Nuchal Translucency Scan

Author: Dr Monica Pahuja*

What are the generally accepted indications for first trimester screening (nuchal translucency scan)?

All women who are pregnant are recommended to have a screening ultrasound at around 11 weeks gestation. (See below: NHMRC Guidelines).

In addition to screening for foetal genetic abnormalities (e.g. Down syndrome), other indications include:

• confirming viability;
• dating and confirming due date;
• assessing early foetal anatomy;
• checking position of developing placenta;
• measuring nuchal translucency to obtain a formal risk assessment;
• prior pregnancy with foetal abnormality (chromosomal or structural);
• maternal medical conditions or drug therapy;
• bleeding in early pregnancy.

What are the prerequisites for having a nuchal translucency scan done?

NHMRC guidelines: Genetics in Family Medicine: The Australian Handbook for General Practitioners: (www.nhmrc.gov.au p.13-14)

All pregnant women from 11 weeks 4 days to 13 weeks 6 days gestation are suitable for screening for foetal chromosomal abnormalities with a nuchal translucency (NT) scan.

It is recommended that the NT screening test be carried out in conjunction with a maternal blood test. Two proteins present in the maternal blood are measured. These are PAPP-A (pregnancy associated plasma protein) and free ß-subunit of human chorionic gonadotrophin (free ß-hCG). Levels of these proteins vary, but tend to be different in women who are carrying foetuses with

Down syndrome or trisomy 18. Increased free ß-hCG with decreased PAPP-A is suggestive of Down syndrome, while decreased levels of both analytes is suggestive of trisomy 18.

By having the blood test in combination with the NT screening test (‘combined risk’), approximately 85–90% of babies who have Down syndrome, and occasionally other problems, will be picked up, compared with 70% or less using NT on its own.

Approximately 5% of combined first trimester screening tests give an increased risk result. This figure varies depending on maternal age. Women with an increased risk result should be offered a diagnostic test. The majority of increased risk results are not due to Down syndrome, and most of these babies will be healthy.

Results are usually available on the day of the NT ultrasound, if blood was collected before the ultrasound at 10–12 weeks gestation, although this will depend on the local provider.

Results are provided as risks for Down syndrome and the other chromosomal trisomy 18, at the time of screening (note: in Western Australia the risk figure is adjusted to give the risk at the time of delivery). Note then that this is not a risk of delivering an affected fetus. Approximately 30% of babies with Down syndrome do not survive to term.

Depending on the state/territory, combined first trimester screening is not always available in the public sector, and there might be private out-of-pocket costs for the patient.

How do I arrange combined first trimester screening?

Nuchal Translucency Scan

Tests should be arranged a couple of weeks in advance to allow time to coordinate the blood test and ultrasound. The blood test should ideally be carried out first.

Arrange the ultrasound scan between 11 weeks 4 days and 13 weeks 6 days with a Fetal Medicine Foundation (FMF) accredited operator. This varies in individual states, but is usually carried out in hospital fetal medicine units or in accredited private radiology/obstetric practices.

Arrange a non-fasting blood test between 10 and 12 weeks gestation. Blood can be collected at the local pathology service, but the request should have clear instructions for the sample to go to the screening lab. This process varies between states, private and public sectors, and metropolitan and regional centres. For specific details regarding coordinating the results of the blood and nuchal translucency test, contact your local FMF accredited operator of choice.

Completing maternal blood test request form.

Ensure that all relevant clinical information is provided on the request form. This is necessary to calculate the ‘combined risk’. In particular:

• LMP/EDD;
• current maternal weight;
• maternal age;
• history of previous child with chromosomal abnormality;
• date and location of ultrasound screening test;
• insulin dependent diabetes;
• any other information requested on the form; for example, ethnicity, IVF details, number of foetuses.

Ensure the completed request form and blood specimen are sent to your local accredited screening laboratory.

What are the absolute contraindications for a nuchal translucency scan?

There are no absolute contraindications.

What are the relative contraindications for a nuchal translucency scan?

Obesity might result in technically poor image quality, which can make accurate measurement difficult. If transabdominal and transvaginal scans fail to provide an accurate measurement of nuchal translucency, the patient can be rescanned at 15 weeks gestation if there is clinical concern.

What are the adverse effects of a nuchal translucency scan?

There are no adverse effects. However, patient anxiety that can result from a positive screening result needs to be considered.

Is there any specific post-procedural care required following a nuchal translucency scan?

There is no specific post –procedural care required.

Are there alternative imaging tests, interventions or surgical procedures to a nuchal translucency scan?

Some alternative tests are available in the second trimester if first trimester screening is not carried out:

• maternal serum blood test between 15 and 17 weeks gestation: detects 75% of Down syndrome and 85% of neural tube defects;
• second trimester ultrasound anatomy scan (see 18–20 week Pregnancy Ultrasound);
• chorion villous sampling;
• amniocentesis;
• MRI, but this is usually carried out in specialised imaging units.

Further information about nuchal translucency scans:

Combination of serum screening with the ultrasound risk assessment has higher negative predictive value for Down syndrome than does carrying out ultrasound risk assessment alone.

Useful websites about nuchal translucency:

NHMRC Guidelines: www.nhmrc.gov.au/_files_nhmrc/file/your_health/egenetics/practioners/gems/sections/03_testing_and_pregnancy.pdf

For Further Diagnostic information and Results Interpretation see: Diagnostic Imaging Pathways www.imagingpathways.health.wa.gov.au/index.php/imaging-pathways/obstetric-gynaecological/first-trimester-screening#pathway-home; First Trimester Screening.

References

1. NHMRC guidelines: Genetics in Family Medicine: The Australian Handbook for General Practitioners: (www.nhmrc.gov.au p.13-14)

*The author has no conflict of interest with this topic.

Page last modified on 18/8/2017.