Nuchal scan (Down's Syndrome Test)

Nuchal scan - The combined ultrasound and biochemistry scan (CUBS)

This assessment is for women who wish to have a screening test for the prediction of a baby with Down's Syndrome. The test combines the woman's age, the measurement of 2 proteins (biochemical markers) in the mother's blood and a measurement of the skinfold behind the baby's neck at 11 and a half to 14 weeks' of pregnancy. A new development in first trimester screening is the assessment of the nasal bone. If you are expecting twins then the Down's risk is calculated from the nuchal ultrasound measurement and the mother's age alone.

What is the CUBS test?

The CUBS (combined ultrasound and biochemistry scan) test is based on the screening test developed by the Fetal Medicine Foundation in London.

The test is best performed between 11 and a half and 14 weeks. The scan is similar to the first 'booking' or early pregnancy scan but may take 20-30 minutes because of the additional detail required to make the measurements. The scan is usually best performed with an empty bladder as the consultant may need to press firmly on the abdomen to get the best views of the baby's neck.

The fetal nasal bone can be visualized by sonography at 11-13+6 weeks of gestation. Several studies have demonstrated a high association between absent nasal bone at 11–13+6 weeks and trisomy 21, as well as other chromosomal abnormalities. In the combined data from these studies on a total of 15,822 foetuses the fetal profile was successfully examined in 97.4% cases and the nasal bone was absent in 1.4% of the chromosomally normal foetuses and in 69% of foetuses with trisomy 21.

The results of the first 6000 screening CUBS tests have revealed that the test has a greater than 90% detection rate for Down's syndrome which is appreciably better (more accurate) than the routinely available blood test offered at 15 to 20 weeks. CUBS testing is the current 'gold standard' screening test for Down syndrome; the HeaIth Technology Assessment, an evaluation carried out by the Health Department of the Scottish Executive published in 2004 (www.nhshealthquality.org) has recommended that CUBS testing be made available to all women having babies in Scotland. It is not yet widely available in Scottish maternity hospitals.  The addition of nasal bone assessment to the CUBS test is estimated to improve the detection rate for Downs syndrome to 95%.

The blood sample will be taken by one of the clinic staff and women will be informed of their results by telephone after 2-4 days. A written report is posted to your address.

The result of the screening test is reported in two ways. Firstly, the result is categorised into a 'low risk' or a 'high risk' result. The cut-off for determining the two risk categories is 1 in 250. Ninety-five percent of women will have a 'low risk' result and no further action is usually recommended. The actual risk of Down's syndrome is also provided; the lowest risk that can be quoted is less than 1 in 20,000 whilst the highest is greater than 1 in 2.

Should a high risk of Down's syndrome be received, your consultant will discuss the implication of the result with you.

The integrated test

There is high-quality evidence to indicate that combining results of first- and second-trimester screening tests improves the diagnostic performance for Down’s syndrome and other chromosomal anomalies and is better than when either of them is used alone.

The integrated test seems to have a higher detection rate and a lower FPR compared with other currently used combined screening tests. The performance of all screening tests is well reviewed in the recently published NICE guideline on Antenatal care (www.NICE.org)

The test is a 2 stage procedure incorporating the Nuchal scan as outlined above followed by the blood test performed at 15 weeks. The results are available after 15 weeks.