Edward Syndrome—Trisomy 18
Trisomy 18, also known as Edward syndrome is a chromosomal condition in which there are three copies of the chromosome 18 in the cells of the body instead of the normal two. Trisomy 18 occurs in about one in 5500 live births and is associated with approximately one per cent of all miscarriages.
Characteristics of Trisomy 18
The term syndrome is used to describe a collection of distinctive characteristics that are often seen together. Babies with Trisomy 18 have recognisable characteristics referred to as Edward syndrome. Dr John Edward first described the specific features associated with this chromosomal abnormality.
Babies with Edward syndrome or Trisomy 18 are likely to have many of the following characteristics (but not necessarily all of them):
- Severe feeding difficulties with poor growth and weight gain.
- Poor muscle tone and episodes when spontaneous breathing may temporarily stop.
- Facial features which may include widely spaced eyes with drooping eyelids, an upturned nose, low-set malformed ears and a small jaw and mouth. The back of the head is also usually prominent.
- Hand and feet abnormalities including overlapping fingers, webbing of the toes and talipes (clubfoot).
- A small pelvis with limited hip movement and a short breast bone.
- Kidney and heart defects which are often severe.
- An increased risk of spina bifida and facial clefts.
- Severe developmental delay and intellectual disabilities.
These characteristics are common for babies with Trisomy 18, but it is important to remember that each baby will be different with their own unique appearance.
Cause of Trisomy 18
Trisomy 18 is caused by an extra copy of the chromosome number 18. It appears to affect females three to four times more frequently than males. The chance of having a baby with Trisomy 18 increases with maternal age. It also increases slightly if the mother has had a previously affected pregnancy. It is estimated that every three to four fertilised eggs out of ten have a chromosomal problem and this rate increases with the mother's age. The majority miscarry early often before they have even been recognised as a pregnancy.
Short life expectancy
Sadly the prognosis for babies with Trisomy 18 is very poor. At least 30 per cent of babies with Trisomy 18 are stillborn and there is generally a higher chance of males being stillborn. Some babies will survive pregnancy and labour but few of these will survive the first week of life. It is very rare for a baby with Trisomy 18 to survive the first year of life.
Normally egg and sperm cells have 23 chromosomes each which join to form a future baby with 46 chromosomes (23 plus 23) in each cell. During fertilisation each of the chromosomes from the egg and the sperm fuse and separate. If this separation does not occur properly, a cell with three copies of the chromosome is the result. This cell starts rapidly copying itself to eventually form a baby with each of its cells having the extra copy of the chromosome 18. It is this extra chromosome that causes the intellectual and physical characteristics of Trisomy 18.
Mosaicism and translocation
In occasional cases of Trisomy 18 the extra copy of the chromosome 18 is not in every cell. This situation is called
Mosaicism (mixture) and is likely to result in less severe characteristics of Edward syndrome.
Very rarely, either of the parents are carriers of a copy of the chromosome attached to a different chromosome. This is called a translocation (rearrangement) and may not affect the parents. However future pregnancies are more likely to be affected when further rearrangements of the chromosomes may occur. In these instances genetic counselling is recommended.
Antenatal screening tests for Trisomy 18 have been developed and are performed in conjunction with Down syndrome screening tests. These tests include the first trimester combined screening test (involving the nuchal translucency ultrasound measurement and blood test between 11+0 and 13+6 weeks), the second trimester triple test (blood test) and obstetric ultrasound between 18 and 20 weeks. Approximately 90 per cent of affected babies are detected at screening.
Trisomy 18 can be suspected prenatally (before birth) or after birth; however the diagnosis can only be made after chromosomal analysis. Diagnosis can be confirmed prenatally by chromosomal analysis on samples obtained following ultrasound-guided procedures such as chorionic villus sampling (CVS) or amniocentesis. For further information on prenatal testing please refer to Mater Mothers' Hospital's brochure: Prenatal diagnostic procedures.
Trisomy 18 is a chromosomal abnormality which cannot be treated or prevented. If born alive, admission to the Neonatal Critical Care Unit is usually necessary for a baby to survive after birth.
Some parents may instead elect perinatal palliative care for their baby, keeping their baby with them or nearby until they pass away.
All families with an affected baby will be offered counselling by a neonatologist or baby doctor regarding the management of their baby
There is approximately a one per cent chance of conceiving a baby with Trisomy 18 in a future pregnancy. This percentage progressively increases for mothers over the age of 35.
Genetic Health Queensland (GHQ) provides a genetic counselling service three days a week at Mater's Centre for Maternal Fetal Medicine. A genetic counsellor is available to discuss all aspects of screening and diagnostic testing for Trisomy 18. To speak to a genetic counsellor please telephone 07 3163 1593.
For further information, we recommend you speak to your doctor. Alternatively, you may find the following websites useful:
Centre for Maternal Fetal Medicine
Mater Mothers' Hospital
South Brisbane Qld 4101
Phone: 07 3163 1896
Fax: 07 3163 1890
Mater acknowledges consumer consultation in the development of this patient information.
Last modified 16/11/2015.