Trisomy disorders

Genes are the blueprint for our bodies#

Almost every cell in the body holds a copy of the body’s blueprint, stored inside a structure called the nucleus. Genes sit along chromosomes, which are tightly bundled strands of the chemical deoxyribonucleic acid (DNA). Humans usually have 23 pairs of chromosomes. One pair is the sex chromosomes, which determine sex, while the other 44 chromosomes guide other features such as growth and how the body works.

A chromosome condition is caused by a change in the number or structure of chromosomes. Trisomy, which means “three bodies”, describes having three copies of a particular chromosome instead of the usual two. A person with a trisomy therefore has 47 chromosomes rather than 46.

Down syndrome, Edward syndrome and Patau syndrome are the most common forms of trisomy. Children affected by a trisomy often have a range of birth differences, which may include delayed development and intellectual disability. The extra chromosome usually appears spontaneously around the time of conception.

The reason this happens is not known, and it cannot be prevented.

Maternal age is the main risk factor#

The chance of a trisomy condition increases with the mother’s age. Women in their late 30s and 40s have a higher chance of having a pregnancy affected by a trisomy.

Down syndrome is also known as Trisomy 21, because the person has three copies of chromosome 21 instead of two. It is the most common trisomy at birth.

There are three types of Down syndrome#

• Standard Trisomy 21 is the most common form. Here the father’s sperm or the mother’s egg cell carries the extra chromosome.
• Mosaic Down syndrome occurs when the extra chromosome appears spontaneously as the embryo develops.
• Translocation Down syndrome, which accounts for about five per cent of cases, can be inherited.

Some of the physical features of Down syndrome may include a flatter facial profile and a smaller body size. Babies with Down syndrome are often smaller and weigh less at birth than other babies.

Children with Down syndrome tend to grow more slowly and are commonly smaller than other children of the same age, and adults with the condition are commonly smaller than other adults. Everyone with Down syndrome will have some delay in their development and some level of learning difficulty.

Edward syndrome (Trisomy 18)#

Edward syndrome is also known as Trisomy 18, because the person has three copies of chromosome 18 instead of two. Some of its features may include:

• malformations of the hands and feet, such as missing thumbs or club feet
• webbing between the fingers or toes (syndactyly)
• a neural tube defect in which the spinal cord and blood vessels protrude through a gap in the vertebrae (myelomeningocele)
• malformations of the sex organs

Survival beyond the newborn period is uncommon for babies with Edward syndrome.

Patau syndrome (Trisomy 13)#

Patau syndrome is also known as Trisomy 13, because the person has three copies of chromosome 13 instead of two. Some of its features may include:

• a neural tube defect in which the spinal cord, its coverings (meninges) and blood vessels protrude through a gap in the vertebrae (myelomeningocele)
• malformations of the sex organs

Survival beyond the newborn period is uncommon for babies with Patau syndrome.

Signs during pregnancy#

Sometimes signs of a trisomy condition can be seen during pregnancy. These may include:

• congenital differences such as cleft palate or heart irregularities seen on an ultrasound scan
• too much amniotic fluid around the baby (polyhydramnios)
• only one artery in the umbilical cord
• a smaller than expected placenta
• a baby that is small for its gestational age or less active than expected

Tests that can help detect trisomy conditions#

• ultrasound scans, which use sound waves to create a picture of the baby
• maternal serum screening, a specialized blood test
• non-invasive prenatal testing (NIPT), a screening test that measures fetal DNA circulating in the mother’s blood
• chorionic villus sampling, in which a sample of cells is taken from the chorion, the tissue that will become the placenta, and examined
• amniocentesis, in which a sample of amniotic fluid is taken and examined

Genetic counseling#

If your child has been diagnosed with a trisomy condition, it may help to speak to a genetic counselor. Genetic counselors are health professionals qualified in both counseling and genetics.

As well as offering emotional support, they can help you understand your child’s condition, what causes it and what the diagnosis may mean for your child’s health and development. Genetic counselors are trained to provide information and support that is sensitive to your family’s circumstances, culture and beliefs. They can also help connect you with support groups and with other individuals and families affected by trisomy conditions.

Key points#

• a chromosome condition is caused by a change in the number or structure of chromosomes
• Down syndrome, Edward syndrome and Patau syndrome are the most common forms of trisomy
• the cause is unknown and the condition cannot be prevented
• the most important risk factor for trisomy conditions is maternal age
• Down syndrome is also known as Trisomy 21, because the person has three copies of chromosome 21 instead of two

Where to get help#