7 facts about prenatal screening (eFTS, NIPT) and Trisomy 13, 18 & 21

Prenatal screening options explained by the experts

At Markham Fertility Centre, we have a dedicated genetics program, led by Diane Myles-Reid, Genetic Counsellor and former President of the Canadian Association of Genetic Counsellors.

In this post we will cover; 

1. What is prenatal screening
2. Types of prenatal screening tests
3. Differences between eFTS and NIPT
4. Why consider prenatal screening
5. Understanding chromosomal conditions
6. Special considerations for prenatal screening
7. Cost and insurance coverage

What is Prenatal Screening?

Prenatal screening is a type of genetic testing offered to all pregnant individuals, regardless of age. It is a non-invasive test designed to assess whether there is a higher or lower chance that a pregnancy may be affected by a chromosomal condition. It is important to emphasize that prenatal screening is not diagnostic—it does not confirm a condition but only indicates the probability of a genetic difference.

If a screening result comes back abnormal, further diagnostic testing such as amniocentesis or chorionic villus sampling (CVS) is required to confirm whether a chromosomal condition is present.

Types of Prenatal Screening Tests

1. Enhanced First Trimester Screening (eFTS)

• What it is: A combination of a blood test and an ultrasound.
• Timing: Performed between 11.5 to 13.5 weeks of pregnancy.
• What it screens for: Down syndrome (Trisomy 21) and Trisomy 18.
• Accuracy: Detects about 89-90% of pregnancies affected by Down syndrome, with a false positive rate of 5-6%.
• Risk Calculation: Provides a specific risk assessment (e.g., 1 in 3,000 or 1 in 200). A risk higher than 1 in 350 is considered screen positive.

2. Non-Invasive Prenatal Testing (NIPT)

• What it is: A blood test analyzing cell-free DNA (cfDNA) from the placenta circulating in the pregnant person’s bloodstream.
• Timing: After 9 or 10 weeks of pregnancy.
• What it screens for:
  • Down syndrome (Trisomy 21)
  • Trisomy 18 and Trisomy 13
  • Sex chromosome differences
• Accuracy: Detects 99% of pregnancies with Down syndrome, with fewer false positives compared to eFTS.
• Additional Screening Options: Some NIPT panels can screen for microdeletion syndromes, though these tests have higher false-positive rates and lower detection rates.

Differences Between eFTS and NIPT

Feature	eFTS	NIPT
Method	Blood test + Ultrasound	Blood test (cfDNA)
Timing	11.5 – 13.5 weeks	After 9-10 weeks
Conditions Screened	Trisomy 21, Trisomy 18	Trisomy 21, Trisomy 18, Trisomy 13, sex chromosome differences (microdeletions optional)
Accuracy for Down Syndrome	89-90%	>99%
False Positive Rate	5-6%	Lower than eFTS
Covered by Insurance (OHIP – Ontario)	Yes	Only for certain high-risk pregnancies

Why Consider Prenatal Screening?

Prenatal screening provides valuable information for expecting parents. Many people opt for screening to:

• Gain reassurance about their baby’s health.
• Prepare emotionally and medically if an increased risk is detected.
• Ensure the appropriate medical support is available at birth if needed.

Some individuals may decline prenatal screening because:

• They feel comfortable with the uncertainty.
• They would not consider terminating the pregnancy regardless of the result.

Understanding Chromosomal Conditions

Down Syndrome (Trisomy 21)

• Caused by an extra copy of chromosome 21.
• Leads to lifelong developmental and intellectual delays.
• Individuals may have heart defects, gastrointestinal issues, and other medical conditions.
• Life expectancy and quality of life have improved with early intervention and medical care.

Trisomy 18 (Edwards Syndrome)

• Caused by an extra copy of chromosome 18.
• Results in severe developmental delays and multiple birth defects.
• High risk of miscarriage or stillbirth.
• Babies born with Trisomy 18 often do not survive beyond early infancy.

Trisomy 13 (Patau Syndrome)

• Caused by an extra copy of chromosome 13.
• Leads to severe developmental disabilities and birth defects.
• High risk of miscarriage or stillbirth.
• Babies with Trisomy 13 usually have a short life expectancy.

Special Considerations for Prenatal Screening

1. Pregnancies Conceived via IVF and PGT-A

• If preimplantation genetic testing for aneuploidy (PGT-A) has been performed, the embryo has already undergone chromosomal screening.
• However, NIPT is still an option for added reassurance.
• eFTS is not recommended for individuals who transferred a chromosomally normal (euploid) embryo.

2. Twins and Higher-Order Multiples

• NIPT is the preferred screening test for twins.
• Triplets or higher-order multiples: Only the nuchal translucency (NT) ultrasound is available, as eFTS and NIPT are not validated for triplets.
• Vanishing twin syndrome: eFTS and NIPT may be less accurate due to the lingering DNA from the lost twin.

3. Egg or Sperm Donation & Gestational Carriers

• The age of the egg donor is an important factor in risk calculation.
• Prenatal screening options remain the same for sperm donation.
• Screening for a gestational carrier follows standard prenatal screening protocols.

Cost and Insurance Coverage (Ontario, Canada)

• eFTS is fully covered by OHIP for all pregnant individuals.
• NIPT is covered in specific cases, such as:
  • Egg provider is 40+ years old.
  • Abnormal ultrasound findings.
  • Previous pregnancy with Down syndrome, Trisomy 18, or Trisomy 13.
  • Twin pregnancies.
• If not covered, NIPT costs $500–$800, depending on the testing panel.

Prenatal screening is an optional but valuable tool for expecting parents. Whether choosing eFTS, NIPT, or diagnostic testing, understanding the benefits and limitations of each test is key to making an informed decision. If a high-risk result occurs, consulting with a genetic counselor can help navigate next steps and additional testing options.

For those interested in more in-depth discussions on genetic screening, check out previous podcast episodes on:

• Expanded Carrier Screening (Episode 29)
• Preimplantation Genetic Testing for Aneuploidy (PGT-A) (Episode 30)
• Preimplantation Genetic Testing for Monogenic Disorders (Episode 36)