Why do they think that our baby has Down syndrome?

Babies with Down syndrome do look slightly different from other babies and therefore they can usually be identified at birth.

Some of the typical features include:

  • eyes that slant upwards
  • tiny folds covering the inner corners of the eyes (epicanthic folds)
  • small white flecks on the iris (Brushfield spots)
  • a small nose with a broad, flat bridge
  • a small mouth which makes the tongue appear large
  • small low set ears
  • a single crease on the palm of the hand
  • short hands and fingers
  • inclining pinkie
  • wider gap between the big toe and second toe
  • A slightly flattened appearance of the back of the head.
  • Other Features

  • Unusual looseness of the joints
  • Poor muscle tone (hypotonia) making the baby feel and appear “floppy”
  • Loose skin folds at the back of the neck
  • Heart defects occur in about 50% of cases
  • Eye defects occur in 60% of cases
  • Hearing defects may occur and can affect speech and language
    Developmental delay (intellectual disability varies from mild to moderate)

Not all these traits mentioned are found in every child with Down syndrome. Like other children, children with Down syndrome resemble their parents, e.g. hair and eye colour but they also have some of the typical features of Down syndrome. Furthermore each person with Down syndrome is a unique individual and can vary enormously in appearance, temperament and ability. It is also important to note that a person’s appearance has nothing to do with his intellectual ability. Therefore a person who has more of these above mentioned characteristics, is not necessarily more intellectually impaired. Down syndrome is not diagnosed only on the basis of physical characteristics, it must be confirmed by means of a chromosome analysis. This is a special type of laboratory test performed on a sample of blood from the baby. Your doctor will be able to tell you more about this test.

Where does the name “Down syndrome” come from?

Down syndrome is named after Dr Langdon Down, the physician who first described its features in 1866. The word “syndrome” means “a collection of signs and symptoms usually found in combination.

What causes Down syndrome?

Down syndrome is caused by extra genetic material. The genetic material is present in every human body cell and is arranged in tiny structures called chromosomes. They are the building blocks, which give us our individual characteristics. For example they determine the color of our hair and our eyes and many other characteristics. Each human body cell contains 23 pairs (46) chromosomes, except the sex cells (egg cells of a woman and sperm of a man), which each contains only 23 chromosomes. Egg cells and sperm are formed by a special cell division process during which the chromosome pairs divide and only one chromosome of each pair is included in a sex cell. The egg cell and sperm therefore each receives only one copy of each chromosome pair. During fertilization a sperm of the father fuses with an egg cell of the mother. The fertilized egg cell then contains 23 pairs of chromosomes (46 in total). One chromosome of each pair comes from the father and the other one of each of the pairs from the mother. The fertilized egg cell divides rapidly in the womb and eventually the baby is formed. Each of the body cells of the baby also contains 23 pairs (46) chromosomes. When a blood sample of a baby is analysed in a laboratory, a photograph of the chromosomes is taken under a microscope. The chromosomes on the photo are then cut out, numbered and arranged according to pairs. Such a chromosome arrangement is called a karyotype. Note that the chromosomes are arranged and numbered according to a specific pattern, namely from large to small. One of the smallest chromosomes is numbered as number 21. It is the chromosome number 21 that is involved in the origin of Down syndrome.

It is important to understand that Down syndrome is not caused by anything the mother or father did or failed to do during the pregnancy.

There are different types of Down syndrome:

Trisomy 21 type of Down syndrome

Trisomy 21 is the most common type of Down syndrome and is the cause of approximately 90-95% of all cases of Down syndrome. Trisomy type of Down syndrome is not hereditary! In young mothers the chance of a second baby being born with Down syndrome is therefore very small. Trisomy 21 (also known as non-disjunction type) occurs when the two number 21 chromosomes fail to separate and both instead of one, become incorporated into either the egg cell or the sperm. This cell then has 24 chromosomes instead of the normal 23. This phenomenon of the chromosome not separating is called non-disjunction. With fertilisation, the sex cell of one of the parents (egg cell or sperm) with the two number 21 chromosomes (and therefore 24 chromosomes in total), fuses with the normal sex cell of the other parent (egg cell or sperm with 23 chromosomes) to form a fertilized egg cell with 47 chromosomes. When this fertilized cell starts to divide, it eventually develops into a baby who has an extra number 21 chromosome in each body cell and hence Down syndrome. Trisomy 21 may be caused by non-disjunction during the formation of either the egg cells of the mother or the sperm of the father. The chances of Trisomy 21 however seem to be increased in women in their late thirties and older.

The Mosaic type of Down syndrome

This type of Down syndrome is very rare, not hereditary and the chance of a second baby born with it is small. Mosaic type is also known as Mosaicism. This is caused by non-disjunction of the chromosome pair number 21 shortly after fertilisation. The cells of the person with this type of Down syndrome have a mosaic pattern. The result is that some cells contain an extra chromosome number 21 (and therefore 47 chromosomes) while the other cells contain only 46 chromosomes (the normal number). Such a baby may only show partial features of the condition. Some of these babies look less affected than other babies with Down syndrome, but not all of them do.

The Translocation type of Down syndrome

A baby sometimes has all the clinical symptoms of Down syndrome but only 46 chromosomes in every cell. With Translocation, the baby has a normal number of chromosomes but extra chromosomal material. In these cases it is found that, over and above a normal pair number 21, these babies also have an extra part of a chromosome number 21, attached to another chromosome, which usually is chromosome number 14. This is called translocation type of Down syndrome. This type of Down syndrome can be hereditary! Either parent can carry a translocation without showing any symptoms. In this case the risk to have another child with this type of Down syndrome for the parents concerned, or for other members of the family, can be quite high. It is strongly advised that a chromosome test is done on both parents and that they seek genetic counseling if they plan further pregnancies. It is important to remember that the type of Down syndrome that is diagnosed makes no difference to the eventual development and potential of the child.