Amniocentisis

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Amniocentisis

Client group: Pregrant women

Amniocentisis is a diagnostic test carried out during pregnancy. It can assess whether the foetus could develop, or has developed, an abnormality or serious health conditions.

It can test for:

  • Down's syndrome - a genetic condition that affects a person's physical appearance and mental development.
  • Spina bifida - a series of birth defects that affect the development of the spine and nervous system.
  • Sickle cell anaemia - a genetic blood disorder in which red blood cells, which carry oxygen around the body, develop abnormally.
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Ultrasound scanning

An ultrasound is carried out before the amniocentisis and this will be continued throughout the procedure. The ultrasound uses high-frequency sound waves to produce an image of the womb that is relayed to a moniter. The scan will allow health care practitioners to:

  • check position of the foetus
  • find the best place to remove some amniotic fluid
  • ensure the needle can pass safely through the walls of the abdomen and womb.
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How amniocentesis is performed

Amniocentesis is usually carried out during weeks 15-20 of pregnancy. The abdomen is cleaned with an antiseptic to prevent infection. A long thin needle is inserted through the abdominal wall using the ultrasound as a guide. The needle is passed into the amniotic sac that surrounds the foetus. A syringe is used to extract a sample of amniotic fluid which contains cells shed from the foetus that can be examined and tested for a number of conditions. The fluid is then sent to a lab for analysis.

8 in 100 women who have amniocentesis need to have the test again because not enough fluid is drawn. The procedure takes around 10 minutes, however, it will take longer if the foetus moves. Most women feel uncomfortable during the procedure but are not in pain.

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Types of testing in the lab

After the amniocentesis procedure, the sample of amniotic fluid will be taken to a lab for testing.

There are two types of test:

  • a rapid test
  • a full karotype test.
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Rapid test

A rapid test will look for abnormalities on specific chromosones. It can identify a number of chromosonal conditions that cause physical and mental abnormalities.

This will diagnose down's syndrome which is caused by an extra chromosome 21.

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Full karotype

A full karotype checks all 23 chromosomes.

The cells in the sample of amniotic fluid are grown for up to 10 days in a lab before being examined under a microscope to check for the number of chromosomes and the apearance of the chromosomes. It can detect:

Spina bifida - the fluid will be tested for the presence of alpha fetoprotein. Although amniocentesis cannot reveal the severity of spina bifida, finding high levels of alpha fetoprotein (AFP) and other proteins may indicate that the disorder is present.

Sickle cell anaemia - the fluid can be tested for this disorder which is usually caused by a single abnormal gene. At present, tests for these are usually carried out when there is someone with the condition in the family of the couple is known to be at risk for a baby with a particular genetic disease such as sickle cell anaemia. Sickle cell disease is caused by a mutation in the haemoglobin-beta gene found on chromosome 11.

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Positive result

If the result is positive, it means the baby has the disorder that was being tested for.

If the woman recieves a positive result, the implications will be fully discussed with them. Generally, there is no cure for genetic conditions, so the parent(s) will need to consider their options carefully. The options may include continuing with the pregnancy while getting information and the advice about the condition so they are prepared for caring for the baby or ending the pregnancy.

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Negative result

For most women who have amniocentesis, the results of the procedure will be negative. This means their baby will not have any of the disorders that were tested for.

It is still possible to have a negative result from amniocentesis but the baby may still be born with the condition tested for or another chromosonal condition. This is because a normal test result does not exclude every chromosomal disorder. As there are so many genes, it is not possible to test them all for every genetic mutation, so a baby may be born with a condition that was not detected.

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The importance of testing

Amniocentesis is an important test because it can assess whether the foetus could develop, or has developed an abnormality or serious health condition. 

Things that increase the risk of abnormality include:

  • the mother's age
  • the mother's medical history
  • a familty history of genetic conditions

After they get their test results, parents can then make informed decisions about their pregnancy.

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