Amniocentesis

Introduction

Amniocentesis is a diagnostic test carried out during pregnancy to assess whether the unborn baby could develop an abnormality or serious health condition.

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

Things that increase the risk of an abnormality include:

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

Read more about when amniocentesis is used.

Amniocentesis can be used to detect a number of conditions such as:

  • Downs syndrome – a genetic condition that affects a persons 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

Read more about why amniocentesis is used

What happens?

Amniocentesis is usually carried out during weeks 15-20 of pregnancy. A needle is used to extract a sample of amniotic fluid, the fluid that surrounds the foetus (the developing baby) in the womb (uterus). Amniotic fluid contains cells shed from the foetus that can be examined and tested for a number of conditions.

Read more about how amniocentesis is performed.

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

Risks of amniocentesis

Diagnostic tests, such as amniocentesis, are usually only offered to women when there is a significant risk 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 risk of miscarriage (the loss of the pregnancy). This risk is estimated to be 1 in 100.

Read more about complications of amniocentesis.

If amniocentesis is recommended, the healthcare professional who carries out the test will explain why they think the procedure is necessary, as well as benefits and risks involved.

Diagnostic tests are voluntary, and counselling should be provided to make it easier to cope with the results of amniocentesis.

Why amniocentesis should be done

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

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

Amniotic fluid

Amniotic fluid (the fluid that surrounds the foetus in the womb) contains cells shed from the skin of the developing baby, and the waste products from the baby.

Every cell in the amniotic fluid from the baby contains a complete set of the baby’s DNA. Therefore, the cell samples obtained during amniocentesis are very useful for assessing the developing babys health and diagnosing potential problems.

Amniocentesis gives healthcare professionals direct information about how likely the baby will develop one or more conditions, which may be genetic (inherited) or develop during the pregnancy.

Results

If the results of amniocentesis indicate 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 enables parents to choose whether to continue with the pregnancy or terminate it at an early stage.

Conditions

Amniocentesis can diagnose many different conditions. Some are described below.

Chromosomal conditions

Chromosomal conditions are conditions that affect the chromosomes (parts of the body’s cells that carry genes). For example:

  • Downs syndrome – a condition that affects a persons physical appearance, mental development and learning ability; it is the result of an extra chromosome, known as trisomy-21
  • Edwards syndrome – a condition that causes severe physical and mental abnormalities; it 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
  • haemophilia – a condition that affects the bloods ability to clot

Neural tube defects

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

A neural tube defect can lead to conditions 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 muscles to gradually weaken, resulting in an increasing level of disability.

Other genetic conditions

As well as helping diagnose chromosomal conditions, blood disorders, neural tube defects and musculoskeletal disorders, amniocentesis can also 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, there is a family history of the condition – then your GP, midwife or genetic counsellor will explain the risk to you and your partner. A genetic counsellor is a healthcare professional who helps people understand and deal with genetic conditions.

Read more information about genetic testing and counselling.

When amniocentesis should be done

Amniocentesis is usually performed during weeks 15 to 20 of the pregnancy.

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 because it may increase the risk of causing a miscarriage or club foot in the foetus.

Read about the complications of amniocentesis for more information on the risks of amniocentesis.

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 the 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 Downs syndrome
  • an earlier antenatal screening test has suggested there may be a problem – for example, a high level of alpha-fetoprotein (AFP – a protein 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 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 GP or midwife can discuss appropriate tests with you, and explain why they might be necessary.

In some cases, you may be referred to a genetic counsellor (a healthcare professional who helps people understand and deal with genetic conditions). They will discuss the likelihood of you passing on certain genetic conditions to your children. They will be able to advise you on what to do when you get the results.

Read more information about genetic testing and counselling and why do I need a genetic consultation?

How amniocentesis is performed

During amniocentesis, a small sample of amniotic fluid (the fluid that surrounds the unborn baby) is taken for testing in a laboratory.

During amniocentesis, a small sample of amniotic fluid is removed for testing in a laboratory. Amniotic fluid is the fluid that surrounds the foetus (unborn baby) in the womb (uterus). The fluid contains cells shed by the foetus, which are analysed to get information about the health of your baby.

Before having amniocentesis, a healthcare professional will explain the procedure to you, including benefits and risks. They will also tell you about alternative tests that may be appropriate. If you decide to have amniocentesis, you are usually asked to sign a consent form.

Read about complications of amniocentesis for more information on the risks.

Ultrasound scan

An ultrasound scan will be carried out before you have amniocentesis, and will be continued throughout the procedure. An ultrasound scan uses high-frequency sound waves to produce an image of your womb that is relayed to a television monitor.

The ultrasound scan will allow healthcare professionals to:

  • check the position of the foetus
  • find the best place to remove some amniotic fluid
  • ensure the needle can pass safely through the walls of your abdomen (tummy) and womb

Painkiller

Before the needle is inserted into your abdomen, the area may be numbed with anaesthetic. This involves having a small injection into your tummy that may sting slightly. However, anaesthetic is not usually necessary because research suggests that it does not have much effect in most cases.

The procedure

Firstly, your abdomen will be cleaned with an antiseptic solution to prevent infection. A long, thin needle will be inserted through your abdominal wall. This may cause a sharp, stinging sensation.

Using the ultrasound image as a guide, the needle will be passed into the amniotic sac that surrounds the foetus. A syringe removes a small sample of the amniotic fluid, which will be sent for analysis in a laboratory.

For about eight women in every 100 who have amniocentesis, not enough fluid is removed the first time the needle is inserted. If this happens, the needle is inserted again. 

Amniocentesis usually takes around 10 minutes. However, it can take slightly longer if the position or movement of the foetus makes it difficult to take a sample.

Amniocentesis is not usually painful, although you may feel uncomfortable during the procedure. Some women describe feeling a pain similar to period pain, or feeling pressure when the needle is taken out.

Recovery

After having amniocentesis, you are usually advised to rest for 24 hours. You may have cramps (similar to menstrual cramps) for a few hours and ‘spotting’  that is, drops of blood from your cervix (the neck of the womb).

You should seek urgent medical attention if you:

  • feel shivery
  • have a high temperature (fever) of 38ºC (100.4ºF) or above
  • are bleeding from your vagina
  • have persistent lower back pain or abdominal pain
  • have clear watery fluid coming from your vagina (that is not urine)
  • have contractions (when your abdomen tightens then relaxes)

Complications of amniocentesis

Amniocentesis is a fairly common and safe procedure. However it is important to be aware of the possible complications during or after amniocentesis.

Amniocentesis is a fairly common and safe procedure. For most women, the benefits of having amniocentesis – in terms of diagnosing problems with the developing baby – significantly outweigh the risks.

However, to make an informed decision, it is important to be aware of the possible complications during or after amniocentesis. These are outlined below.

Injury from the needle

During amniocentesis, the placenta (the organ that links a pregnant womans blood supply to her unborn babys) may be punctured by the needle. Sometimes this is necessary to access the amniotic fluid. If this happens, the puncture wound usually heals without any more problems developing.

Ultrasound (where high frequency sound waves are used to create images) is now commonly used to guide the needle. This significantly reduces the risk of injury.

Infection

In very rare cases, an infection may develop if the procedure introduces bacteria to your amniotic sac (the sac surrounding the foetus that contains amniotic fluid). This can cause:

  • a high temperature (fever) of 38ºC (100.4ºF) or above
  • tenderness of your abdomen (tummy)
  • contractions (when your abdomen tightens then relaxes)

You should seek medical attention if you have any of these symptoms. The risk of developing a serious infection from amniocentesis is estimated to be less than 1 in 1,000.

Rhesus disease

Rhesus disease is a condition where proteins in a pregnant woman’s blood attack her babys blood cells.

Rhesus disease is only possible if your blood is rhesus-negative and your babys blood is rhesus-positive. If this is the case, amniocentesis could trigger rhesus disease if your blood is exposed to your baby’s blood during the procedure.

In this situation, you will be given an injection of anti-D immunoglobulin after having amniocentesis. Immunoglobulin is a solution of antibodies (proteins produced by the body to fight disease-carrying organisms) that can prevent rhesus disease from occurring.

Club foot

Amniocentesis may cause club foot in your baby. This is when the baby is born with a deformed ankle and foot. However, the likelihood of this happening is higher if you have amniocentesis before week 15 of your pregnancy.

Miscarriage

There is a small risk that amniocentesis can cause a miscarriage (the loss of the pregnancy). The risk is estimated to be around 1 in 100.

Inconclusive results

Following the amniocentesis procedure, it can be reassuring if the results indicate that your foetus has normal chromosomes (the parts of the body’s cells that carry genes), and that there are no signs of any developmental problems.

However, be aware that amniocentesis cannot test for every condition or disease, and it cannot guarantee your baby will be born completely healthy.

Read more about the results of amniocentesis

Results of amniocentesis

After you have had the amniocentesis procedure, the sample of amniotic fluid (the fluid that surrounds the unborn baby) will be taken to a laboratory for testing.

After you have had the amniocentesis procedure, the sample of amniotic fluid (the fluid that surrounds the unborn baby in the womb) will be taken to a laboratory for testing.

There are two different types of tests:

  • a rapid test
  • a full karotype

These are described in more detail below.

Rapid test

A rapid test looks for abnormalities on specific chromosomes (the parts of the body’s cells that carry genes). A rapid test can identify a number of chromosomal conditions that cause physical and mental abnormalities. These are:

  • Downs syndrome – caused by an extra chromosome 21
  • Edwards syndrome – caused by an extra chromosome 18
  • Patau’s syndrome – caused by an extra chromosome 13

The results of a rapid test should be ready after three working days. This test is almost 100% accurate, but it only tests for the three conditions listed above.

Full karotype

Each cell in the body contains 23 pairs of chromosomes. A full karotype checks all of these.

The cells in the sample of amniotic fluid are grown for up to 10 days in a laboratory before being examined under a microscope to check for:

  • the number of chromosomes
  • the appearance of the chromosomes

Results from a full karotype will usually be ready in two or three weeks. In about 1 in every 100 tests, the results may not be clear. This could be due to the mother’s blood contaminating the sample of amniotic fluid, which may have prevented the cells from growing properly. 

Negative test results

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

It is possible to have a negative result from amniocentesis but your baby may still be born with the condition tested for or another chromosomal condition. This is because a normal test result does not exclude every chromosomal disorder.

Genetic (inherited) disorders are caused by mutations (changes) in the genes (units of genetic material). Each chromosome contains thousands of genes. This means it is not possible to test for every possible genetic mutation, so a baby may occasionally be born with a condition that was not detected.  

Positive test results

If your test result is ‘positive’ it means your baby has the disorder that was being tested for. If you receive a positive test result, the implications will be fully discussed with you. Be aware there is no cure for the majority of chromosomal conditions, so you need to consider your options carefully.

Your options may include:

  • continuing with your pregnancy while getting information and advice about the condition so you are prepared for caring for your baby
  • ending your pregnancy 

If you are considering ending your pregnancy, talk to your GP or midwife. They will provide you with important information and advice.

For example, your options for ending your pregnancy will depend on how many weeks pregnant you are when you make the decision. If you decide to end your pregnancy, you may wish to talk to a counsellor afterwards. Your GP or midwife will be able to arrange this for you.

Read more about genetic testing and counselling.