While Down syndrome is often diagnosed shortly after birth, it is also increasingly being diagnosed during pregnancy (prenatally) due to abnormal ultrasound (sonogram) findings, an abnormal result on a blood test done mom (maternal serum screening test), or by amniocentesis or chorionic villi sampling (CVS) testing. Chorionic villi sampling or CVS is a test done on the placenta between 10 and 12 weeks of pregnancy
Screening versus Diagnostic Testing
Both ultrasound and maternal serum tests are screening tests. In contrast, amniocentesis and CVS are considered diagnostic tests. A screening test can never give you a firm diagnosis - a screening tests simply tells you if you have a higher risk to have a baby with Down syndrome. A diagnostic test on other hand, gives you a firm diagnosis.
An ultrasound, also know as a sonogram, is a test done during pregnancy which uses sound waves to generate a picture or image of the fetus. Occasionally, but not always, infants with Down syndrome show subtle signs on an ultrasound that can make your doctor suspect that the fetus has Down syndrome.
Some of these subtle signs include a decrease in femur length (a bone in the leg), an increase in the skin behind the neck (called nuchal translucency), or choroid plexus cysts (cysts in a section of the brain that produces spinal fluid). These are what are often referred to as “soft signs” because none of these ultrasound findings will cause the fetus any problems by themselves. However they may make the doctor suspicious that the fetus has Down syndrome. Other more serious signs that can also be seen on an ultrasound can include heart defects and intestinal blockages.
Down syndrome cannot be diagnosed based on ultrasound findings alone. These findings simply suggest that there is a higher risk that the fetus may have Down syndrome. Based on this increased risk, your doctor will recommend that your consider having an amniocentesis or CVS to confirm the diagnosis. It is up to you to decide whether or not to have this testing.
It is important to note that most fetuses with Down syndrome show no abnormalities on ultrasound. Often people are falsely reassured by a normal ultrasound because they believe it means that there are no problems with the baby. Unfortunately this is simply not true. Having a normal ultrasound is great and is somewhat reassuring, but it is not a guarantee that everything is perfect.
Maternal serum screening tests
A blood test (done on mom) often called the "triple screen" can be done between the 15th and the 20th weeks of pregnancy. The triple screen looks at three substances in the mother’s blood stream - alpha-fetoprotein (AFP), unconjugated estriol and human chorionic gonadotropin (hCG). AFP is produced in the yolk sac and fetal liver, and estriol and hCG are produced by the placenta. When a fetus has Down syndrome, the levels of these substances in the mother’s bloodstream can be different from normal. AFP and unconjugated estriol are about 25 percent lower than normal levels, and maternal serum hCG is approximately two times higher than the normal hCG level.
The triple screen is - as it’s name implies - just a screening test. It cannot tell for sure if your baby has Down syndrome, but it can tell if your risk is higher. If your screening test is positive, it simply means your risk of having a baby with Down syndrome is higher than other women your age. But even with a positive screening test, most women will have babies without Down syndrome. Conversely, a negative triple screen means that the chance of Down syndrome is low but not zero. It doesn't guarantee a baby without Down syndrome.
The triple test can detect approximately 60 percent of the pregnancies affected by trisomy 21 and will miss about 40% of cases.
If a screening blood test or an ultrasound shows an increased risk, or if you are over age 35, you will be offered an amniocentesis. An amniocentesis is a test that is usually done between 15-20 weeks of pregnancy. It involves using a thin needle inserted through your abdomen (not through your belly button), to obtain some of the amniotic fluid surrounding the fetus. This amniotic fluid contains some of the fetus’s skin cells. These skin cells can be used to obtain a fetal karyotype - a picture of the fetus’s chromosomes. If the fetus is is discovered to have an extra number 21 chromosome, then the diagnosis of Down syndrome is made.
Chorionic villi sampling or CVS is a procedure that is done at about 10-12 weeks of pregnancy. In this procedure, a thin needle or catheter is inserted into the placenta to obtain some cells. The placenta is derived from the same cells that make up the fetus, so by looking at the cells of the placenta, you are also looking at the cells of the fetus. These cells are then sent to the lab for chromosome analysis. If the karyotype reveals that the fetus has an extra number 21 chromosome, then the fetus will have Down syndrome.
Amniocentesis and CVS are optional tests during pregnancy - they are not something that you have to do. It is up to you to decide whether the benefits of knowing about the fetus’s chromosomes outweigh the risk of the procedure. Both procedures have a small risk for miscarriage. The risk is less that 1% for amniocentesis and about 1-2% for CVS. Some women prefer CVS to amniocentesis because they want to get information earlier in their pregnancy, and others prefer amniocentesis because of the lower risk of miscarriage. Only you can decide what is best for you.
National Institute of Child Health and Human Development (NICHD), Facts about Down Syndrome, 2008
Newberger, D, (2000) Down Syndrome: Prenatal Risk Assessment and Diagnosis, American Family Physician.