Amniocentesis is a test you may be offered during pregnancy to check if your baby has a genetic disorder, such as Down's syndrome.

It involves removing and testing a small sample of cells from amniotic fluid, which surrounds the foetus in the womb (uterus).

When amniocentesis is offered

Amniocentesis isn't routinely offered to all pregnant women. It's only offered if there's a high risk your baby could have a genetic condition.

This could be because:

It's important to remember that you don't have to have amniocentesis if it's offered. It's up to you to decide whether you want it.

Your midwife or doctor will speak to you about what the test involves, and let you know what the possible benefits and risks are, to help you make a decision.

Read more about why amniocentesis is offered and deciding whether to have it.

How amniocentesis is performed

Amniocentesis is usually carried out between the 15th and 20th weeks of pregnancy, but may be performed later than this if necessary.

Although it can be performed earlier, this may increase the risk of complications of amniocentesis and is usually avoided.

During the test, a long, thin needle is inserted through your abdominal wall, guided by an ultrasound image. The needle is passed into the amniotic sac that surrounds the foetus and a small sample of amniotic fluid is removed for analysis.

The test itself usually takes about 10 minutes, although the whole consultation may take about 30 minutes.

Amniocentesis is usually described as being uncomfortable rather than painful. Some women describe experiencing a pain similar to period pain or feeling pressure when the needle is taken out.

Read more about what happens during amniocentesis.

Getting your results

The first results of the test should be available within three working days and this will tell you whether Down's, Edward's or Patau's syndrome has been discovered.

If rarer conditions are also being tested for, it can take two to three weeks or more for the results to come back.

If your test shows that your baby has a serious inherited or genetic disorder, the implications will be fully discussed with you. There's no cure for most of the conditions amniocentesis finds, so you'll need to consider your options carefully.

You may choose to continue with your pregnancy, while gathering information about the condition so you're fully prepared, or you may consider having a termination (abortion).

Read more about the results of amniocentesis.

What are the risks of amniocentesis?

Before you decide to have amniocentesis, the risks and possible complications will be discussed with you.

One of the main risks associated with amniocentesis is miscarriage, which is the loss of the pregnancy in the first 23 weeks. This is estimated to occur in 1% of women who have amniocentesis.

There are also some other risks, such as infection or needing to have the procedure again because it wasn't possible to accurately test the first sample that was removed.

The risk of amniocentesis causing complications is higher if it's carried out before the 15th week of pregnancy, which is why the test is only carried out after this point.

Read more about the possible complications of amniocentesis.

What are the alternatives?

An alternative to amniocentesis is a test called chorionic villus sampling (CVS). This is where a small sample of cells from the placenta (the organ that links the mother's blood supply with her unborn baby's) are removed for testing.

It's usually carried out between the 11th and 14th weeks of pregnancy, although it can be performed later than this if necessary.

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

If you're offered tests to look for a genetic disorder in your baby, a specialist involved in carrying out the test will be able to discuss the different options with you, and help you make a decision.

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Why it is used

Amniocentesis is offered to pregnant women at risk of having a baby with a serious genetic disorder. It will diagnose any problems at an early stage.

Amniocentesis isn't routinely offered to all women during pregnancy. It's offered if your test results or medical or family history suggest you have an increased chance of having a baby with a genetic disorder.

You don't have to have the test if it's offered – it's up to you to decide whether you want it.

What conditions can amniocentesis detect?

Amniocentesis can be used to diagnose a number of serious genetic disorders, including:

  • Down's syndrome –  a condition that typically causes some level of learning disability and a characteristic range of physical features
  • Edward's syndrome and Patau's syndrome – conditions that can result in miscarriagestillbirth or (in babies that survive) severe physical problems and learning disabilities
  • cystic fibrosis – a condition in which the lungs and digestive system become clogged with thick, sticky mucus
  • muscular dystrophy – a condition that causes progressive muscle weakness and disability
  • sickle cell anaemia – where the red blood cells develop abnormally and are unable to carry oxygen around the body properly
  • thalassaemia – a condition that affects the red blood cells, which can cause anaemia, restricted growth and organ damage

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

Deciding whether to have amniocentesis

If you're offered amniocentesis, ask your doctor or midwife what the procedure involves and what the risks and benefits are before deciding whether to have it.

You may also find it helpful to contact a support group, such as Antenatal Results and Choices (ARC). ARC is a charity that offers information, advice and support on all issues related to screening during pregnancy.

Reasons to have amniocentesis

The test will usually tell you whether your baby will be born with any of the genetic conditions that were tested for.

If no problem is found, it may be reassuring. A result showing that a genetic condition was detected will give you plenty of time to decide how you want to proceed with your pregnancy. Read about the results of amniocentesis for more information.

Reasons not to have amniocentesis

There is a 1% chance you could have a miscarriage after the procedure. You may feel this risk outweighs the potential benefits of the test. Read more about the risks of amniocentesis.

Some women decide they would rather find out when their baby is born.

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What happens?

Amniocentesis involves taking a small sample of amniotic fluid so the cells it contains can be tested. Amniotic fluid surrounds the foetus (unborn baby) in the womb (uterus).

Preparing for amniocentesis

You won't usually need to do anything special to prepare for amniocentesis. You can eat and drink as normal beforehand.

In some cases, you may be advised to avoid going to the toilet for a few hours before the test, because it's sometimes easier to perform when your bladder is full. Your doctor or midwife will tell you about this before you attend your appointment.

You may want to bring a partner, friend or family member for support when you have the test.

Ultrasound scan

You'll have an ultrasound scan before and during amniocentesis. An ultrasound scan uses high-frequency sound waves to produce an image of your womb that's relayed to a monitor.

The ultrasound scan allows 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 (belly) and womb


Before the needle is inserted into your abdomen, the area may be numbed with anaesthetic. This involves having a small injection into your belly that may sting slightly.

However, anaesthetic isn't usually necessary because research suggests that it doesn't have much effect in most cases.

How amniocentesis is performed

An antiseptic solution will first be used to clean your abdomen to minimise the risk of infection. A long, thin needle is then inserted through your abdominal wall. It may cause a sharp, stinging sensation.

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

In about 8 out of every 100 women who have amniocentesis, not enough fluid is removed the first time the needle is inserted. If this happens, the needle will be inserted again. 

Is amniocentesis painful?

Amniocentesis isn't usually painful, but you may feel uncomfortable during the procedure.

Some women describe experiencing a pain similar to period pain or feeling pressure when the needle is taken out.

How long does it take?

The procedure usually takes around 10 minutes to perform.

Afterwards, you will be monitored for up to an hour, in case the test causes any side effects, such as heavy bleeding. You can then go home to rest.

It's a good idea to arrange for someone to drive you home, as you might not feel up to it yourself.

Recovering after amniocentesis

After amniocentesis, it's normal to have cramps similar to period pain and light vaginal bleeding called "spotting" for a day or two.

You can take over-the-counter painkillers such as paracetamol (but not ibuprofen or aspirin) if you experience any discomfort.

You may wish to avoid any strenuous activity for the rest of the day.

Contact your midwife or the hospital where the procedure was carried out for advice as soon as possible if you develop any of the following symptoms after the procedure:

  • persistent or severe pain
  • a high temperature of 38C (100.4F) or more
  • chills or shivering
  • discharge or clear fluid coming from the vagina
  • contractions (when your abdomen tightens then relaxes)

Getting the results

The first results should be available within a few days, and this will tell you whether a major chromosome problem has been discovered.

If rarer conditions are also being tested for, it can take two to three weeks or more for the results to come back.

You can usually choose whether to get the results over the phone or during a face-to-face meeting at the hospital or at home.

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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 threadlike structures that carry genetic information in the form of genes). A rapid test can identify a number of chromosomal conditions that cause physical and mental abnormalities. These are:

  • Down's syndrome – caused by an extra chromosome 21
  • Edward's 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

In most cases, the results of amniocentesis are negative. This means that the baby doesn't have any of the disorders tested for.

However, it's possible to have a negative result from amniocentesis, but your baby may still be born with the condition tested for, or they may be born with another chromosomal condition. This is because a normal test result doesn't exclude every chromosomal disorder.

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

Positive test results

A positive test result means your baby has the disorder that was tested for. The implications of a positive test result will be fully discussed with you.

You should be aware that there's no cure for the majority of chromosomal conditions, so you need to consider your options carefully. These 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.

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Before you decide to have amniocentesis, you'll be told about the risks and possible complications.

The main risks associated with the procedure are outlined below.


There's a small risk of miscarriage (loss of the pregnancy) occurring in any pregnancy, regardless of whether or not you have amniocentesis.

If you have amniocentesis after 15 weeks of pregnancy, the chance of having a miscarriage is estimated to be about 1%. The risk is higher if the procedure is carried out before 15 weeks.

It's not known for certain why amniocentesis can lead to a miscarriage. However, it may be caused by factors such as infection, bleeding or damage to the amniotic sac that surrounds the baby.

Most miscarriages that happen after amniocentesis occur within 72 hours of the procedure. However, in some cases a miscarriage can occur later than this (up to two weeks afterwards).

Inconclusive results

After having amniocentesis, it can be reassuring if the results indicate that your developing baby has normal chromosomes and that there are no signs of any developmental problems.

However, you should be aware that amniocentesis can't test for every condition or disease, and it can't guarantee your baby will be born completely healthy.

Read more about the results of amniocentesis

Injury from the needle

During amniocentesis, the placenta (the organ that links the mother's blood supply to her unborn baby's) may be punctured by the needle.

It's sometimes necessary for the needle to enter the placenta to access the amniotic fluid. If this happens, the puncture wound usually heals without any more problems developing.

An ultrasound scanner is now commonly used to guide the needle, significantly reducing the risk of injury.


As with all types of surgical procedures, there's a risk of infection during or after amniocentesis. Infection can occur if there are bacteria on your skin or on the instruments being used.

However, severe infection occurs in less than 1 in every 1,000 procedures.

Rhesus disease

If your blood type is rhesus (RhD) negative, but your baby's blood type is RhD positive, it's possible for sensitisation to occur during amniocentesis.

This is where some of your baby's blood enters your bloodstream and your body starts to produce antibodies to attack it. If it's not treated, this can cause the baby to develop rhesus disease.

If you don't already know your blood type, a blood test will be carried out before amniocentesis to see if there's a risk of sensitisation. An injection of a medication called anti-D immunoglobulin can be given to stop sensitisation occurring, if necessary.

Read more about preventing rhesus disease.

Club foot

Having amniocentesis early (before week 15 of the pregnancy) has been associated with an increased risk of the unborn baby developing club foot.

Club foot, also known as talipes, is a deformity of the ankle and foot that's present at birth (congenital).

Because of the increased risk of a baby developing club foot, amniocentesis isn't recommended before 15 weeks of pregnancy.

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The information on this page has been adapted by NHS Wales from original content supplied by NHS Choices.
Last Updated: 10/05/2016 09:43:42