Prenatal Genetic Testing
By George D. Carson, M.D. FRCSC
on March 15, 2011
Today all pregnant women can be tested to determine their risk of having a baby with a chromosome abnormality.
What Are Chromosomes?
- Chromosomes contain the information directing the development and function of every cell.
- Usually, we have 23 pairs of chromosomes.
- The chromosomes line up in pairs in the egg and sperm and then separate so that each receives just one out of each pair of chromosomes.
- With the union of the egg and sperm, the fertilized egg then has a full set of chromosomes.
Sometimes the normal orderly sorting goes wrong and produces an abnormality of development. There is no way to prevent this but there are two types of available testing to find an abnormality: Screening and Diagnostic.
Prenatal Genetic Screening Tests
Screening tests indicate the chance of disease and assists a pregnant woman in deciding whether or not to have an invasive diagnostic testing.
The usual cause of Down Syndrome is to have an extra copy of chromosome number 21. We don't know what causes the abnormality of chromosome sorting but this occurs with increasing age of the mother.
- The chance of this occurring in a child is about 1/ 800 for all pregnancies for women of all ages
- The risk is 1/ 350 for a woman who is 35 at the time of delivery
- 1/50 for a woman of 45
What Are Some Of The Screening Tests?
This test measures the fluid under the skin at the back of the neck of the fetus between 11 and 14 weeks. Fetuses with Down Syndrome tend to have an increased amount of fluid, indicating increased risk.
Serum Screen, "Triple Screen" or "Quad Screen"
This is a test for certain chemicals made by the fetus that appear in the blood of the mother in somewhat different amounts than normal when the fetus has Down Syndrome. It is performed at about 15-20 weeks.
Eighteen Weeks Structural Ultrasound
This test is routinely offered in Canada. Benefits include being able to look for structural abnormalities. There are "markers" for Down Syndrome, which may also be present in normal fetus. These abnormalities or markers are seen in at least half the fetuses with Down Syndrome.
When combined with the nuchal translucency and a serum screen at about 15 weeks about 90 percent of cases of Down Syndrome can be identified.
What Are The Diagnostic Tests?
This is the most common test, usually done at about 16 weeks. Cells from the fetus are recovered by putting a needle through the abdominal wall into the fluid surrounding the fetus and the chromosomes get counted. It takes two to three weeks to get an answer. This is the definitive diagnostic test that can rule in or out the presence of an abnormal number of chromosomes. The risk with amniocentesis is a one in 200 chance of loss of the pregnancy.
There are two important limitations:
- A normal result does not guarantee a normal child. There is still about a two percent risk of an abnormality.
- An abnormal result cannot be cured.
Chorionic villous sampling is less readily available and samples tissue from the placenta. A tube or biopsy forcep is placed through the cervix or a needle is placed through the abdominal wall into the placenta, to remove a portion of placental tissue. This test can be done at around 11-12 weeks.
The risk of pregnancy loss due to CVS testing is a little bit higher than amniocentesis, but the benefit is a definite diagnosis at an earlier gestational age.
By George D. Carson, M.D. FRCSC|
March 15, 2011