Second Trimester Screening (STS) | Prenatal Screening Ontario

You can get publicly-funded Second Trimester Screening (STS) in the second trimester of pregnancy. This test is safe because it uses only a blood test. It can tell you the chance of having a baby with trisomy 21 (Down syndrome) or trisomy 18 (Edwards syndrome). This screening can be done if you are not able to get enhanced First Trimester Screening (eFTS).

How to get STS

Talk with your health-care practitioner if you would like to get STS. In Ontario, this test can be ordered by:

doctors (such as family doctors, obstetricians)
nurse practitioners
midwives

If you don’t have a family doctor, you can still arrange this test through a walk-in clinic. Use Health811 to find walk-in clinics near you.

The blood test for STS can be done from 14 weeks to 20 weeks 6 days of pregnancy.

Who should not have STS?

STS should not be done if you:

are in the first trimester of pregnancy and are able to get a nuchal translucency ultrasound. Think about having enhanced First Trimester Screening (eFTS) instead of STS.
have already received a "low risk" result from Non-Invasive Prenatal Testing (NIPT), or you are waiting for your NIPT results.
are expecting more than one baby (twins, triplets, quadruplets etc).

For Health-Care Practitioners

Requisitions and How To Order

What does STS look for?

STS can detect most pregnancies with trisomy 21 and trisomy 18. Like other screening tests, STS cannot detect all pregnancies with these chromosome differences. STS does not look for other conditions that the baby might have.

How does STS work?

This screening test will combine all the information below to tell you about the chance to have a baby with trisomy 21 and trisomy 18.

Your age at the time of birth

You have a higher chance for chromosome differences like trisomy 21 and trisomy 18 as you get older. If you got pregnant through in vitro fertilization, it is the age of the egg that is used. The egg may be your own or from a donor.

Blood test result

The blood test measures 4 chemicals or proteins made by your pregnancy.

Other information about you

Your health-care practitioner will ask you some questions about you that improve the accuracy of test. You will be asked about your racial identity, diabetes status, smoking history in the pregnancy, weight and whether the pregnancy happened through in vitro fertilization.

An ultrasound isn’t required for this test, but it makes STS more accurate.

How long does it take to get results?

The results are expected to be received by your health-care practitioner within approximately 7 working days. Talk to your health-care practitioner about how they plan to give you these results.

If your health-care practitioner does not receive the results within this time, they are encouraged to fax a request to the screening laboratory the blood sample was sent to:

Mount Sinai Hospital: 416-586-4640

North York General Hospital: 416-756-6108

Trillium Health Partners / Credit Valley Hospital: 905-813-4335.

What will the results mean?

When you get your STS report, it will most likely say that you have a result that is “screen negative”. This means that you have a lower chance to have a baby with trisomy 21 and trisomy 18. It does not mean "no chance".

Some people will get a “screen positive” result, which means that there is a higher chance to have a baby with trisomy 21 or trisomy 18. It does not mean your baby definitely has one of these chromosome differences. You would have more options for testing, including publicly-funded NIPT and diagnostic testing.

How does STS compare to NIPT?

You might have the option of doing another prenatal screening test, called Non-Invasive Prenatal Testing (NIPT), instead of STS. In this section, you will learn how STS and NIPT compare to help you make a decision.

Source

The results presented in this visual are based on the sensitivity and specificity of STS and NIPT from actual Ontario pregnancies with an estimated date of delivery between September 2016 and March 2023 that underwent prenatal screening (twins were excluded). The incidence of trisomy 21 in this group was 1 in 394. The figures were applied to this fictitious population of 50,000 people. View more details about the performance of prenatal screening tests.

Accessible version of above visuals (text only)

STS and NIPT side-by-side

Questions to Consider	STS	NIPT
When in the pregnancy is it done?	Between 14 weeks and 20 weeks 6 days	After 9 or 10 weeks of pregnancy, depending on the laboratory
What does it include?	Blood work	Blood work
What does it screen for?	Trisomy 21 Trisomy 18	Trisomy 21 Trisomy 18 Trisomy 13 Sex chromosome differences (optional)
What does it not screen for?	Only above conditions are tested. A 18-22 week ultrasound is recommended for extra information about the baby's health	Only above conditions are tested. A 11-14 week (NT) and a 18-22 week ultrasound are recommended for extra information about the baby's health
Is it funded by public funds?	Yes	Yes, in certain situations. For example: you are 40 years or older at the time of birth or you have a "screen positive" STS result. Private-pay is an option if the test cannot be publicly-funded
What is the chance that the test will fail to give a result?	STS is only rarely not able to give a result	Sometimes NIPT fails to give a result or gives a result that is not typical
How accurate is it?	Detects most pregnancies with trisomy 21 and trisomy 18. STS is less accurate than NIPT	Detects most pregnancies with trisomy 21 and trisomy 18. NIPT is more accurate than STS

More about accuracy

Screening tests can give false positive or false negative results.

False positive

A false positive is when a pregnant individual gets a "screen positive" or "high risk" result but the baby does not actually have trisomy 21. Both screens have false positives. However, there are more false positives through STS than NIPT.

False negative

A false negative is when a pregnant individual gets a "screen negative" or "low risk" result but the baby actually has trisomy 21 (missed case). Both screens have few false negatives. However, there are more false negatives through STS than NIPT.

To help you understand the accuracy of STS and NIPT, view the type of results that 50,000 people can get if they choose prenatal screening.

Results of 50,000 people who have STS or NIPT

Type of Result	Number of people
False Positive	3,771 people (7.5%) get a "screen positive" result, but the baby does not actually have trisomy 21. The bottom line is that most "screen positive" results are false positives.	35 people (less than 0.1%) get a "high risk" result, but the baby does not actually have trisomy 21.
True Positive	108 people (0.2%) get a "screen positive" result, and the baby does have trisomy 21.	124 people (0.2%) get a "screen positive" result, and the baby does have trisomy 21. The bottom line is that most people who get a "high risk result" do have a baby with trisomy 21, but there are some false positives.
False Negative	17 people (less than 0.1%) get a "screen negative" result, and the baby does have trisomy 21.	1 person (less than 0.1%) gets a "low risk" result, and the baby does have trisomy 21. The bottom line is that only very few people who get a "low risk" result in fact have a baby with trisomy 21, and are therefore missed with NIPT.
True Negative	46,104 people (92.2%) get a "screen negative" result, and the baby does not have trisomy 21.The bottom line is that most people who get a "screen negative" result do not have a baby with trisomy 21, but there are some false negatives (missed cases).	49,840 people (99.7%) get a "low risk" result, and the baby does not have trisomy 21. The bottom line is that most people who get a "low risk" result do not have a baby with trisomy 21.

Frequently Asked Questions

I got pregnant through In-Vitro Fertilization and had Preimplantation Genetic Testing (PGT-A) during this process. Can I still have prenatal genetic screening?

PGT-A involves screening embryos for chromosome differences to help decide which embryos to transfer into the womb. STS is not recommended for pregnancies that had PGT-A, since PGT-A is a better screen for trisomy 21, trisomy 18 and other chromosome differences. You can speak to a genetic counsellor about the benefits and limitations of having Non-Invasive Prenatal Testing (NIPT) in your situation.