Understanding the Diagnosis

Prenatal Testing & Diagnosis

There are two types of tests for Down syndrome that can be performed before your baby is born: screening tests and diagnostic tests.

Prenatal Screenings

Prenatal screenings estimate the chance of the fetus having Down syndrome. These tests do not tell you for sure whether your fetus has Down syndrome. They only provide a probability.

There is an extensive menu of prenatal screening tests now available for pregnant women. Most of these screening tests involve a blood test and an ultrasound. The blood tests (or serum screening tests) measure quantities of various substances in the blood of the mother, including alpha-fetoprotein, inhibin A, plasma protein A, estriol, and human chorionic gonadotropin. Together with a woman's age, these are used to estimate her chance of having a child with Down syndrome. Typically offered in the first and second trimesters, maternal serum screening tests are only able to predict about 80% of fetuses with Down syndrome. It is important to note that none of these prenatal screens will be able to definitively diagnose Down syndrome. Instead, mothers should expect results such as, "You have a 1 in 240 chance of having a child with Down syndrome" or "You have a 1 in 872 chance of having a child with Down syndrome."

These blood tests are often performed in conjunction with a detailed ultrasound to check for "markers" (characteristics that some researchers feel may have a significant association with Down syndrome). Recently, researchers have developed a maternal serum/ultrasound/age combination that can yield a much higher accuracy rate at an earlier stage in the pregnancy. Even with the ultrasound, however, the screen will not definitively diagnose Down syndrome.

There are several companies that offer screening tests, which can only be ordered through a physician. These involve blood being taken from the expectant mother at 10 weeks of gestation. It relies on the detection of cell-free DNA that circulates between the fetus and the expectant mother. According to the latest research, this blood test can detect up to 98.6% of fetuses with trisomy 21. A "positive" result on the test means that there is a 98.6% chance that the fetus has trisomy 21; a "negative" result on the test means that there is a 99.8% chance that the fetus does not have trisomy 21.  All mothers who receive a "positive" result on this test can confirm the diagnosis with one of the diagnostic procedures described below.

Prenatal screening tests are now routinely offered to women of all ages. If the chance of having a child with Down syndrome appears high during prenatal screening, doctors will often advise a mother to undergo diagnostic testing.

Diagnostic Procedures

The diagnostic procedures available for prenatal diagnosis of Down syndrome are chorionic villus sampling (CVS) and amniocentesis. These procedures, which carry up to a 1% risk of causing a spontaneous termination (miscarriage), are nearly 100% accurate in diagnosing Down syndrome. Amniocentesis is usually performed in the second trimester between 15 and 22 weeks of gestation, and CVS in the first trimester between 9 and 14 weeks. Amniocentesis and CVS are also able to distinguish between the genetic types of Down syndrome: trisomy 21, translocation Down syndrome, and mosaic Down syndrome.

Diagnosing Down Syndrome

Since many expectant parents choose to forgo prenatal tests, most cases of Down syndrome are diagnosed after the baby is born. Doctors will usually suspect Down syndrome if certain physical traits are present.

Some of the Traits Common to a Baby With Down Syndrome Are:

  • Low muscle tone
  • A flat facial profile
  • A small nose
  • An upward slant to the eyes
  • A single deep crease across the center of the palm
  • An excessive ability to extend the joints
  • Small skin folds on the inner corner of the eyes
  • Excessive space between large and second toe

Not all babies with Down syndrome have all these characteristics, and many of these features can be found, to some extent, in individuals who do not have the condition. Therefore, doctors must perform a special test called a karyotype before making a definitive diagnosis.  To obtain a karyotype, doctors draw a blood sample to examine your baby's cells. They use special tools to photograph the chromosomes and then group them by size, number and shape. By examining the karyotype, they can determine accurately whether or not your baby has Down syndrome.


The information featured in this section is reproduced via an exclusive arrangement with National Down Syndrome Society [ONLINE] Available at http://www.ndss.org