Screening and diagnosis for Down syndrome

What is the pre-natal screening process for Down syndrome detection?

Prenatal care now screens for Down syndrome, among other genetic conditions, as part of the normal routine, regardless of a woman’s age. It is standard practice, particularly if a woman is over the age of 35, a father is over 40 or a doctor is aware of a family history of Down syndrome.

Before performing any tests, a doctor will have a detailed discussion with an expecting woman about the various advantages and disadvantages of performing the tests. This is to ensure that a pregnant woman is well aware of all risks and can make the most informed decisions regarding the health of her pregnancy and growing baby.

How does screening take place?

• First trimester: Once a pregnancy has been confirmed, a blood test and ultrasound evaluation will be performed. The blood sample will be evaluated for the pregnancy hormone, hCG (human chorionic gonadotropin and the pregnancy-associated plasma protein-A (or PAPP-A) levels will be measured. Abnormal levels detected in the evaluation process indicate a problem with the baby. An ultrasound evaluation (or nuchal translucency test) checks and measures a specific area on the back of a baby’s developed neck. If there is more fluid than is normal, a doctor will be able to easily pick up an accumulation of this neck tissue during the scan, thus flagging a developmental problem with the baby.
• Second trimester: Towards the middle stages of pregnancy, a doctor will recommend further screening via an ultrasound scan and a QMS (quadruple marker screen). This is to assess possible defects in the brain and spinal cord of the foetus (which will have already formed) by looking at the blood level of four primary pregnancy-associated substances. These are hCG, estriol, alpha fetoprotein and inhibin A. The tests can help to identify possible Down syndrome as well, and are usually done between 15 and 20 weeks of pregnancy. Abnormal results of these tests place a foetus at higher risk for birth defects.

Diagnostic tests during pregnancy

Results for any of these screening tests merely show abnormalities (or the lack of these) and an increased likelihood for birth defects and genetic disorders such as Down syndrome. None can offer conclusive diagnostic criteria for Down syndrome. This can only be achieved with diagnostic tests either during pregnancy or at birth.

Diagnostic tests will be highly recommended if found to be necessary (i.e. if the risk is perceived to be high). A low risk of Down syndrome, does not necessarily mean that a baby will not be born with down syndrome either. The decision to go ahead with a diagnostic test is purely up to the expectant mother / parents.

Diagnostic tests do carry some risk, the most concerning one being the possibility of miscarriage. The reason for this is that these tests are somewhat invasive. For some women / parents, this is enough to not go ahead with diagnostic tests and rather opt to prepare for a baby with possible birth defects through counselling (most often genetic counselling).

Diagnostic tests during pregnancy are:

• First trimester: Around the 10 to 13-week mark, if a doctor feels it necessary, he or she may recommend a CVS (chorionic villus sampling) test. This involves extracting cells from the placenta for analysis through inserting a needle into the abdominal wall or inserting a long, fine instrument through the cervix (with a local anaesthetic and the use of an ultrasound to guide it). The primary aim is to examine foetal chromosomes from the extracted sample. Risk for miscarriage is minimally increased (up to 1%) when having this test.
• Second trimester: It can be difficult to detect Down syndrome earlier in a pregnancy, but if any abnormalities are picked up in screening, a doctor may recommend an amniocentesis (a small sample of amniotic fluid which contains foetal tissues is extracted for testing with a fine needle through the abdominal wall, into the uterus with the use of an ultrasound as a guide) around the 15th week of pregnancy. Another test which is commonly performed around 18 weeks, is PUBS (percutaneous umbilical blood sampling) which involves a sample of blood from the umbilical cord for chromosomal defect examination. This test only becomes necessary when all others have not shown conclusive results. A doctor will carefully assess the need for this test as it can increase a woman’s risk of miscarriage.

The diagnostic process after birth

Tests once a baby is born can help to determine a diagnosis of Down syndrome. Once a baby is born a doctor will do the following in order to make a diagnosis:

• Physical examination of the baby: Physical characteristics of the baby will be checked for, although they are likely to be subtler in a new-born. A baby with trisomy 21 will be easier to assess than the other types of Down syndrome.
• Karyotype test (blood test): A blood sample will also be taken for chromosomal analysis. It is through this test that the additional chromosome 21 can be detected in either some or all of the body’s cells. This test will not only confirm Down syndrome, but also the type. The test can take a little time, however, to return accurate results. New moms / parents can wait between 1 and 3 weeks before an actual diagnosis for Down syndrome is officially made.