (also called somatic variants) are not passed down from your parents. They happen during your lifetime. They may be caused by things like harmful chemicals or ultraviolet (UV) rays from the sun.

Genetic disorders may be caused by:

• Single gene disorders are caused by changes in one gene. Examples can include sickle cell anemia, where a change in a single gene can cause the condition, and Charcot-Marie-Tooth disease, where a variant in one of many different genes can cause the condition.
• Chromosomal disorders are caused by missing, extra, or altered chromosomes. Chromosomes are structures that carry genes. These disorders involve changes in the number of chromosomes people have or changes in the structure of one or more chromosomes. An example is Down syndrome, which is caused by an extra copy of chromosome 21.
• Complex (multifactorial) disorders are caused by changes in many genes. Each change alone may not cause the disease, but together they increase the risk. Lifestyle and environmental factors, such as exercise, diet, or exposure to pollutants, also play a role. Colon cancer is an example.

Some genetic conditions are passed down through families (inherited) in one of several patterns, depending on the specific gene involved.

Patterns of inheritance can include:

• Dominant means you only need one changed gene to cause the condition.
• Recessive means you need two changed copies of the gene (one from each parent) for the condition to occur.
• X-linked conditions involve genes located on the X chromosome. These conditions often affect males more frequently.
• Mitochondrial conditions are passed down by the mother.

Your health care provider may check for a genetic condition based on:

• A physical examination.
• Your personal medical history.
• Your family health history.
• Laboratory tests, including genetic testing.

NIH: National Library of Medicine