Pregnancy Diagnostic Test: Amniocentesis

Amniocentesis is a diagnostic test that is carried out during pregnancy. It can assess whether the unborn baby (foetus) could develop, or has developed, an abnormality or serious health condition.

Amniocentesis is carried out after week 15 of the pregnancy. A needle is used to extract a sample of amniotic fluid, which is the fluid that surrounds the foetus (the developing baby) in the womb (uterus). The amniotic fluid contains cells shed from the foetus that can be examined and tested for a number of conditions.Amniocentesis can be used to detect a number of conditions such as:

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 condition that causes unusually shaped red blood cells

Chorionic villus sampling (CVS) is an alternative diagnostic test that can be carried out during weeks 10 to 13 of pregnancy.

Factors that could increase the risk of an abnormality developing include:

the mother’s age
the mother’s medical history
a family history of genetic (inherited) conditions

Why amniocentesis should be done

Diagnostic tests, such as amniocentesis, are usually only offered to women when there is a significant risk that their baby will develop a serious condition or abnormality.

This is because the procedure is quite invasive (involves going into the body) and has a small associated risks of miscarriage (the loss of the pregnancy). This risk is estimated to be around one in 100.

Amniocentesis is a diagnostic test to detect a serious, or potentially serious, disorder in an unborn baby (foetus). If amniocentesis is recommended, the healthcare professional who carries out the test will explain why they think the procedure is necessary, as well as the benefits and risks that are involved.

Amniotic fluid

Amniotic fluid (the fluid that surrounds the foetus in the womb) contains cells that have been shed from the skin of the developing baby, as well as the waste products from the baby. Every cell in the amniotic fluid that has been from the baby contains a complete set of the baby’s DNA. Therefore, the cell samples that are obtained during amniocentesis are very useful for assessing the developing baby’s health and diagnosing any potential problems.

Amniocentesis provides healthcare professionals with direct information about the likelihood of the baby developing one or more of a number of conditions, which may either be genetic (inherited) or develop during the pregnancy.

Results

If the results of amniocentesis indicate that there is a problem with the development of the foetus, it may be possible to treat the disorder while the baby is still in the womb.

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

Conditions

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

Chromosomal conditions

Amniocentesis can be used to identify chromosomal conditions. These are conditions that affect the chromosomes (the parts of the body’s cells that carry genes). For example:

Down’s syndrome - a condition that affects a person’s physical appearance, mental development and learning ability and is the result of an extra chromosome, known as trisomy-21
Edward’s syndrome – a condition that causes severe physical and mental abnormalities and is the result of an extra chromosome, known as trisomy-18
Patau’s syndrome – a rare, but serious, condition where babies rarely survive for more than a few days; it is the result of an extra chromosome, know as trisomy-13

Blood disorders

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

sickle cell anaemia - a condition where red blood cells (which carry oxygen around the body) are an unusual shape and texture
thalassaemia - a condition that affects the body’s ability to create red blood cells

Neural tube defects

Amniocentesis can test for neural tube defects. The neural tube is a primitive tissue structure that the embryo (fertilised egg) grows inside during its first month of life. As the embryo develops, the neural tube begins to change and eventually forms the spine and nervous system.

A neural tube defect can lead to a condition such as 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 the muscles to gradually weaken, resulting in an increasing level of disability.

Other genetic conditions

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

If there is a risk of an inherited condition being passed to your baby – for example, because there is a family history of the condition, your doctor, midwife or genetic counsellor will explain the risk to you and your partner. A genetic counsellor is a healthcare professional who helps people to understand and deal with genetic conditions

When amniocentesis should be done

Amniocentesis is usually performed during weeks 15 to 20 of the pregnancy. The procedure can be performed earlier than 15 weeks but, if possible, this is avoided due to an increased risk of it causing a miscarriage or club foot in the foetus (see Amniocentesis – complications).

Occasionally, amniocentesis is performed later in the pregnancy to test for an infection. For example, in rare cases, it may be used to test a pregnant woman who has been exposed to the parvovirus B19 (this causes slapped cheek syndrome – a childhood infection that causes a bright red rash on both cheeks).

Amniocentesis may be recommended when there is a high risk of the unborn baby having a serious inherited condition. This could be because:

you have had a previous pregnancy with foetal problems, such as a baby born with a chromosome abnormality
you have a family history of a condition, such as muscular dystrophy (an inherited condition that causes increasing muscle weakness)
you are over 35 years of age, which means that you have an increased risk of your child having Down’s syndrome
an earlier antenatal screening test has suggested that there may be a problem – for example, a high level of alpha-fetoprotein (AFP – a protein that is found in the blood) may indicate neural tube defects, such as spina bifida (birth defects that affect the spine and nervous system)

Amniocentesis or CVS?

Another diagnostic test, called chorionic villus sampling (CVS), can be carried out slightly earlier than amniocentesis (from about week 10 of the pregnancy). CVS tests a sample of cells that are taken from the placenta (the organ that links the mother’s blood supply with her unborn baby’s).

With CVS, the risk of miscarriage is around 1-2%, which is slightly higher than the risk of miscarriage for amniocentesis. However, because the test can be carried out earlier, you will have more time to consider the results.

Genetic counselling

If you are at risk of passing a genetic condition onto your child, your doctor or midwife will be able to discuss any appropriate tests with you, and explain why they might be necessary. In some cases, you may be referred to a genetic counsellor. They will discuss the likelihood of you passing on certain genetic conditions to your children and they will be able to offer you advice about what to do when you get the results.

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