Glossary

Amniocentesis
Amniocentesis is a procedure to remove a small amount of amniotic fluid from around the baby in the womb in order to look at the baby’s chromosomes.

Cells
The human body is made up of cells. Each cell has chromosomes that produce the materials needed for life.

Chromosomes and genes
Chromosomes are made of genes and carry our genetic information. There are 46 chromosomes (23 pairs) in most body cells.
The pairs of chromosomes are numbered from pair number 1, the largest chromosomes to pair number 22, the smallest. The 23rd pair is the sex chromosomes, XX usually in a female and XY in a male. Genes are units of genetic material. They control how human beings grow, develop and function. Genes are passed on from parents to the children.

Down’s syndrome
Down’s syndrome or trisomy 21 is a genetic condition that is caused by having an extra chromosome (chromosome 21). The extra chromosome is usually present from the time of conception. It is not known what makes this happen. The extra chromosome cannot be removed from cells and so there is no cure even if the condition is diagnosed before the baby is born. Find more about Down’s syndrome

Edwards’ syndrome (trisomy 18)
Edwards’ syndrome or trisomy 18 is a very rare chromosome abnormality. The incidence of this condition is about 1 in 3000 live births. It is less common than Down's syndrome. Edwards' syndrome occurs when there is an additional chromosome 18. Find more about Edwards’ syndrome

Genes
Genes are units of genetic material. They control how human beings grow, develop and function. Genes are passed on from parents to the children.

Karyotype
A karyotype is a description of how a person’s chromosomes look under the microscope. It can identify alterations in the number, size and arrangement of chromosomes. It cannot identify small changes in genetic material.

Karyotype Test
To do this test the cells from the baby that are floating in the amniotic fluid are grown (cultured) in the laboratory and examined under a microscope.

Patau’s syndrome (trisomy 13)
Patau’s syndrome or trisomy 13 is a very rare chromosome abnormality. The incidence of this condition is 1 in 7000 live births. It is less common than Down’s syndrome. Patau’s syndrome occurs when there is an additional chromosome 13. Babies with Patau’s syndrome can have multiple, severe problems such as heart defects, brain abnormalities and severe kidney abnormalities. Most babies do not live beyond the first weeks of life and few survive beyond one year of life. Find more about Patau’s syndrome

PCR Test
The PCR (Polymerase Chain Reaction) test provides results about some of the chromosomes in three working days. It can only tell you whether your baby has Down’s syndrome, Edwards’syndrome or Patau’s syndrome.

Screening test for Down’s syndrome
There are a number of screening tests for Down’s syndrome available. Screening tests do not show definitely whether a baby has Down’s syndrome or not. They show the woman's chance of having a baby with Down’s syndrome. These tests do not pick up all babies affected by Down’s syndrome.

On average, for every 10 babies with Down’s syndrome, depending on the test used, only six to nine will be picked up by screening. This means one to four out of every 10 babies with Down’s syndrome will be missed by the screening tests.
The triple test can be done from 15 to 18 weeks of pregnancy. This test measures three chemicals in your blood: alpha-fetoprotein (AFP), unconjugated oestriol (uE3), and beta human chorionic gonadotrophin (βhCG) (ref 1). Some tests use a measurement of the baby’s neck (a nuchal translucency measurement), to work out the chance of your baby having Down’s syndrome. A computer programme uses the results of the blood test, your age, your weight, and the number of weeks pregnant you are and some tests use a measurement of the baby’s neck (a nuchal translucency measurement), to work out the chance of your baby having Down’s syndrome.

Some people pay privately to have Down’s syndrome screening. Screening done privately is not audited by the NHS. This means that your midwife will have no information about the quality and accuracy of any screening tests offered by private clinics. As with the NHS test, a private test cannot tell you whether or not the baby has Down’s syndrome but will give a chance of this happening.

Ultrasound scans
Ultrasound scans are a way of producing pictures of inside the body using sound waves.