Trisomy 21 Screening Tests
Article updated on december 2020
Trisomy 21 (also called Down’s Syndrome), 18 or 13 are different types of trisomy that refer to problems associated with chromosome configuration. These chromosomes carry genes that will be transmitted to the baby’s genetic code. A chromosomal dysfunction can lead to different repercussions for the future baby’s physical and mental health. But it is impossible to determine these impacts before the baby is born, grows and develops.
In Quebec, approximately 1/770 children will be diagnosed with Down’s Syndrome out of 87,000 births.
The screening is entirely voluntary and consists of 2 blood tests taken from the future mother at two different times, to measure the most common aneuploids. The first test needs to be done between the 10th and 13th week of pregnancy, and the second between the 14th and 16th week of pregnancy to obtain the best results.
The results of these measurements do not diagnose trisomy. They indicate the risk of giving birth to a baby with Down’s Syndrome. The detection rate is around 80-85% with a margin of error. Some pregnant women, in addition to screening tests, will have what is called a nuchal translucency ultrasound at 12 weeks of pregnancy. It measures the fluid at the back of the baby’s neck, which increases measurement reliability and improves the detection rate to more than 95% with a tiny margin of error. The nuchal translucency ultrasound is not universally offered in the health network. It is provided by many private clinics, available to parents who want to take it. Even a dating ultrasound can increase the screening detection rate from blood tests.
According to the SOGC clinical directives, all pregnant women in Canada should be given prenatal screening tests. In the best of worlds, a fetal ultrasound between the 11th and 14th week of pregnancy should be part of these tests. This ultrasound also allows health professionals to evaluate fetal viability, gestational age (dating), the number of fetuses, early anatomy and nuchal translucency depending on the options offered.
Over recent years, private companies have started offering other tests to screen for trisomy 21, 18 and 13, using acellular DNA analyses of the mother’s blood. The test is safe for both mother and baby. It is highly effective, but not generally available to all women given the high cost of performing it. Irrespective of the mother’s age, HarmonyMC, Panorama and MaterniT21® should be considered as private options when deciding to participate in prenatal trisomy screening.
These tests can be done very early during pregnancy, and involve blood tests for the mother around 10 weeks pregnant. Fetal DNA fragments, which are released into the blood, are then studied to identify the baby’s genetic code. The detection rate is 98-99%, but the test costs between $500-$900. Parents have to pay the full price unless they have private coverage.
If the screening test results indicate a high trisomy risk, your health professional will help you choose the best possible option. If you want to have further screening tests, you have two options. The first is an amniocentesis (removal of amniotic fluid and fetal cells with a needle) for more in-depth analysis. The second is chorionic villus sampling, where the baby’s chromosomes will be examined from a small sample of the placenta.
Research is ongoing for tests that don’t require the use of a needle. Known as NIPS (non-invasive prenatal genetic screening), it involves taking a blood sample from the mother. Her DNA, as well as DNA from the placenta (which includes fetal DNA (5-15%)), is isolated. Technicians can validate the presence of trisomy risk with a false negative margin of error of about 0.5-1%, which is very respectable.
I know that it’s not easy for parents to know what is best for them in a case like this. But I hope the information they receive from health professionals at the start of their pregnancy can help them make informed decisions. Always ask questions, get the answers you need, and then decide together.
I hope this article has been helpful.
The Baby Expert