Pregnancy: Trisomy 21 screening tests

In Quebec, a prenatal screening test is used to determine the probability of having a child with trisomy 21. The test is free for all pregnant women under the Québec Prenatal Screening Program. Your health care provider will discuss this option with you during your first pregnancy follow-up. However, you are not required to do the test. It’s up to you.

What is trisomy 21 (Down syndrome)?

Trisomy 21 is commonly known as Down syndrome. It is one of the most common chromosomal anomalies in humans. Individuals with this condition are born with an extra copy of chromosome 21.

Children born with trisomy 21 experience mild to severe intellectual delays. However, there is no way to know the limits of their intellectual development and level of autonomy in advance. They vary from person to person. The stimulation and support that a child receives from the time they’re very young can support their development and help push these limits further.

People with Down syndrome have the potential to develop strong, loving relationships, despite their intellectual disability. What’s more, these people can thrive. With the right stimulation and support, some adults with Down Syndrome are able to work and live relatively independently.

There is no treatment for this chromosomal anomaly. This means that the majority of affected adults require some level of support throughout their lives. The life expectancy for people with Down syndrome is 60 to 70 years.

Health problems associated with Down syndrome

Certain health problems are often associated with trisomy 21. Issues affecting a person’s vision (e.g., nearsightedness, strabismus, and cataracts), hearing (deafness), heart, digestive system, and hormones (e.g., hypothyroidism), as well as epilepsy, are common.

In fact, between 40% and 50% of newborns with trisomy 21 have a heart defect. Around 5% to 10% of these newborns have gastrointestinal defects. Early intervention and regular medical check-ups can help minimize the impact of these issues.

People with Down syndrome are also 3 to 5 times more likely to develop symptoms of Alzheimer’s disease.

What is the probability 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 probability of carrying a child with trisomy 21 increases with the age of the expectant mother. For example, the probability of carrying a child with trisomy 21 is lower for a 20-year-old woman than for a 40-year-old woman (see table below).

What are the screening tests for trisomy 21 and when should they be done?

The prenatal screening test is offered free of charge in Quebec to women carrying one or two babies. These women must have a Québec Health Insurance Plan (RAMQ) card. The test can detect trisomy 21, but also trisomy 18 and trisomy 13.

The prenatal screening test includes:

• A biochemical test (blood test) with or without nuchal translucency measurement (ultrasound)
• A non-invasive prenatal genomic test (NIPT) (blood test).

Biochemical test with or without nuchal translucency measurement

Biochemical test

This test requires a blood sample, which is taken between the 10th and 13th week of pregnancy (1st trimester). A blood sample can be taken between the 14th and 16th week (2nd trimester), but it will provide results later in the pregnancy.

The biochemical test is safe. There is no risk of miscarriage.

Nuchal translucency measurement

Your doctor, primary care specialized nurse practitioner (PCSNP), or midwife may recommend that you undergo an ultrasound between the 11th and 13th week of pregnancy. This will allow your care provider to assess the age of the fetus, see if you are carrying more than one fetus, and check for certain anomalies.

This test can also be used 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 probability that the baby will have trisomy 21. There is also an increased probability of heart defects or other chromosomal abnormalities.

To be reliable, the nuchal translucency measurement must be combined with the results of the biochemical test (blood test). However, the biochemical test can be performed even if nuchal translucency is not measured.

The nuchal translucency measurement is safe. There is no risk of miscarriage.

Results of the biochemical test (with or without nuchal translucency)

The results of the biochemical test, with or without nuchal translucency, will indicate the probability of trisomy 21 in the fetus. You’ll know whether you have a high or low probability of giving birth to a child with trisomy 21. The biochemical test can also be used to determine the probability of the fetus being affected by trisomy 18.

If the probability of trisomy 21 is low (less than 1 in 300):

• You won’t have to undergo any further tests. However, it is not guaranteed that your baby won’t have the condition. There is always some probability.
• The biochemical test, with or without nuchal translucency, cannot detect all fetuses with trisomy 21. These tests are not without their limitations, and each person’s medical profile is unique.

If your probability of giving birth to a baby with trisomy 21 is high (1 in 300 or more):

• This doesn’t mean that your baby will definitely have Down Syndrome.
• The doctor, PCSNP, or midwife monitoring your pregnancy may then recommend that you undergo a genomic test.
• Your health care professional may also recommend that you directly undergo a diagnostic test (amniocentesis or chorionic villus biopsy).

Non-invasive prenatal genomic test (NIPT)

The genomic test is used to determine the probability of the fetus being affected by trisomy 21, trisomy 18, or trisomy 13. It is performed by taking a blood sample and can be done as early as the 10th week of pregnancy. If you are expecting twins, genomic testing can be done once you’ve reached the 12th week of pregnancy.

The genomic test is reliable and safe. There is no risk of miscarriage.

Your doctor, PCSNP, or midwife may recommend genomic testing if the biochemical test found a high probability of trisomy. They may also recommend it instead of the biochemical test in the following cases:

• If you have ever been pregnant with a baby with trisomy 21, trisomy 18, or trisomy 13
• If you will be aged 40 or over at the time of delivery
• If you are carrying twins
• Following genetic counselling

Results of the genomic test

The results of the genomic test indicate the probability of trisomy 21, trisomy 18, or trisomy 13 in the fetus. You’ll know whether it’s unlikely or very likely that your baby will be born with one of these trisomies.

If your probability is low:

• It is unlikely that your baby will be born with a form of trisomy. This result is over 99% reliable.
• You won’t have to undergo any further tests.

If your probability is high:

• It is very probable that your baby will be born with a form of trisomy, but not 100% guaranteed.
• The professional monitoring your pregnancy may recommend a diagnostic test to confirm whether the fetus has trisomy 21.

Diagnostic tests

Diagnostic tests can reliably confirm whether or not your baby has a form of trisomy. They can also be used to diagnose other chromosomal abnormalities, as well as neural tube defects such as spina bifida.

Two diagnostic tests are available:

• Amniocentesis: During this test, a fine needle is inserted into the woman’s womb to collect a small amount of amniotic fluid. The cells found in this fluid can then be used to analyze the fetus’s chromosomes. This test can be performed from the 15th week of pregnancy.
• Chorionic biopsy: This test involves taking a sample of the placenta through the abdomen or vagina. Chorionic biopsies are performed between the 11th and 14th week of pregnancy.

Risks associated with amniocentesis and chorionic biopsy

Diagnostic tests can lead to complications, including miscarriage. This is why they are only offered to women who have a high probability of having a baby with a form of trisomy.

The risk of miscarriage following amniocentesis is estimated at 0.2% (1 miscarriage out of around 500 tests). For chorionic biopsy, this risk is estimated at 1% (1 miscarriage out of approximately 100 chorionic biopsies).

The choice to get tested

Prenatal screening is not obligatory. If you’re unsure about whether to do the test, talk about it with your partner and loved ones. You can also discuss it with the professional who is monitoring your pregnancy. They can answer your questions and direct you to the appropriate resources.

You can also contact a group of parents to children with trisomy 21 (e.g., RT21), or speak to people who decided to terminate a pregnancy following a fetal diagnosis of trisomy 21.

Take time to consider the emotional aspect of this screening test. Understanding how you feel will help you come to a decision that resonates with your values, beliefs, and personal experience.

Here are a few questions to think about:

• Is knowing the probability of having a child with trisomy 21 important to me?
• How might knowing that I have a low or high probability affect me?
• Am I comfortable taking a test that’s based on probability?
• If my baby’s probability of trisomy 21 is high, would I be willing to undergo a diagnostic test knowing that it carries a risk of miscarriage?
• If the diagnostic test confirms the presence of a form of trisomy, would I be ready to choose between terminating the pregnancy and carrying it to term?
• How do I envision my life with a child with trisomy 21 (link in French)?

Answering these questions isn’t easy. But thinking about them will help you make an informed decision. If necessary, the professional monitoring your pregnancy can refer you to psychological services to help you think things through.

Things to keep in mind

• The prenatal screening test is offered free of charge in Quebec, but it is not mandatory.
• The test consists of a blood test with or without ultrasound (nuchal translucency). A genomic test (blood test) and a diagnostic test can also be performed.
• The probability of carrying a child with trisomy 21 increases with the age of the expectant mother.

Scientific review: Paméla Farman, M.Sc., nurse clinician and lecturer at the Université Laval faculty of nursing Research and copywriting:The Naître et grandir team Updated: April 2026

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