Down syndrome is often detected before birth. Prenatal testing includes screening tests and diagnostic tests. Screening tests identify a higher likelihood of a given defect but cannot definitively diagnose it. A woman who receives a positive result on a screening test may elect to have a diagnostic test, which can confirm the presence of Down syndrome in more than 98 percent of cases.

Prenatal screening tests include:

• Maternal blood tests. Identify and measure substances in the mother’s blood that reveal information about her fetus. There are a number of these tests available, including the triple screen test and quadruple screen test. The triple screen identifies the levels of alpha-fetoprotein (AFP), human chorionic gonadotropin (HCG) and estriol. The quadruple screen tests levels of those three elements plus the inhibin A protein. They are usually performed in the 16^th to 18^th weeks of pregnancy.
  
  
• Detailed ultrasound. Uses sound waves to create an image of inside a mother's womb. This can be used to identify certain physical features common in Down syndrome, but it is not very accurate without supplementation with other screening tests.
  
  
• Nuchal translucency testing. Ultrasound is used to examine the back of the fetus's neck. Fluid tends to accumulate in this area when Down syndrome or other chromosomal conditions are present. This gives a swollen appearance on the ultrasound. This test may be performed between 11 and 14 weeks of pregnancy. Recent research indicates that nuchal translucency testing combined with a test for HCG and another protein can identify more than 80 percent of Down syndrome cases before 13 weeks of gestation, much earlier than some diagnostic tests.

The American College of Obstetricians and Gynecologists began recommending in 2007 that all pregnant women be offered the choice of having screening tests for Down syndrome, not just higher-risk mothers.

Prenatal diagnostic tests are nearly 99 percent accurate in identifying Down syndrome and certain other chromosomal conditions, according to the American Academy of Family Physicians. However, they are associated with complications, including a risk of miscarriage and preterm labor. These tests are typically recommended only for women with a high risk of having a child with Down syndrome, including those over age 35, those with a family history of genetic problems and those who had abnormal results on screening tests.

Prenatal diagnostic tests include:

• Amniocentesis. A needle is inserted through the abdomen and into the uterus. Then, a small amount of amniotic fluid (the fluid in the uterus) is removed for testing. This test is generally performed between 16 and 20 weeks into the pregnancy.
  
  
• Chorionic villus sampling (CVS). A sample of the placenta is removed for testing. It may be obtained using either a needle through the abdomen (as in amniocentesis) or a tube inserted through the vagina. This test can be performed between eight and 12 weeks of pregnancy.
  
  
• Percutaneous umbilical blood sampling (PUBS). A needle is used to remove a sample of blood from the fetus's umbilical cord. This test carries higher risks of complications than amniocentesis or CVS and is typically used only when diagnosis must be made quickly. It is typically performed later in pregnancy, after 20 weeks.

The results of screening and diagnostic tests may provide pregnant women with valuable information about the fetus. Some women may choose to terminate an affected pregnancy. Others use the information to prepare to care for an infant with special needs.

After an infant has been born, Down syndrome can usually be diagnosed by a physical examination, or even just looking at the infant. The diagnosis is confirmed with a karyotype. This is a chart of the child's chromosomes created using a blood or other tissue sample.