NT Scan

nt scan 2Personalised risk for Down Syndrome

Babies with Down syndrome and heart failure tend to have extra fluid at the back of the neck (arrow).

Only sonographers with the “Certificate of Competence from the Fetal Medicine Foundation in London” are provided with the Computer Evaluation Programme to

perform this Down syndrome risk assessment.

Each woman will be given an estimate of her individual risk for Down syndrome in this pregnancy.

This is calculated by taking into account the age of the mother, measurement of two hormones in the mother’s blood  and the scan findings. This test can detect about 90%-95% of fetuses with Down syndrome.

fetalThe factors assessed at the 1st Trimester NT scan are summarised as

  • Age of the mother
  • Amount of fluid behind the neck of the fetus (nuchal translucency)
  • Presence or absence of the fetal nasal bone
  • Fetal heart rate
  • Blood flow through the tricuspid valve of the fetal heart
  • Blood flow through the ductus venosus in the fetal liver
  • Presence or absence of any major structural fetal abnormality
  • Level of two hormones (free ß-hCG and PAPP-A) in the mother’s blood

What is Downs Syndrome?

The term ‘syndrome’ is used to describe a collection of features which are often seen together. Down syndrome was first identified by Dr John Langdon Down in 1866, who noticed a similarity in appearance in some of his patients. These individuals possessed a broad, flat face, a thick tongue and a small nose and were intellectually impaired to a variable degree.

However, there are more differences than similarities between people with Down syndrome. Many babies with the condition will have one or all of the following features at birth: low muscle tone (a floppy baby), a face that appears flatter with eyes slanting upward, small ears and a wider neck than usual, a crease across the palm of the hand and a gap between the toes. Some may have heart problems. While intellectual disability is a feature of the syndrome, those with the condition will develop and learn throughout life, but at a slower pace than usual.

There is no cure for a child born with this condition but many symptoms can be treated and special early intervention programs are enabling these individuals to develop their potential. A child with Down syndrome can usually do most things that any young child can do such as walking, talking, dressing and being toilet trained although they may do these things later than other children.
child

Chromosomes

In each human cell, except for the egg and sperm cells, there are 46 chromosomes, made up of 23 pairs. The chromosomes are numbered according to their size. When egg and sperm cells are formed the number of chromosomes they contain is halved to 23 with only one copy of each pair. A baby is formed when the sperm from the father fertilises the egg from the mother. The baby will now have 46 chromosomes, just like the parents, with one copy from each parent.

Sometimes, when the egg and sperm are forming, a mistake occurs so that the chromosome pairs do not separate in an ordered fashion. The result is an egg or sperm cell that has only 22 chromosomes while others have 24 chromosomes. If an egg or sperm carrying 24 chromosomes combines with an egg or sperm carrying the usual 23 chromosomes, the result will be an individual with cells in which there are 47 chromosomes instead of the usual 46. Thus there will be three copies of a particular chromosome in the cells rather than two.

Down syndrome is caused by an extra copy of chromosome number 21. For this reason it is also known as Trisomy 21. The presence of the extra chromosome causes the mental and physical characteristics of Down syndrome. In 95% of cases the extra chromosome is present in all the cells of the baby. In about 1% of cases, some of the baby’s cells will contain the usual chromosome number of 46 and others will contain the extra chromosome 21. This situation is known as ‘mosaic’ Trisomy 21. In about 4% of cases, the extra copy of chromosome 21 is attached (translocated) to another chromosome. This is called ‘translocation’ Trisomy 21. This type of Downs syndrome can be inherited. So where there is a previous child with Down syndrome or a close family history of Down syndrome, a discussion with a genetic counsellor can provide information about the risk of having an affected child and the availability of testing.

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