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What happens during nuchal translucency screening?

There is a collection of fluid under the skin at the back of every foetus' neck. This pocket of fluid can be measured using ultrasound when the foetus is between 11 and 13 weeks plus six days gestation. A great number of foetuses with Down's syndrome have larger amount of fluid.

A nuchal translucency (NT) scan is a screening test for Down's syndrome. A screening test is only an estimate of the risk of the baby having Down's syndrome. A diagnostic test, such as CVS or amniocentesis gives a definite diagnosis, but also runs a small risk of miscarriage. But based on the results of the NT, you decide can decide whether or not to have a diagnostic test.

Before 11 weeks gestation, the scan is difficult to do because the foetus is so small and after 14 weeks, the developing lymphatic system may absorb any excess fluid. The scan is usually done through your abdomen, but on occasion it may be necessary to have a vaginal scan. Vaginal scans give better views. There are no risks to having a NT scan.

The sonographer will measure the foetus from the top of the head to the bottom of the spine in order to date your pregnancy accurately. They will then measure the width of the NT. The skin looks like a white line and the fluid under the skin will look black. You will be able to see the foetus' head, spine, limbs, hands and feet on the screen. It is recommended that you also have a detailed anomaly scan at about 20 weeks gestation.

At about 11 weeks an NT measurement of up to 2mm is normal and by the end 13 weeks up to about 2.8mm. The NT should normally grow in proportion with the foetus. Some foetuses without Down's have increased fluid, so an increased NT does not necessarily mean there is a problem. Nine out of 10 babies with a measurement between 2.5mm and 3.5mm will be completely normal. The risk of Down's syndrome and other chromosomal abnormalities increases as the NT increases.

The NT scan also shows the risks of two other chromosomal abnormalities, Edwards' syndrome and Patau's syndrome, which are much rarer than Down's syndrome, and most of these pregnancies will miscarry. The NT scan is accurate up to up about 75 per cent in detecting Down's syndrome. Sometimes, a screening test can give a false positive, where a normal foetus has been predicted to have Down's. The false positive rate for an NT scan is about five per cent. A more accurate result can be achieved by combining an NT scan with a blood test. The blood test measures the levels of the protein PAPP-A and the hormone free beta-HCG. Foetuses with Down's syndrome tend to have low levels of PAPP-A and high levels of HCG. When the tests are combined, the detection rate jumps to 90 percent.
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