Nuchal scan

Nuchal Scan is a sonographic prenatal screening scan (ultrasound) to help identify higher risks of Down syndrome in fetuses, 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 Down tend to have a higher amount of fluid around the neck.

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.

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Indication

All women, whatever their age, have a small risk of delivering a baby with a physical and/or mental handicap. The most common genetic disorder is Down 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 or chorionic villus sampling, such tests carry an approximate 1% risk of causing a miscarriage, whether or not the fetus is normal or affected with Down. Most women, especially those with a low risk of having a Down-affected fetus, may wish to avoid the risk to the fetus and the discomfort of invasive testing.

The aim of the nuchal scan is to estimate the risk of the fetus having Down syndrome more accurately than calculating 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 the fetal loss rate associated with amniocentesis are advised to proceed to invasive testing.

Procedure

Nuchal scan is performed between the 11th and 13th week of gestation, because the accuracy is best in this period. The scan is obtained with the fetus in sagittal section and a neutral position of the fetal head (neither hyperflexed nor extended, either of which can influence the nuchal translucency thickness). The fetal image is enlarged to fill 75% of the screen, and the maximum thickness is measured, from leading edge to leading edge. It is important to distinguish the nuchal lucency from the underlying amnionic membrane.

Normal thickness depends on the Crown-rump length (CRL) of the fetus. Among those fetuses whose nuchal translucency exceeds the normal values, there is a relatively high risk of significant abnormality.

Accuracy

Between 65 and 85% of trisomic fetuses will have a large nuchal thickness. Further, other, non-trisomic abnormalities may also demonstrate an enlarged nuchal transparency. This leaves the measurement of nuchal transparency as a potentially useful 1st trimester screening tool. Abnormal findings allow for early careful evaluation of chromosomes and possible structural defects on a targeted basis.

At 12 weeks of gestational age, an "average" nuchal thickness of 2.18mm has been observed, however, up to 13% of chromosomally normal fetuses present with a nuchal luncency of greater than 2.5mm, and thus 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 hormones - primarily hCG and PAPP-A. In pregnancies affected by Down syndrome there is a tendency for the levels of hCG to be increased and PAPP-A to be decreased. Over all detection rate for using the combined method is 85% (4.2% of these are advised to have an amniocentesis).

Development of nuchal translucency

The translucent area measured (the nuchal translucency) is only usable to measure between 10 and 14 weeks’ gestation, when the fetal lymphatic system is developing and the peripheral resistance of the placenta is high. After 14 weeks the lymphatic system is likely to have developed sufficiently to drain away any excess fluid, and changes to the placental circulation will result in a drop in peripheral resistance. So after this time any abnormalities causing fluid accumulation may seem to correct themselves and can thus go undetected by making a nuchal scan.

See also

• Prenatal diagnosis which further disusses the reasons for prenatal screening and the ethics of such testing.
• List of obstetric topics
• Obstetric ultrasonography

References

1. ^ Kypros Nicolaides. Patient Information - First trimester screening for Down syndrome. Fetal Medicine Centre. Retrieved on 2006-07-04.