There are two types of tests for Down syndrome that can be performed before your baby is born: screening and diagnostic tests. Prenatal screenings estimate the chance of the fetus having Down syndrome. These tests do not tell you for sure whether your baby has Down syndrome; they only provide a risk assessment. Diagnostic tests, on the other hand, can provide a definitive diagnosis with almost 100 percent accuracy.
There are two types of prenatal screening tests available: maternal serum screening and ultrasound (sonogram) screening. Maternal serum screening tests measure quantities of various substances in the blood of the mother, including alpha-fetoprotein and the hormones 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 between 15 and 20 weeks of gestation, maternal serum screening tests are only able to accurately detect about 60 percent of fetuses with Down syndrome. Many women who undergo these tests will be given false-positive readings, and some will be given false-negative readings.
Because maternal serum screening tests are of limited value, they are often performed in conjunction with a detailed sonogram 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.
Prenatal screening tests are routinely offered to women over the age of 35, due to their increased chances of giving birth to a child with a disability; however, pregnant women of any age can request a test or choose not to have it done. If the estimate determined by prenatal screening is high, doctors will often advise a mother to undergo diagnostic testing.
The diagnostic procedures available for prenatal diagnosis of Down syndrome are chorionic villus sampling (CVS), amniocentesis and percutaneous umbilical blood sampling (PUBS). These procedures, which carry a small risk of miscarriage, are about 98 to 99 percent accurate in the detection of Down syndrome. Amniocentesis is usually performed between 15 and 22 weeks of gestation, CVS between 9 and 14 weeks, and PUBS after 18 weeks.
If you have any questions about these procedures, do not hesitate to ask your doctor. It is important that you receive accurate information and understand all your options. Whether or not to undergo a prenatal screening or diagnostic test is a personal decision, and expectant parents must make the choice that is best for them.
Diagnosing Down syndrome
Even though there are many prenatal tests available for Down syndrome, most cases of Down syndrome are diagnosed after the baby is born. Doctors will usually suspect Down syndrome if certain physical characteristics are present. Some of the traits common to babies with Down syndrome include:
- 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.