Nuchal scan

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Nuchal Scan is a prenatal screening ultrasound scan to help identify higher risks of a Downs syndrome affected fetus, particularly for older mothers who have higher risks of such pregnancies. The scan is carried out at 11-13 weeks pregnancy and assesses the amount of fluid behind the neck of the fetus - also known as 'the nuchal translucency'. Babies at risk of Downs tend to have a higher amount of fluid around the neck.

All women, whatever their age, have a small risk of delivering a baby with a physical and/or mental handicap. The commonest genetic disorder is Downs syndrome with the risk rising with maternal age from 1 in 1530 pregnancies at aged 20, to 1 in 30 at aged 44.^[1] Whilst the only way to know for sure whether or not the fetus has a chromosomal abnormality is by having an invasive test such as an amniocentesis, such formal tests carry an approximate 1% risk of causing a miscarriage, whether or not the fetus is normal or affected with Downs. Women with a low risk of having a Downs affected fetus may wish to avoid the discomfort and risks of formal testing.

The aim of the nuchal scan is to more accurately estimate the risk of the fetus having Downs syndrome than relying upon calculations based on maternal age alone. Only those women with significantly higher risks than that predicted for their age group, or those with an estimated risk above that of amniocentesis's miscarriage rates then need undergo the more formal testing.

For even greater accuracy of predicting risks, the outcome of the nuchal scan may be combined with the results of simultaneous maternal blood tests. The blood test is used to measure the levels of certain hormones - usually free ß-hCG and PAPP-A. In pregnancies affected by Downs syndrome there is a tendency for the levels of free ß-hCG to be increased and PAPP-A to be decreased. Over all detection rate using NT, free ß-hCG and PAPP-A is 85% (4.2% of these are advised to have an amniocentesis).

Whilst the nuchal scan, like any obstetric ultrasound-scan, may help confirm both the accuracy of the pregnancy dates and the foetal viability, its high definition imaging may also detect other less common chromosomal abnormalities.