Down syndrome

Down syndrome is usually diagnosed at birth on the basis of the typical facial features, hypotonia, and single palmar crease. Down syndrome is the most common chromosomal disorder. An estimated 1 in 800 infants are born with Down syndrome. Down syndrome is also a common cause of miscarriage. Down syndrome is a genetic disorder that results in varying degrees of physical and mental retardation. The condition varies in severity, causing developmental problems that range from mild to severe. The disorder occurs as a result of extra genetic material.

Screening and diagnosis 

There's currently no way to screen for Down syndrome before pregnancy.

Screening tests during pregnancy
Doctors generally perform screening tests that can help identify if your fetus has an increased possibility of Down syndrome around the 16th week of pregnancy.

Blood tests such as the triple screen or the maternal serum alpha-fetoprotein (MSAFP) test identify concentrations of certain biochemical markers, such as human chorionic gonadotropin (HCG), alpha-fetoprotein (AFP) and estriol, in the mother's bloodstream. When a fetus has Down syndrome, these values can be altered from those that occur in a normal pregnancy. However, a positive result on a screening test doesn't mean that your baby has Down syndrome. It simply identifies the possibility, which may warrant further investigation with more-invasive tests.

A normal blood-screening test done during pregnancy doesn't guarantee that the fetus doesn't have Down syndrome, only that the likelihood is reduced. In fact, only 60 percent of babies with Down syndrome are identified by an abnormal blood-screening test.

Diagnostic tests during pregnancy
If a pregnancy-screening test is positive and suggests an increased likelihood of Down syndrome, you might consider further testing. These tests may confirm a suspicion of Down syndrome in the fetus.

Diagnostic tests for identification of 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 A sample of the amniotic fluid surrounding the fetus is withdrawn through a needle inserted into the mother's uterus.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 between 14 and 18 weeks of gestation. The test carries a risk of miscarriage ranging from one in 200 to one in 400, and it may take as long as two weeks for results.

• Chorionic villus sampling (CVS). Cells taken from the mother's placenta can be used to analyze the fetal chromosomes. Typically performed between the 10th and 12th week of pregnancy, this test carries a risk of miscarriage of one in 100. Test results may be available in as little as two days, though it commonly takes a week or longer for final results.

• 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 20 weeks of gestation.

These three tests are all between 98 percent and 99 percent accurate in diagnosing Down syndrome before birth.

Researchers are working on developing less-invasive methods of detecting Down syndrome before birth using blood tests or ultrasound.

Diagnostic tests for newborns
After birth, the initial diagnosis of Down syndrome is often based on the baby's appearance. Your doctor will probably order a test called a karyotype if your child displays some or all of the characteristics of Down syndrome. A karyotype 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.