Amniocentesis is a diagnostic test to detect a serious or potentially serious disorder in an unborn baby (foetus).

Amniotic fluid

Amniotic fluid surrounds the foetus in the womb. It contains cells shed from the skin of the developing baby and the baby's waste products.

Every cell in the amniotic fluid contains a complete set of the baby's DNA. This means that cell samples obtained during amniocentesis can be used to assess the health of the developing baby and diagnose potential problems.

Amniocentesis provides healthcare professionals with important information about the likelihood of a baby developing one or more inherited genetic conditions, or conditions that develop during the pregnancy.

If a serious abnormality is detected, amniocentesis enables parents to choose whether to continue with the pregnancy or terminate it at an early stage.

Conditions

Amniocentesis can be used to help diagnose many different conditions. Some are described below.

Chromosomal conditions

Chromosomal conditions affect the chromosomes (the threadlike structures inside cells that carry genetic instructions in the form of genes). For example:

• Down's syndrome – a syndrome that affects physical appearance, mental development and learning ability; it's caused by an extra chromosome 21, giving it it's other name, trisomy 21
• Edward's syndrome – a syndrome that causes severe physical and mental abnormalities; it's caused by an extra chromosome known as trisomy 18
• Patau's syndrome – a rare but serious syndrome where babies usually don't survive for more than a few days; it's caused by an extra chromosome known as trisomy 13 

Blood disorders

Amniocentesis can also be used to check for inherited blood disorders such as:

• sickle cell anaemia – an inherited genetic condition that causes a person's red blood cells to develop abnormally 
• thalassaemia – a group of inherited blood disorders where the part of the blood known as haemoglobin is abnormal; the abnormality affects the red blood cells' ability to function normally 
• haemophilia – a condition that affects the blood's ability to clot

Neural tube defects

Amniocentesis can test for neural tube defects.

The neural tube is a collection of cells that form very early in the development of an embryo. The top of the neural tube eventually forms the brain and the remainder becomes the spinal cord.

If the neural tube isn't completely closed, it can cause a defect to develop, such as a hole in the spinal column. These defects often occur within the first month of pregnancy.

The most common neural tube defect is spina bifida, which can cause learning difficulties and paralysis (weakness) of the lower limbs.

Musculoskeletal disorders

Amniocentesis can also be used to diagnose conditions that affect the musculoskeletal system (your bones and muscles), such as muscular dystrophy.

Muscular dystrophy is an inherited condition that causes muscles to gradually weaken, resulting in an increasing level of disability.

Other genetic conditions

As well as helping identify chromosomal conditions, blood disorders, neural tube defects and musculoskeletal disorders, amniocentesis can also help diagnose a number of genetic conditions, such as Marfan syndrome. Marfan syndrome affects the tissues that provide support and structure in the body.

If there's a risk of an inherited condition being passed to your baby – for example, if there's a family history of the condition – your GP, midwife or genetic counsellor will explain the risk to you and your partner.

Genetic counsellors are healthcare professionals who help people understand and deal with genetic conditions.

Read more about genetic testing and counselling.