Nuchal Translucency Screening

Please note: If you have been referred to Maternal Fetal Medicine you will be offered First Trimester Combined Screening which includes the nuchal translucency screening and a blood test. A First Trimester Combined Screen brochure and information video is available on this website.

What is nuchal translucency screening?

The nuchal translucency screening uses prenatal ultrasound to measure the amount of fluid at the back of your baby’s neck. This measurement, together with your age, is used to estimate the chance that your baby could have Down syndrome or two other chromosome conditions called trisomy 13 and trisomy 18.

What is my chance for having a baby with Down syndrome?

All women are at risk for having a baby with a chromosome condition such as Down syndrome. This risk increases as a woman’s age increases (see Table 1).

Why is nuchal translucency screening called a screening test?

Nuchal translucency screening test tells you what the chance is for having a baby with Down syndrome, trisomy 13, or trisomy 18. The nuchal translucency screen answers the question: “What is the chance my baby has a chromosome condition?”

Prenatal diagnostic tests such as amniocentesis and chorionic villus sampling give a “yes” or “no” answer about chromosome conditions. Prenatal diagnostic tests answer the question: “Does my baby have a chromosome condition?”

When is nuchal translucency screening done?

Nuchal translucency screening can only be done between 11 and 13 weeks 6 days of pregnancy. An ultrasound measurement of the baby called a crown to rump length, which measures the baby from the top of the head to their bottom, is used to determine your dates and make sure your pregnancy is between 11 weeks and 13 weeks 6 days.
  • If your pregnancy is earlier than about 11weeks along, your nuchal translucency screen may be re-booked
  • If you are more than 13 weeks 6 days pregnant, your prenatal care provider can give you information about other prenatal screening tests available.

How accurate is nuchal translucency screening?

The detection rate is 70%, meaning that if a baby has Down syndrome, nuchal translucency screening will detect it about 70 percent of the time.

Nuchal translucency screening does not detect all chromosome conditions, birth defects or other genetic conditions.

How is nuchal translucency screening done?

The nuchal translucency measurement is done by ultrasound. The ultrasound at this early stage in pregnancy is also used to:
  • confirm that the baby is alive
  • confirm the dates of your pregnancy
  • diagnose a multiple pregnancy
  • check for birth defects that may be visible this early in pregnancy

A measurement of the fluid underneath the skin along the back of the baby's neck will be taken. This measurement is called the nuchal translucency measurement. All babies will have some fluid here, but in some pregnancies with chromosome conditions or other health concerns, there tends to be more fluid. This nuchal measurement, along with your age and dates of your pregnancy, are entered into a computer program that calculates your risk for Down syndrome, trisomy 13 and trisomy 18.

What type of results should I expect from nuchal translucency screening?

Based on the risk assessment, you will be told whether you have:
  • A reduced-risk or “screen negative” result. This result means that the risk for your pregnancy is equal or greater than 1/300.
  • An increased-risk or “screen positive” result. This result means that the risk for your pregnancy is lower than 1/300.

As a guide, prenatal diagnostic testing (amniocentesis or chorionic villus sampling) is offered to women who are 35 years or older or women who have a risk equal or greater than 1/300.

What are the benefits to nuchal translucency screening?

For many women who receive a reduced-risk result, this risk may be very reassuring information; providing peace of mind for the rest of their pregnancy.

For women who receive an increased-risk result, the risk assessment gives them the information they need to help them make an informed choice about having a diagnostic test such as amniocentesis or chorionic villus sampling.

What are the limitations of nuchal translucency screening?

  • About 1 in 20 women will receive an increased-risk result. It is normal to be worried if you have a positive screen result.
  • An increased-risk result does not mean that your baby has a problem. Most women will have a healthy baby.
  • A reduced-risk result does not guarantee a normal baby.

How do I get my nuchal translucency screening results?

If your screening is done at the Southern Alberta Centre for Maternal Fetal Medicine, your results will be explained to you by the nurse or doctor right after your screen. This time will give you a chance to ask any other questions or talk about any concerns. As well, you will be given a copy of your screen report. The report is also mailed to your prenatal care provider.

If you are having nuchal translucency from another center in Calgary certified to do nuchal translucency screening, ask the center how you will receive your results.

Is there another screening test available this early in pregnancy?

Yes. The First Trimester Combined Screen is available in Calgary.

Tell me about prenatal diagnostic tests

Amniocentesis or chorionic villus sampling (CVS) are the two prenatal diagnostic tests that will give you a definite answer about your baby’s chromosomes.

A CVS is done between 11 and 13 weeks and an amniocentesis is done after 15 weeks. Most women who have an amniocentesis or chorionic villus sampling do not have complications following the procedure and receive reassuring chromosome results. However, these tests do place a pregnancy at increased risk for miscarriage.

How do I get more information about the prenatal screening options?

The Prenatal Genetics Clinic provides a weekly Prenatal Testing Options session that is open to all pregnant women or women considering a pregnancy. This session outlines all prenatal testing options available and addresses the most commonly asked questions. An appointment for genetic counselling in Calgary can be arranged by calling (403) 943-8375.

Table 1: Risk for having a baby with a chromosome condition based on maternal age*
Age At Delivery Down syndrome risk at mid-trimester of pregnancy Down syndrome risk at term Term risk for any significant chromosome abnormality
(including Down syndrome)
25   1/1250 1/476
26   1/1176 1/476
27   1/1111 1/455
28   1/1053 1/435
29   1/1000 1/417
       
30 1/630 1/952 1/384
31   1/909 1/384
32 1/460 1/769 1/322
33   1/625 1/317
34 1/310 1/500 1/260
       
35 1/250 1/385 1/204
36 1/200 1/294 1/164
37   1/227 1/130
38 1/120 1/175 1/103
39   1/136 1/82
       
40 1/70 1/106 1/65
41   1/82 1/51
42 1/40 1/64 1/40


* The risk at mid-trimester of pregnancy is greater than term because pregnancies carrying babies with chromosome conditions are more likely to be lost to a miscarriage in pregnancy than pregnancies carrying babies with normal chromosomes.