Any pregnant individual has a chance to have a baby with trisomy 21 (Down syndrome) and trisomy 18 (Edwards syndrome). This chance increases with maternal age (or the age of the egg donor).

The purpose of enhanced First Trimester Screening is to provide you with a personalized chance for having a baby with one of these two chromosome differences, by incorporating information from an ultrasound and blood work in addition to the maternal age. 

How the test works

If you choose to have enhanced First Trimester Screening (eFTS), this testing can be arranged by your health care provider in the first trimester of your pregnancy, within a certain time window: from 11 weeks 2 days to 13 weeks 3 days gestation. You will be scheduled for a nuchal translucency (NT) ultrasound and a blood test within this time window.

While same day blood sampling is ideal and preferred, it is NOT essential. Blood samples can be accepted until 13 weeks and 3 days gestation, without affecting the accuracy of the screening.

Depending on where your screening is performed, the blood test will measure the levels of 3 or 4 different hormones (or proteins) that are present in your blood at this point in pregnancy:

• pregnancy-associated plasma protein A (Papp-A) – a protein produced by the placenta
• human chorionic gonadotropin (hCG) – a hormone made by the placenta during pregnancy
• alpha-fetoprotein (AFP) – a protein produced by the baby's liver
• placental growth factor (PlGF) – a protein produced by the placenta

The results of the NT ultrasound and the blood test will be combined with your age (or the age of your egg donor) at delivery, to give you information about the chance to have a baby with trisomy 21 (Down syndrome) or trisomy 18.

If Non-Invasive Prenatal Testing (NIPT) has already been initiated, eFTS should not be done. However, an NT ultrasound is still recommended as it can offer additional information about the health of the baby. 

The benefits of eFTS

• screening tests pose no risk to pregnancy
• early results allow for earlier decision-making about further testing & care

The limitations of eFTS

• eFTS will give information about the chance of having a baby with trisomy 21 or trisomy 18, but does not give a definitive "yes" or "no" answer about these chromosome difference. Screening tests are not 100% accurate – false positive and false negative results are possible
• Does not specifically screen for conditions other than trisomy 21 or trisomy 18, although additional information about the baby's health can be discovered during the screening process
• Cannot be done in a vanishing twin scenario
• Some pregnancies are conceived after in vitro fertilization (IVF), and the embryos may be screened with Preimplantation Genetic Testing for Aneuploidy (PGT-A) during that process.  PGT-A involves screening embryos for chromosome differences, including trisomy 21 and trisomy 18 and it is a highly effective screen, although it is not diagnostic.  Traditional screening, such as eFTS, is not recommended for pregnancies that had PGT-A, since PGT-A is considered to be a better screen¹. 

Screening results

Your eFTS result will be shared with your provider within approximately 5 working days. Talk to your provider about how they plan to share these results with you.

When you receive your eFTS 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.  Alternatively, your result will be  “screen positive”, which means that you have a higher chance to have a baby with trisomy 21 or trisomy 18. 

The report will also give you a personalized likelihood that your baby has trisomy 21, and you can ask your provider for the specific number on your report. For example, a possible result may be a 1 in 50 or 1 in 700 chance that the baby has trisomy 21. Your provider will review the result with you and make a plan for follow-up if necessary.

Detection rate

The detection rate of eFTS is approximately 89% for trisomy 21. This means that 89% of pregnant individuals carrying a baby with trisomy 21 will get a screen positive result using eFTS. Screening cannot detect all cases: approximately 11% of pregnancies with trisomy 21 will receive a screen negative result using eFTS. More information about screening performance can be found here. 

References: 

¹Committee Opinion No 406: prenatal testing after IVF with preimplantation genetic testing for aneuploidy. J Obstet Gynaecol Can 2020