Most pregnant women offered antenatal testing will probably be concerned about the risk of Down’s syndrome. But there are two other less well known conditions which may also be identified in antenatal tests.

These conditions are as common as cystic fibrosis and have a higher occurance rate than muscular dystrophy, yet few people have ever heard of Trisomy 18 or Trisomy 13.

In the UK, Trisomy 18 (also known as Edward's syndrome after the doctor who identified it) occurs in one out of every 3,000 live births, with three times as many girls affected. Trisomy 13 (or Patau's syndrome) occurs in one in 4,000 live births, affecting boys and girls equally.

These syndromes are the most common chromosome disorders after Down's syndrome, which is also known as Trisomy 21.

How do trisomies occur?

The ‘tri’ in the word trisomy means three. At conception, a baby inherits 23 chromosomes from its father and 23 chromosomes from its mother. These combine to create a baby with 46 chromosomes in each cell; two of each type.

Each chromosome holds a particular "position" in the cell and carries certain genetic material. A trisomy occurs when a baby has three chromosomes in one position instead of the normal two.
 

On average, 400 babies a year are born with Trisomy 13 or 18, with many cases diagnosed late in pregnancy. Most trisomies (around 95 per cent) are full trisomies. That is, the extra chromosome occurs in every cell in the baby's body. This type of trisomy is not hereditary, and is not due to anything the parents did or did not do. 

Because up to half of all pregnancies are miscarried – frequently before a woman even knows she is pregnant - many couples do not know their baby had a chromosomal abnormality.

Unlike Down’s syndrome, Trisomy 18 and Trisomy 13 are usually fatal, with most affected babies dying before birth, and those who do make it to birth typically living only a few days. However, a small number of babies (less than 10 per cent) live at least one year, and some children have survived into their teens.

Experts know how a trisomy happens but not why it happens. What they do know is that the risk of having a baby with trisomy increases with age. However, more babies with trisomy are born to mothers in their twenties because more babies are born to women in this age group.

Characteristics of Trisomy 18 and 13

The characteristics of Trisomy 18 include heart and kidney defects, oesophageal atresia (the oesophagus does not connect to the stomach, meaning the baby cannot eat by mouth), clenched hands, deformed feet – known as rocker bottom feet, micrognathia (small jaw), as well as severe developmental delays.

As with Trisomy 18, babies born with Trisomy 13 may have heart and kidney defects and rocker bottom feet. Other characteristics include extra fingers and toes, a small head (microcephaly), failure of the brain to divide into halves during gestation (holoprosencephaly), absent or malformed nose, cleft lip and/or cleft palate and severe mental deficiency.

Diagnosis
If any likely abnormalities are identified during an ultrasound scan, then the chance of a trisomy is increased and the parents will be offered further tests to check (e.g. an amniocentesis or placental biopsy).

Both tests involve a small needle being passed into the uterus. The drawback with both these diagnostic procedures is the associated risk of miscarriage. Only you and your partner can decide whether the chance of having a child with trisomy is high enough to accept the increased miscarriage risk if you undergo further tests. Some couples see a risk of 1 in 100 as a 99 per cent chance of things being OK and so make the decision not to have further diagnostic tests. Another couple might think a risk of 1 in 1000 is enough to warrant more investigation. The decision really has to come from you. 

Further information and advice

A good resource for information and support is the Support Organisation for Trisomy 18, 13 and other Related Disorders (S.O.F.T.).

Last Modified: 13/05/2008