Down syndrome is defined as a genetic condition that results in mental retardation and particular physical characteristics. The three main types of Down syndrome are trisomy 21, mosaic Down syndrome, and translocation Down syndrome.

The main cause of developing Down syndrome is the abnormal cell division that involves chromosome 21. Risk factors that may lead to conceiving a child with Down syndrome include advancing maternal age, previous conceiving of a child with Down syndrome, and presence of genetic translocation for Down syndrome.

A process called nondisjunction is responsible for Down syndrome and it occurs when the genetic materials fail to separate during a crucial part of the formation of gametes, resulting in an extra chromosome (called trisomy 21). Symptoms of Down syndrome vary from person to person and ranging from mild to severe.

The physical characteristics that distinguish people with Down syndrome from the general population may include floppiness (hypotonia or low muscle tone) and below-average weight and length at birth. Before the baby is born, there are two categories of tests that can be carried out to diagnose Down syndrome. The first category is screening tests and the second category is diagnostic tests. After birth, the initial diagnosis of Down syndrome is based often on the baby's appearance.

There is no available medical treatment for Down syndrome. However, early developmental interventions and medical assistance beginning during infancy may benefit children diagnosed with Down syndrome.

Certain complications can appear in some people with Down syndrome include heart defects, hearing problems, and digestive problems. Genetic counseling for individuals with a family history of Down syndrome is recommended by experts especially for those who wish to have a baby.

The prognosis of Down syndrome is inconstant and it is largely based on the possible complications such heart defects. Depending on the severity of health problems, an individual with Down syndrome is expected to live more than 60 years. From an epidemiological standpoint, the global estimation for the incidence of Down syndrome is between 1 in 1,000 to 1 in 1,100 live births, according to World Health Organization. 

Down syndrome is a genetic condition that causes mental retardation, some level of learning disability, and certain physical characteristics.The person develop Down syndrome when he/she has 47 chromosomes instead of the usual 46. Down syndrome is named after the doctor who first identified the syndrome “John Langdon Down”.

There are three types of Down syndrome that can be studied separately:

• Trisomy 21: This type is responsible for the vast majority of Down syndrome cases. In this type, the person has three copies of chromosome 21 instead of the usual two copies in all cells. This type occurs because of an abnormal cell division during the development of the sperm cell or the egg cell. The extra chromosome is replicated in every cell of the body as the embryo develops.
• Mosaic Down syndrome: This is an uncommon type of Down syndrome. In this type, a person has only some cells with an extra copy of chromosome 21.Mainly, abnormal cell division after fertilization is responsible for this mosaic of normal and abnormal cells. Clinical observations showed that individuals with mosaic Down syndrome suffer from fewer traits of Down syndrome compared to those with other types of Down syndrome.
• Translocation Down syndrome: This type occurs before or at conception, when a portion of chromosome 21 becomes attached (translocated) onto another chromosome or when additional full or partial copy of chromosome 21 attaches to another chromosome, usually chromosome 14.

Abnormal cell division involving chromosome 21 is the main cause behind Down syndrome. The result of these cell division abnormalities is an extra partial or full chromosome 21. The characteristics associated with Down syndrome are caused by this additional genetic material that alters the course of development.

The cause of the extra full or partial chromosome is still unknown. The mother's age is the main thing that increases the chance of having a baby with Down's syndrome. For a 35-year-old woman, the chance of conceiving a child with Down syndrome is 1 in 350, and by age 40, the chance increases gradually to 1 in 100. The incidence becomes approximately one in thirty at age 45. The majority of Down syndrome cases are not inherited; it can be attributed to a one-off genetic change in the sperm or egg.

Certain factors can play a role in increasing the risk of conceiving a child with Down syndrome including:

• Maternal age: Since it is widely known that older eggs have a greater risk of abnormal chromosome division, a woman's chances of giving birth to a child with Down syndrome increase with age. Having a child with Down syndrome increases after the woman age exceeds 35 years.
• Previous conceiving of a child with Down syndrome: The risk of conceiving a child with Down syndrome increases in case of couples who previously conceived one child with Down syndrome and in case of couples who have a translocation themselves.
• Presence of genetic translocation for Down syndrome: The genetic translocation for Down syndrome can be passed on to children.

The genetic material is stored in genes inside a nucleus that exist in every cell in the human body. All of our inherited traits are carried by the genes that grouped along thread-like structures called chromosomes. In the normal circumstances, the cells contain 46 chromosomes – 23 from the mother and 23 from the father. An individual with Down syndrome has 47 chromosomes instead of the usual 46.

A process called nondisjunction is responsible for Down syndrome and it occurs when the genetic materials fail to separate during a crucial part of the formation of gametes, resulting in an extra chromosome (called trisomy 21). The exact cause behind nondisjunction is not fully recognized, although investigators suggesting a correlation with advancing maternal age. The presence of the additional genetic material influences development and it lead to the state known as Down syndrome.

Symptoms of Down syndrome vary from person to person and ranging from mild to severe. People with Down syndrome suffer from slower mental and physical developments. The physical characteristics that distinguish people with Down syndrome from the general population may include:

• Floppiness (hypotonia or low muscle tone), loose ligaments, excessive flexibility,
• Small mouth with a tongue that may stick out,
• Their palm may have only one crease across it,
• Below-average weight and length at birth,
• Eyes that slant upwards and outwards,
• White spots in the iris of the eye,
• Small nose and flat nasal bridge,
• Broad hands with short fingers,
• Abnormally shaped ears,
• Small hands and feet,
• A flat back of the head.

Children with Down's syndrome may also have delayed mental and social development resulting in various problems such as:

• Short attention span,
• Impulsive behavior,
• Poor judgment,
• Slow learning.

Before the baby is born, there are two categories of tests that can be carried out to diagnose Down syndrome. The first category is screening tests and the second category is diagnostic tests.Screening tests are performed to determine the likelihood or chances that a mother is carrying a baby with Down syndrome. However, these tests cannot indicate with full confidence whether the baby has Down syndrome or not. Screening tests include:

-  The first trimester combined test: This test involves two steps including:

• Blood test: This test performed to test any abnormal levels for pregnancy-associated plasma protein-A (PAPP-A) and the pregnancy hormone known as human chorionic gonadotropin (HCG). Abnormal changes in levels of PAPP-A and HCG may indicate a problem with the child.
• Nuchal translucency test: This test uses an ultrasound to measure a specific area on the back of the child's neck. Detecting abnormalities may indicate an excessive fluid that tends to collect in the neck tissue.

- The integrated screening test:  This test performed in two parts during the first and second trimesters of pregnancy. The collected results are combined together to estimate the risk that the baby has the syndrome.

• First trimester: Blood test will be carried out to measure PAPP-A, and an ultrasound will be performed to measure nuchal translucency.
• Second trimester: The quad screen will be performed to measure the blood level of four pregnancy-associated substances: inhibin A, estriol, alpha-fetoprotein, and HCG.

If the screening test indicates positive or disturbing results, then, the mother has to consider herself at a high risk of having a baby with Down syndrome. More testing to confirm the diagnosis can be performed including:

• Chorionic villus sampling (CVS): This test performed to analyze the fetal chromosomes in the first trimester, typically between 10 and 13 weeks of pregnancy.This test carried out by obtaining cells from the placenta. The risk of miscarriage from a CVS is very low.
• Amniocentesis: This test performed to analyze the chromosomes of the fetus usually in the second trimester. This test carried out by obtaining a sample of the amniotic fluid surrounding the fetus through a needle inserted into the mother's uterus.
• Percutaneous umbilical blood sampling (PUBS): This test performed after week 18. This test carried out by obtaining a sample of the baby's blood from the umbilical cord for testing.

After birth, the initial diagnosis of Down syndrome is often based on the baby's appearance. Down syndrome is usually identified at birth by observing certain physical traits such as a slightly flattened facial profile, low muscle tone, an upward slant to the eye, and a single deep crease across the palm of the hand.  However, health care provider is more likely to suggest a test called a chromosomal karyotype to confirm diagnosis due to the fact that some of the traits associated with Down syndrome can be found in babies without Down syndrome.

Karyotype is carried out by obtaining a blood sample to examine the baby's cells. Experts can photograph the chromosomes and classify them into different groups based on shape, size, and number. Similar principles can be applied to another genetic test called FISH to confirm the diagnosis in a shorter amount of time.

The medical treatment for Down syndrome is not available.However, early developmental interventions and medical assistance beginning during infancy may benefit children diagnosed with Down syndrome. Major surgeries may be performed immediately after birth in case of children born with a gastrointestinal blockage or those with certain heart defects. Babies with Down syndrome should be well supported and fully awake during breastfeeding. Many babies with Down syndrome can successfully breastfeed.However, some leakage due to poor tongue control may present in many babies.

Overweight and obesity may be regarded as a significant problem among older children and adults diagnosed with Down syndrome. Avoiding high-calorie foods and practicing regular physical activities are important to prevent obesity. However, before practicing sports activities, careful examination for the child's neck and hips should be performed. Dealing with frustration, anger, and compulsive behavior that associated with Down syndrome can be accomplished by proper behavioral training that can help individuals with Down syndrome and their families.

Proper medical care can be provided by a team of specialists that can help individuals with Down syndrome develop skills as fully as possible. Such team of specialists may include primary care pediatrician to coordinate and provide routine child care, pediatric cardiologist, audiologist, and physical/occupational therapists.

Individuals with Down syndrome are vulnerable to develop certain complications include:

• Heart defects: the septal defect is known widely to be the most common defect that affects children with Down syndrome. This defect takes the form of a hole inside one of the walls that separate the four chambers of the heart.
• Immune problems: People with Down syndrome tend to have abnormalities in their immune systems which increase the risk of developing autoimmune disorders as well as some forms of cancer.
• Hearing problems: A common cause of temporary hearing problems in children with Down's syndrome is the glue ear (build-up of fluid in the middle ear).
• Vision problems : Common eye problems include: Squint or strabismus (eyes do not look in the same direction),  eye infections ( such as conjunctivitis), cataracts (lens of an eye becomes cloudy and affects vision) , nystagmus (eyes move uncontrollably ), ,keratoconus ( eye’s cornea becomes thin and bulges out), short-sightedness ( distant objects appear blurred),  long-sightedness( nearby objects appear blurred), and glaucoma ( increased pressure in the eye).
• Digestive problems: The most common digestive problems are constipation, diarrhea, and indigestion. Other serious problems may include small bowel obstruction (blocking the passage of food from the stomach into the large bowel), coeliac disease (adverse reaction to gluten presents in grass-related grains), imperforate anus (absence of anal opening at birth),Hirschsprung's disease (affects the large intestine and can cause problems with passing stool , and reflux (bringing up milk during or shortly after feeding).
• Thyroid problems: Hypothyroidism (underactive thyroid gland) is common in most people with Down's syndrome who have a problem with their thyroid gland. Symptoms of hypothyroidism can include weight gain and slow physical and mental reactions.
• Sleep apnea: People with Down syndrome are at greater risk of obstructive sleep apnea because of soft tissue and skeletal changes that lead to the obstruction of their airways.
• Spinal problems: Atlantoaxial instability (misalignment of the top two vertebrae in the neck) is a major concern in many Down syndrome cases.
• Infections: It is more likely for people with Down's syndrome to develop infections such as pneumonia and other lung infections.
• Dementia: Dementia tends to develop in people with Down's syndrome at a younger age, usually from about the age of 40 onwards. In addition, the risk of developing Alzheimer's disease is high in people with Down syndrome.
• Leukemia: The risk of developing leukemia increases among young children with Down syndrome.
• Obesity: People with Down syndrome are at high risk of being obese and overweight.
• Other problems: Other health conditions may associate with Down syndrome including endocrine problems, dental problems, seizures, autism spectrum disorder (ASD), and attention deficit hyperactivity disorder (ADHD).

Genetic counseling for individuals with a family history of Down syndrome is recommended by experts especially for those who wish to have a baby. As the woman gets older, the risk of conceiving a child with Down syndrome increases gradually.

Women age 35 and older are at higher risk of conceiving a child with Down syndrome. Having another baby with the condition is probable in case of those couples who already have a baby with Down syndrome. Diagnostic tests to check for Down syndrome such as ultrasound, nuchal translucency, amniocentesis, or chorionic villus sampling can be performed on a fetus during the first few months of pregnancy.

The outlook of Down syndrome is inconstant and it is largely based on the possible complications such heart defects. Many children with Down syndrome are able to live independent and productive lives although the presence of physical and mental limitations. Heart problems such septal defect and endocardial cushion defects can be present in about half of children with Down syndrome at birth.

Early death can result from critical heart problems and from certain types of leukemia. Variation in the intellectual disability have been observed among Down syndrome cases but it is usually moderate. Moreover, a dramatic increase in lifespans for people with Down syndrome has been noticed through the years. Depending on the severity of health problems, an individual with Down syndrome is expected to live more than 60 years.

Many adults with Down syndrome are living healthy lives and can participate more actively in society due to early intervention and therapy.

• According to WHO, the global estimation for the incidence of Down syndrome is between 1 in 1,000 to 1 in 1,100 live births. 60 to 80 % of children with Down syndrome have hearing deficits. 40 to 45 % of children with Down syndrome have congenital heart disease.
• According to Centers for Disease Control and Prevention in the USA, about 6,000 babies are born with Down syndrome, which is about 1 in every 700 babies born. In addition, it is estimated that in 2008 about 250,700 children, teens, and adults were living with Down syndrome in the United States.
• According to Down Syndrome Australia organization, The number of Australian babies born per year with Down syndrome has been over 270 since 2007 and the population of people with Down syndrome in Australia is now over 13,000.
• According to the Canadian official reports, the birth prevalence of Down syndrome in Canada (excluding Quebec province) has been stable in the last decade, averaging 15.8 per 10,000 total births, between 2005 and 2013.