Second Trimester Screening (STS)

Update: Maternal Serum Screening (MSS) has been renamed "Second Trimester Screening", or STS.

Second Trimester Screening (STS) is a screening test available in the second trimester of pregnancy (when you are about four or five months pregnant). STS involves a blood test, and therefore poses no risk to the pregnancy. This testing can tell you the chance for having a baby with trisomy 21 (Down syndrome) and trisomy 18 (Edwards syndrome).

How to Get STS

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

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

The blood test for STS can be done from 14 weeks 0 days to 20 weeks 6 days of pregnancy.

For Health-Care Providers

Requisitions and How To Order

Who Should Not Have STS?

STS should not be done when:

you are in the first trimester and have access to a nuchal translucency ultrasound. Consider having enhanced First Trimester Screening (eFTS) instead of STS.
you have already received a "low risk" result from Non-Invasive Prenatal Testing (NIPT), or you are waiting for your NIPT results.
there is more than one baby (twins, triplets, quadruplets etc).
the pregnancy happened after in vitro fertilization (IVF) and the embryos were screened through Preimplantation Genetic Testing (PGT-A) during that process. PGT-A involves screening embryos for chromosome differences, including trisomy 21 and trisomy 18. PGT-A is a highly effective screen, which means it can pick up most embryos with trisomy 21 and trisomy 18. STS is not recommended for pregnancies that had PGT-A, since PGT-A is considered to be a better screen.

How the Test Works

The test will combine all the information below to tell you about the chance to have a baby with trisomy 21 (Down syndrome) and trisomy 18. Even though an ultrasound is not necessary for this test, a regular ultrasound improves the accuracy of STS and how the pregnancy is looked after.

Your age at the time of birth

There is a higher chance for chromosome differences like trisomy 21 and trisomy 18 as the age of the pregnant individual increases. If the pregnancy happened through IVF, it is the age of the egg that is taken into account (whether it is your own or a donor egg).

Blood test result

The blood test for STS will measure the amount of 4 different hormones (or chemicals) that are present in your blood:

human chorionic gonadotropin (hCG) – a hormone made by the placenta during pregnancy
alpha-fetoprotein (AFP) – a protein made by the baby's liver
inhibin-A – a protein made by the baby and the placenta
unconjugated estriol (uE3) – a form of estrogen that increases during pregnancy

Other information about you

Your health-care provider 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.

How Long Does It Take to Get Results?

The STS result will be shared with your health-care provider within approximately 5 working days. Note that there may be a delay between the time that the report is sent to your provider and the time when the provider receives it. Talk to your provider about how they plan to share these results with you.

What Will the Results Mean?

When you receive 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. If your report does not include information about trisomy 18, you can assume you have a "screen negative" result for this chromosome difference as well.

Some individuals will get a “screen positive” result, which means that there is a have a higher chance to have a baby with trisomy 21 or trisomy 18. More testing will be offered to you.

In both "screen negative" and "screen positive" situations, the report will also give you a specific chance that your baby has trisomy 21. For example, a possible result may be a 1 in 50 or 1 in 700 chance that the baby has trisomy 21.

How Does STS Compare to NIPT?

You may 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.

STS and NIPT Side-by-Side
	STS	NIPT
When in the pregnancy is it done?	14 weeks 2 days to 20 weeks and 6 days	9 or 10 weeks and later
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
Is it funded by OHIP?	Yes	Yes, in certain situations
Detection rate (trisomy 21)	79%²	More than 99%³
False positive rate (trisomy 21)	6%⁴	0.1%⁵
Detection rate (trisomy 18)	34 to 88% (cannot give a precise figure due insufficient Ontario data at this time)	96%
False positive rate (trisomy 18)	Less than 1%	Less than 0.1%
Can it be done for a twin pregnancy?	No, it can only be done if you are expecting one baby	Yes, it can be done if you are expecting one or two babies

Notes

The detection rate and false positive rate for the screening tests were obtained from Ontario data (twin pregnancies were excluded). View more details about the performance of these tests.

²79% of people that are carrying a pregnancy with trisomy 21 will get a “screen positive” result using STS. Screening cannot detect all cases: approximately 12% of pregnancies with trisomy 21 will be missed with this test.

³More than 99% of people that are carrying a pregnancy with trisomy 21 will get a “high risk” result using NIPT. Rarely (less than 1% of the time), people that are carrying a pregnancy with trisomy 21 will be missed with this test.

⁴6% of people that are not carrying a pregnancy with trisomy 21 will get a “screen positive” result using STS.

⁵Less than 0.1% of people that are not carrying a pregnancy with trisomy 21 will get a “high risk” result using NIPT.

Benefits

There are a few benefits of STS:

Non-invasive - as any screening test, it poses no risk to pregnancy
Nuchal translucency (NT) ultrasound is not needed -therefore, STS is an option in areas where this specialized ultrasound is not as easily available.

Limitations

STS has some of the following limitations:

Not diagnostic - STS can tell you the chance of having a baby with trisomy 21 or trisomy 18, but will not give you a definitive "yes" or "no" answer about these chromosome differences. Screening tests are not 100% accurate – the test can miss pregnancies with trisomy 21 or trisomy 18, or it can tell you the baby has one of these chromosome differences when in fact, the baby does not.
Does not screen for "everything" - this test does not specifically screen for conditions other than trisomy 21 or trisomy 18.
STS will not pick up any differences in the growth or development of the baby (such as spina bifida). A detailed anatomy ultrasound scheduled between 18-22 weeks of pregnancy will look at the development of the spine and other parts of the body.