Pregnancy: Trisomy 21 screening tests

Pregnancy: Trisomy 21 screening tests
Trisomy 21, also known as Down syndrome, is the most common chromosomal abnormality. The condition affects approximately 1 in 800 babies.

Quebec’s trisomy 21 prenatal screening program provides all pregnant women and couples in the province with access to a free trisomy 21 screening test. Your health care provider will discuss this option with you during your first pregnancy follow-up. Participation in the program is completely voluntary.

What is trisomy 21?

Trisomy 21, also known as Down syndrome, is the most common chromosomal abnormality. Individuals diagnosed with this condition have an extra copy of chromosome 21.

Children born with trisomy 21 experience mild to moderate intellectual delays. There is no way to predict the degree of impairment, as every case is different. The stimulation and support that a child with trisomy 21 receives also contributes to their development.

About 40 to 50 percent of babies born with trisomy 21 have a heart defect, and 5 to 10 percent have gastrointestinal defects. Early medical intervention and regular follow-ups can help minimize these issues.

Despite their limited cognitive development, individuals with trisomy 21 have the potential to build strong emotional relationships and lead a fulfilling life. As adults, given the right amount of support and stimulation, some are able to work and live relatively independently. However, because there is no cure for the condition, the majority of adults require some level of support throughout their lives.

Trisomy 21 is also associated with a number of health problems, including epilepsy and heart, vision (e.g., myopia, strabismus, cataracts), hearing (e.g., deafness), hormonal (e.g., hypothyroidism), and digestive issues. In addition, individuals with the condition are three to five times more likely to develop symptoms of Alzheimer’s disease. The life expectancy for people with trisomy 21 is 55 years.

The risk of carrying a fetus with trisomy 21

Any woman could potentially carry a fetus with trisomy 21, as the condition is generally not hereditary. However, the risk increases with the mother’s age. For example, as outlined in the table below, a 20-year-old woman is at lower risk of carrying a child with trisomy 21 than a 40-year-old.


The risk of carrying a child with trisomy 21
(full-term pregnancy)
Age of expectant mother
20 years
1 in 1,500
30 years
1 in 900
35 years
1 in 385
40 years
1 in 100


Screening procedure

Blood tests

The prenatal screening test for trisomy 21 is offered free of charge in Quebec. The procedure consists in taking two blood tests. The first is taken between weeks 10 and 13 (the first trimester), and the second is taken between weeks 14 and 16 (the second trimester). The results of the blood work, combined with the risks associated with your age, determine whether you have a low or high probability of giving birth to a child with trisomy 21.

For more accurate results, it is recommended to take both blood tests. While it’s possible to make a calculation based on only one blood sample, this leads to less reliable results: the risk of obtaining a false positive increases significantly, and the likelihood of detecting the abnormality decreases.

Ultrasound: Nuchal translucency test

Between the 11th and 13th week of pregnancy, your health care provider may suggest taking an ultrasound to measure the nuchal translucency of the fetus, which is the amount of fluid under the skin behind the fetal neck. If the fluid buildup is thicker than normal, there is a higher risk the baby will have trisomy 21.

There is also an increased risk of heart defects or other chromosomal abnormalities. For more accurate screening results, it’s best to have the nuchal translucency ultrasound as well as both blood tests. It’s important to know that pregnant women expecting more than one child are not eligible for Quebec’s trisomy 21 prenatal screening program, as the procedure is not adapted to multiple pregnancies.

Test results

The prenatal screening test results will indicate the probability—either low or high—of your child being born with trisomy 21.

  • If the probability is low (less than 1 in 300), you will not have to undergo further testing. However, it is not guaranteed that your baby won’t have the condition. There is always some risk. Due to a number of variables, not all fetuses affected by trisomy 21 can be detected through blood samples and nuchal translucency testing.
  • If the probability is high (equal to or greater than 1 in 300), it doesn’t necessarily mean that your baby will have trisomy 21. The health care professional monitoring your pregnancy will recommend amniocentesis with chromosome analysis. This is the only test that can definitively diagnose whether your baby has trisomy 21, some other form of chromosomal abnormality, or a neural tube defect. However, it also comes with the risk of miscarriage. In most cases, women who have amniocentesis see normal results and give birth to a healthy baby.
Private testing
Some private clinics offer a trisomy 21 screening test that involves taking a sample of the mother’s blood to analyze DNA fragments from the placenta. This test is more accurate than the Quebec screening program, with a detection rate above 99 percent and very few false positives. It can be performed once you’ve reached your 10th week of pregnancy, and the results are usually available after two weeks. If the test is positive, amniocentesis must be done to determine a diagnosis. Cost: $500 to $650.
The other tests listed in this fact sheet are also offered by private clinics. While similar to the Quebec screening program in terms of accuracy, they deliver faster results. Cost: $350 to $500.

The choice to get tested

Taking the trisomy 21 screening test is not mandatory. If you’re unsure about whether to take the test, talk about it with your partner and loved ones.

Your doctor, midwife, or nurse can also offer advice and point you to the right resources. Lastly, you may wish to get in touch with a group for parents of children with trisomy 21 (e.g., RT21) or for parents who chose to end their pregnancy.

It’s important to think about the emotional side of the screening process. Understanding how you feel will help you come to a decision that matches your values, beliefs, and personal experience.

Here are a few questions to guide your reflection:

  • Is knowing the probability of having a child with trisomy 21 important to me?
  • How will knowing that I have a low or high probability affect me?
  • Am I comfortable taking a test that’s based on probability?
  • If the result is a high probability of trisomy 21, am I willing to have amniocentesis despite the risk of miscarriage?
  • How do I envision my life with a child with trisomy 21?
  • Am I willing to have an abortion if the result of the amniocentesis test is positive for trisomy 21?

These aren’t easy questions to answer, but thinking about them will help you make an informed decision.


Things to keep in mind

  • The trisomy 21 screening test is free in Quebec.
  • The test involves giving two blood samples and, in some cases, having an ultrasound (nuchal translucency test).
  • The risk of carrying a child with trisomy 21 increases with age.
  • The screening test is voluntary; whether you take it is entirely your choice.


Naître et grandir

Scientific review: Amélie Guay, M. Sc., IP(C), nurse clinician, and Bi Lan Wo, obstetrician-gynecologist
CHUM Birthing Centre
Research and copywriting: The Naître et grandir team
Updated: November 2019


Photo: 123 rf: 10289562



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  • Health and Welfare Commissioner. Rapport de consultation : consultation sur les enjeux éthiques du dépistage prénatal de la trisomie 21, ou syndrome de Down, au Québec [Consultation report: Consultation on the ethics of prenatal screening for trisomy 21, or Down syndrome, in Quebec]. Quebec, 2008, 120 pp.
  • Doré, Nicole and Danielle Le Hénaff. From Tiny Tot to Toddler: A practical guide for parents from pregnancy to age two. Quebec, Institut national de santé publique du Québec, 2019, 776 pp.
  • Ministère de la Santé et des Services sociaux. “Programme québécois de dépistage prénatal de la trisomie 21” [Trisomy 21 prenatal screening program of Quebec].
  • Alzheimer Society of Canada. Down syndrome and Alzheimer’s disease. 2016.
  • The Society of Obstetricians and Gynaecologists of Canada. “Prenatal genetic screening.”