Kel the Downs scan is called the Nuchal Translucency NT for short.
In the fetus fluid collects behind the neck, much like it does in dependent ankle oedema in later life. This occurs partly because of the fetus’s tendency to lie on its back and partly because of the laxity of the skin of the neck. As with ankle oedema this accumulation of fluid can represent the end point of several pathological processes, including heart failure. Fluid collecting behind the neck can be detected as nuchal translucency by ultrasound scanning, and it can be measured. The more fluid that has accumulated, the greater the risk of an abnormality being present.
Chromosomal abnormalities—for example, Down’s syndrome—can cause fluid accumulation. Chromosome 21 contains the gene that codes for type VI collagen. In trisomy 21 one subunit of this collagen can be overexpressed, resulting in connective tissue that has a more elastic composition.
Failure of fetal movements is also likely to cause nuchal thickening. Neuromuscular abnormalities can cause poor breathing and body movements, both of which may lead to fluid collecting as happens in peripheral oedema. One example is arthrogryposis, which causes contractures and flexion deformities and can be fatal. Other causes are intrathoracic and extrathoracic compressive syndromes. If the thoracic cage is abnormally narrow or an intrathoracic lesion is present (such as a diaphragmatic hernia) the vessels in the fetus’s head and neck become congested and oedema occurs. Likewise, if heart failure occurs (due to congenital cardiac abnormalities or cardiac dysfunction) more extravascular fluid will form.
There is a brief opportunity between 10 and 14 weeks’ gestation (when the fetal lymphatic system is developing and the peripheral resistance of the placenta is high) to detect abnormal fluid collections. 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 measuring nuchal translucency.
About 90% of fetuses with a nuchal translucency measurement of 3mm (“high”) at 12 weeks’ gestation are normal at birth, while 10% have major abnormalities. Only 10% of babies with a measurement of 6mm (“very high”) at this time will be found to be normal. In centres that perform nuchal translucency measurements, once a fetus has been identified as having a high nuchal translucency, mothers are usually offered chorionic villus sampling and a repeat scan about two weeks later to exclude chromosomal or major physical abnormalities.