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Down syndrome

Human cells normally contain 23 pairs of chromosomes. One chromosome in each pair comes from your father, the other from your mother.

The cause of Down syndrome is one of three types of abnormal cell division involving the 21st chromosome. All three abnormalities result in extra genetic material from chromosome 21, which is responsible for the characteristic features and developmental problems of Down syndrome. The three genetic variations that can cause Down syndrome include:

Trisomy 21. More than 90 percent of cases of Down syndrome are caused by trisomy 21. A child with trisomy 21 has three copies of chromosome 21 — instead of the usual two copies — in all of his or her cells. This form of Down syndrome is caused by abnormal cell division during the development of the sperm cell or the egg cell.
Mosaic Down syndrome. In this rare form of Down syndrome, children have some cells with an extra copy of chromosome 21, but not all. This mosaic of normal and abnormal cells is caused by abnormal cell division after fertilization.
Translocation Down syndrome. Down syndrome can also occur when part of chromosome 21 becomes attached (translocated) onto another chromosome, before or at conception. Children with translocation Down syndrome have the usual two copies of chromosome 21, but they also have additional material from chromosome 21 stuck to the translocated chromosome. This form of Down syndrome is uncommon.
There are no known behavioral or environmental factors that cause Down syndrome.

Is it inherited?
Most cases of Down syndrome aren't inherited. They're caused by a mistake in cell division during the development of the egg, sperm or embryo.

Translocation Down syndrome is the only form of the disorder that can be passed from parent to child. However, only about 4 percent of children with Down syndrome have translocation. And only about half of these cases are inherited from one of the parents.

In these cases, the mother or father is a balanced carrier of the translocation, which means he or she has some rearranged genetic material, but no extra genetic material. A balanced carrier has no signs or symptoms of Down syndrome, but he or she can pass the translocation on to children.

The chance of passing on the translocation depends on the sex of the parent who carries the rearranged chromosome 21:

If the father is the carrier, the risk is about 3 percent.
If the mother is the carrier, the risk is about 12 percent.
Some parents have a greater risk of having a baby with Down syndrome. Risk factors include:

Advancing maternal age. As a woman's eggs age, there's a greater inclination for chromosomes to divide improperly. So a woman's chances of giving birth to a child with Down syndrome increase with age. By age 35, a woman's risk of conceiving a child with Down syndrome is 1 in 385. By age 40, the risk is 1 in 106. And by age 45, the risk is 1 in 30. However, most children with Down syndrome are actually born to women under age 35 because this younger group of women has far more babies.
Mothers who already have one child with Down syndrome. Typically, a woman who has one child with Down syndrome has about a 1 percent chance of having another child with Down syndrome.
Parents who are carriers of the genetic translocation for Down syndrome. Both men and women can pass the genetic translocation for Down syndrome on to their children.
Screening for Down syndrome is offered as a routine part of prenatal care. A mother's age has traditionally been a factor in the decision to screen for Down syndrome. But now, the American College of Obstetricians and Gynecologists recommends offering various screening tests for Down syndrome to all pregnant women, regardless of age. Your health care provider can help you weigh the pros and cons of these tests for your individual situation.

Screening tests during pregnancy
Various screening tests can help identify whether you have a high risk of carrying a baby with Down syndrome. Blood tests, such as the quad screen, have typically been offered around the 16th week of pregnancy to screen for Down syndrome, spina bifida and various other chromosomal disorders. These tests are still available. But the most effective screening is done in two steps between the 11th and 14th week of pregnancy:

Ultrasound. The doctor uses ultrasound to measure a specific region on the back of a baby's neck. This is known as a nuchal translucency screening test. When abnormalities are present, more fluid than usual tends to collect in this tissue.
Blood tests. Results of the ultrasound are paired with blood tests that measure levels of pregnancy-associated plasma protein-A (PAPP-A) and a hormone known as human chorionic gonadotropin (HCG). Abnormal levels of PAPP-A and HCG may indicate a problem with the baby.
When this two-step screening is done during the 11th week of pregnancy, researchers say it can identify 87 percent of babies with Down syndrome. By 13 weeks, accuracy of the two-step screening approach drops to 82 percent.

If a screening test indicates a high risk of Down syndrome, a more invasive test — such as chorionic villus sampling or amniocentesis — may be used to determine whether the baby actually has Down syndrome.

About one in 20 women will have a positive result with any of these screening tests — far more than those who eventually deliver a baby with a chromosomal abnormality. In fact, most women who have a positive result from a screening test deliver healthy babies.

Diagnostic tests during pregnancy
If your screening tests are positive or worrisome or you're at high risk of having a baby with Down syndrome, you might consider further testing to confirm the diagnosis. Diagnostic tests that can identify Down syndrome include:

Amniocentesis. A sample of the amniotic fluid surrounding the fetus is withdrawn through a needle inserted into the mother's uterus. This sample is then used to analyze the chromosomes of the fetus. Doctors usually perform this test after 15 weeks of gestation. The test carries a risk of miscarriage of one in 200.
Chorionic villus sampling (CVS). Cells taken from the mother's placenta can be used to analyze the fetal chromosomes. Typically performed between the ninth and 14th week of pregnancy, this test carries a risk of miscarriage of one in 100.
Percutaneous umbilical blood sampling (PUBS). Blood is taken from a vein in the umbilical cord and examined for chromosomal defects. Doctors generally perform this test after 18 weeks of gestation. This test carries a greater risk of miscarriage than does amniocentesis or chorionic villus sampling. Generally, this test is only done when speed of diagnosis is essential.
Each of these three tests is 98 percent to 99 percent accurate in diagnosing Down syndrome before birth.

Diagnostic tests for newborns
After birth, the initial diagnosis of Down syndrome is often based on the baby's appearance. If your child displays some or all of the characteristics of Down syndrome, your doctor will probably order a test called a chromosomal karyotype. This test is an analysis of your child's chromosomes. If there's an extra chromosome 21 present in all or some of the cells, the diagnosis is Down syndrome.

Complications
Children with Down syndrome can have a range of complications. These include:

Heart defects. Approximately half of children with Down syndrome are born with some type of heart defect. These heart problems can be life-threatening and may require surgery in early infancy.
Leukemia. Young children with Down syndrome are more likely to develop leukemia than are children who don't have Down syndrome.
Infectious diseases. Because of abnormalities in their immune systems, those with Down syndrome are much more susceptible to infectious diseases. For example, their risk of contracting pneumonia is much higher than that of others without this disorder.
Dementia. Later in life, people with Down syndrome have a greatly increased risk of dementia. Signs and symptoms of dementia often appear before age 40 in people with Down syndrome.
Other problems. Down syndrome may also be associated with a variety of other health conditions, including gastrointestinal blockage, thyroid problems, hearing loss and poor vision.
Life spans have increased dramatically for people with Down syndrome. In 1929, a baby born with Down syndrome often didn't live to age 10. Today, someone with Down syndrome can expect to live to 50 and beyond, depending on the severity of his or her health problems. This age continues to rise because of early interventions and better care.

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