Amniocentesis is a procedure that involves testing the amniotic fluid to detect genetic abnormalities in a developing baby (fetus).
You will meet the doctor carrying out your procedure to discuss your care. It may differ from what is described here as it will be designed to meet your individual needs.
This information was published by Bupa's health information team and is based on reputable sources of medical evidence. It has been peer reviewed by Bupa doctors. The content is intended for general information only and does not replace the need for personal advice from a qualified health professional.
Amniocentesis involves taking a small sample of the amniotic fluid surrounding the baby in your womb. This fluid is then tested in a laboratory.
Amniotic fluid contains cells and various substances from your developing baby that can provide information about his or her health and development.
Amniocentesis is usually used to identify chromosome disorders, such as Down's syndrome (trisomy 21) and Edward's syndrome (trisomy 18).
Chromosomes are structures that contain genes - these contain the instructions for life and are inherited from parents. Normally, our cells contain 46 chromosomes: 23 inherited from each parent. Sometimes a mistake is made during cell division. This is most likely to occur when the sperm or egg is being formed and can cause there to be more chromosomes present. Most people who have Down's syndrome have an extra chromosome 21 and most people with Edward's syndrome have an extra chromosome 18.
Amniocentesis can sometimes be used to test for other conditions as well. If you get an infection during pregnancy, such as cytomegalovirus (CMV), amniocentesis can detect if the baby has also been infected. The procedure may also be done if you have a family history of a specific condition. These conditions include:
- musculoskeletal conditions, such as Duchenne muscular dystrophy
- blood disorders, such as haemophilia, thalassaemia or sickle cell anaemia
You will usually be offered amniocentesis if:
- a screening test has suggested that you have a high risk of your baby having a birth defect or Down's syndrome
- you're over 35 and have missed an earlier screening test - your risk of having a baby with a genetic disorder increases after this age
- you have a history of certain disorders in your family
- there is a possibility of a genetic disorder, for example if you or your partner have an inherited condition such as sickle cell anaemia
Amniocentesis can also be used to find out the baby's sex. This is important if you or your partner is a carrier of a sex-specific disease, such as haemophilia or Duchenne muscular dystrophy (both of these are more likely to occur in boys).
Amniocentesis is sometimes used to check whether your baby is at risk of rhesus (RhD) disease. As well as the blood groups A, B, AB and O, RhD is another way to categorise the type of blood you have. You will either be RhD-positive or RhD-negative - this means respectively that you carry the D antigen on your red blood cells or you don't.
An antigen is a type of protein that can cause your immune system to react. If your immune system doesn't recognise an antigen, it will produce antibodies to attack it. Antibodies are proteins produced by your immune system that usually fight against viruses and bacteria.
Your baby won't necessarily have the same blood type as you. Therefore, you may be RhD-negative but your baby may be RhD-positive. If this happens and your blood comes into contact with your baby's blood, your immune system will react to the RhD antigen on your baby's blood cells and make antibodies against it (anti-D antibodies). These antibodies can then cross into your baby's bloodstream. This is called sensitisation and can cause problems for your baby (for example, anaemia).
Amniocentesis can monitor whether your immune system has reacted against your baby's blood and give more information about his or her health.
If your blood is RhD-negative, you will usually be given an injection of an antibody called anti-D after amniocentesis. This is because during amniocentesis there is a risk that some of your baby's blood may enter your bloodstream, causing your body to make anti-D antibodies. The injection helps to stop your immune system from reacting to your baby's blood but it won't harm your baby. You may be offered further injections if necessary.
Before deciding to have an amniocentesis, it's worth thinking about what the results will mean to you.
If the results show that your baby has normal chromosomes and no other obvious problems, this is reassuring. However, amniocentesis can't test for every disease or guarantee that your baby will be born completely healthy.
If the amniocentesis results are show there is a problem, you will need to make some decisions. These will be based on what the results of the amniocentesis mean for you, your family and your unborn baby. You will need to think about several things including:
- whether to continue with or terminate the pregnancy (abortion)
- where to have your baby - for example, if your baby may need surgery soon after birth, your doctor may advise that you give birth in a hospital where the staff are experienced in this
These decisions may be difficult, and it's a good idea to discuss the issues with your partner, family, doctor, genetic counsellor or your midwife before deciding to have an amniocentesis.
Your doctor or midwife will explain how to prepare for your test.
Amniocentesis is usually done as a day case and can take up to half an hour. The test can be slightly uncomfortable so your doctor may use a local anaesthetic to numb the skin where the needle will be inserted.
Your doctor will discuss with you what will happen before, during and after your procedure, and any pain you might have. This is your opportunity to understand what will happen, and you can help yourself by preparing questions to ask about the risks, benefits and any alternatives to the procedure. This will help you to be informed, so you can give your consent for the procedure to go ahead, which you may be asked to do by signing a consent form.
Your doctor will use an ultrasound device to check the position of your baby and the placenta in your womb. An ultrasound uses sound waves to produce an image of the inside of part of your body. This helps to find the best place to remove some amniotic fluid without damaging the placenta, the umbilical cord or your baby. Your doctor will use ultrasound throughout the procedure to monitor your baby.
Your doctor will insert a long, thin, needle through the wall of your abdomen (tummy) and into the sac of fluid that surrounds your baby. You may feel a sharp, stinging sensation when the needle pierces the amniotic sac but this should only last a few seconds. A small amount of fluid will be removed and sent to the laboratory for testing.
The fluid that is removed is usually an amber or yellow colour. Sometimes the test will need to be repeated, for example if there is blood in the sample.
Certain conditions (for example, Down's syndrome and Edward's syndrome) can be identified within a couple of days of amniocentesis.
For other conditions it can take a laboratory two to three weeks to provide a final result.
For most women the test will give a very clear result as to whether or not the chromosomes are normal, or for other conditions, positive or negative. Occasionally a different condition is detected than the one that was originally being checked for. Rarely, your baby may be born with a disorder despite you having a normal result.
Amniocentesis is commonly performed and generally safe. For many women, the benefits of a clear diagnosis of any problem with their baby are greater than the potential risks. However, in order to make an informed decision and give your consent, you need to be aware of the possible side-effects and the risk of complications of this procedure.
Side-effects are the unwanted but mostly temporary effects you may get after having the procedure. You may have some mild pain or cramps, like period pain, and spotting (light bleeding from the vagina) for a few hours afterwards.
If you have any other side-effects such as feeling shivery and unwell, heavy bleeding or contractions, contact your doctor immediately.
Complications are when problems occur during or after the procedure. Most women are not affected, but the risks of amniocentesis include the following.
- Injury to you or your baby from the needle - using ultrasound to guide the needle reduces this risk. Puncture of the placenta is the most common injury, but this usually heals without any further problems.
- You may develop an infection because the needle has punctured the bowel or because of skin contamination, but this is very rare.
- There is a slight risk that your blood will be exposed to your baby's blood. This is only an issue if your blood is rhesus-negative and your baby's is rhesus-positive. This risk is reduced when you're given an injection of the appropriate antibodies after your test.
In about one in 100 women, one of these complications of amniocentesis leads to miscarriage.
The exact risks are specific to you and differ for every person. Ask your doctor to explain how these risks apply to you.
How can I know if amniocentesis is right for me?
Your doctor will explain to you the risks and benefits of having amniocentesis. Only you can decide whether the test is right for you.
Making a decision about whether or not to have a diagnostic test during pregnancy can be difficult. You may find it helpful to discuss the options with your partner or family.
To help you make the decision that is best for you, your doctor should discuss the following information with you:
- the types of tests available and what the results will tell you
- how reliable the tests are
- the risk of having an uncertain result and needing a repeat test
- your risk of miscarriage from having the test
- how long the results will take and how you will get them
- your options if your baby is found to have a disorder
Your doctor will be able to tell you about the disorders that can be detected by amniocentesis. You may also want information about what is involved in ending a pregnancy and how you may feel afterwards.
If you think you're likely to decide to end your pregnancy if the results are abnormal, you might want to find out sooner about any possible disorders. In this case, you may decide that amniocentesis isn't the best option. You may want to have an alternative diagnostic test such as chorionic villus sampling, which can be done earlier in your pregnancy to detect genetic disorders. Talk to your doctor about your options.
Will amniocentesis harm my baby?
There is a small risk that you may miscarry as a result of having amniocentesis.
Amniocentesis may cause a miscarriage if the needle punctures the placenta or causes an infection in the womb. About one in 100 women who have amniocentesis will miscarry as a result of the procedure.
This risk is also dependent on the skill of the doctor as well as the technique. A more experienced doctor may have a higher success rate and a lower rate of procedure-related miscarriage.
If you have any concerns about amniocentesis, speak to your doctor or midwife.
Can I have amniocentesis if my blood group is rhesus-negative and my baby is rhesus-positive?
Yes, you can have the amniocentesis test if you're rhesus (RhD)-negative and your baby is rhesus (RhD)-positive. You will be given an injection, which helps to stop your immune system from reacting to your baby's blood.
As well as the blood groups A, B, AB and O, RhD is another way to categorise the type of blood you have. If your blood group is RhD-positive, this means you have RhD antigens on the surface of your red blood cells. RhD-negative means you don't have these antigens.
An antigen is a type of protein that can cause your immune system to react. If your immune system doesn't recognise an antigen, it will produce antibodies to attack it. Antibodies are proteins made by your immune system that usually fight against viruses and bacteria. So, if your doctor tells you that you are A positive, it means your blood group is A, and you are RhD-positive.
Your baby won't necessarily have the same blood type as you. Therefore, a mother who is RhD-negative can carry a baby who is RhD-positive. There is a risk that during a procedure such as amniocentesis or chorionic villus sampling (CVS), some of your baby's blood may enter your bloodstream. If this happens, your immune system will react to the RhD antigen on your baby's blood cells and make antibodies against it (anti-D antibodies). This process is called sensitisation or alloimmunisation.
To prevent this, you will usually be given an injection of an antibody called anti-D after amniocentesis. The injection helps to stop your immune system from reacting to your baby's blood but it won't harm your baby. You may be offered further injections if necessary.
As your pregnancy continues (or in a future pregnancy), your immune system may produce more anti-D antibodies. These can cross the placenta and cause red blood cells to be removed from your baby's bloodstream. Your baby can develop anaemia if the red blood cells are removed faster than they are produced. Severe anaemia can cause fetal heart failure, fluid retention or swelling (hydrops) or cause your baby to die soon after birth, although this is rare. During your pregnancy, your baby can have blood transfusions to help reduce the risk of anaemia.
What is the difference between amniocentesis and chorionic villus sampling (CVS)?
Although amniocentesis and CVS are similar, there are some significant differences.
Amniocentesis involves taking a small sample of the amniotic fluid surrounding the baby in your womb. This is done through your abdomen (tummy). CVS involves taking a small sample of the placenta from your womb. This can be done in two ways: through your cervix (neck of the womb) or through your abdomen). Both tests use ultrasound to check the position of your baby.
Amniocentesis is usually done after 15 weeks of pregnancy. CVS is usually done between 10 and 14 weeks of pregnancy. This time difference may affect what is involved if you choose to end the pregnancy.
The risk of having a miscarriage after amniocentesis or CVS is rare. However, the risk is slightly higher after CVS (two in 100) than after amniocentesis (one in 100). The exact risks are specific to you and will differ for every person. Ask your doctor to explain how these risks apply to you.
These decisions may be difficult, and it's a good idea for you to discuss the issues with your partner, family, doctor, genetic counsellor or your midwife before deciding to have either amniocentesis or CVS.
- Royal College of Obstetricians and Gynaecologists (RCOG)
- Midwives Information and Resource Service
- Antenatal Results and Choices (ARC)
- UK Screening Portal
- Amniocentesis: what you need to know. Royal College of Obstetricians and Gynaecologists. www.rcog.org.uk, published June 2006
- Beers MH, Fletcher AJ, Porter R, et al. The Merck Manual of Medical Information: Home Edition. 2nd ed. London: Simon and Schuster Ltd, 2004:1430-34, 1453, 1527-28
- Routine antenatal anti-D prophylaxis for women who are rhesus D negative. National Institute for Health and Clinical Excellence (NICE). 2008. www.nice.org.uk
- Chorionic villus sampling: what you need to know. Royal College of Obstetricians and Gynaecologists. www.rcog.org.uk, published June 2006
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