Amniocentesis is a fairly common and safe procedure. For most women, the benefits of amniocentesis, in terms of providing a diagnosis of any problem with their developing baby, significantly outweigh the risks.

However, to make an informed decision, it is important that you are aware of the possible side effects and the risk of complications.

The side effects of amniocentesis are usually mild and temporary, and they often involve some slight pain and cramps in the abdomen (tummy). Occasionally, unexpected complications can occur during, or after, the procedure. These are outlined below.

Injury to you, or your foetus, from the needle

During amniocentesis, there is a risk that the placenta may be punctured by the needle. However, if this happens, the puncture wound usually heals without any further problems developing. Ultrasound is now commonly used to guide the needle which significantly reduces the risk of injury.

Infection

Infection may develop if the amniocentesis procedure introduces bacteria into your amniotic sac. This can cause a high temperature, contractions of the womb, and tenderness of your abdomen (tummy).

Rhesus disease

There is a small risk that your blood will be exposed to the foetus's blood. However, this is only a problem if your blood is rhesus-negative and your baby's blood is rhesus-positive. In this situation, there is an increased risk that your baby will develop rhesus disease. If there is a risk, following the test, you will be given an injection of antibodies.

Club foot

Amniocentesis may cause club foot in your baby. However, the likelihood of this happening is higher if you have amniocentesis before 15 weeks of pregnancy.

Most women who have amniocentesis are not affected by any of the above complications, and the risk of them causing a miscarriage is very small (approximately 1%).

Following the amniocentesis procedure, it can be reassuring if the results show that your foetus has normal chromosomes, and there are no signs of any developmental problems.

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

During amniocentesis, a small sample of amniotic fluid (the fluid that surrounds the foetus in the womb) is taken for testing in a laboratory. The fluid contains cells that have been shed by the foetus which can be analysed to provide information about the health of your baby.

Before having amniocentesis, a health professional will explain the procedure to you, along with the associated benefits and risks.They will also inform you about any alternative tests that may be appropriate. If you decide to have the amniocentesis test, you will normally be asked to sign a consent form.

The amniocentesis procedure

An ultrasound scan is often carried out before amniocentesis. Ultrasound creates an image of your womb on a TV monitor and enables the health professional to locate the best place to remove some amniotic fluid from inside your womb.

Having an ultrasound scan also ensures that the needle that will be used for amniocentesis can be passed safely through the walls of your abdomen and womb and into the amniotic fluid in which the foetus is floating.

Before the needle is inserted into your abdomen, you may wish to have the area numbed with anaesthetic. This involves a small injection into your tummy which may sting slightly. The health professional will then insert a long, thin needle through your abdominal wall and into the amniotic sac of fluid surrounding the foetus.

A syringe will be used to remove a small sample of amniotic fluid from your amniotic sac for analysis in a laboratory. The test usually takes between 10-30 minutes during which time you may feel slightly uncomfortable. Afterwards, you will usually be advised to rest for 24 hours.

Each year in the UK, approximately 5% of pregnant women are offered a diagnostic test, such as amniocentesis and chorionic villus sampling (CVS). Amniocentesis is the most commonly offered diagnostic test.

What is amniocentesis?

Amniocentesis is a medical test that is carried out during pregnancy in order to assess whether the unborn baby (foetus) could develop or has developed an abnormality or serious condition.

The results of amniocentesis will indicate the likelihood of the baby developing certain chromosomal conditions, such as Down's syndrome, Edward's syndrome, and Patau's syndrome, which are all conditions where the baby is born with an extra chromosome.

Because diagnostic tests such as amniocentesis are quite invasive (involve going into the body) and carry a small risk of miscarriage, they are usually only offered to women when there is a significant risk that their baby will develop a serious condition or abnormality.

Age and history

The mother's age, medical history and family history are factors that could increase the risk of an abnormality developing in their unborn child, and they will be taken into consideration when deciding whether a diagnostic test should be offered.

If amniocentesis is recommended for you, the health professional in charge of your care will tell you why they think the procedure is necessary, and they will outline the benefits and risks involved. However, it is your decision whether or not to go ahead with the procedure. You do not have to have a diagnostic test if you do not want to.

Chorion villus sampling versus amniocentesis for prenatal diagnosis. Alfirevic Z et al. The Cochrane Library, 4, 2001.

Early amniocentesis versus transabdominal chorion villus sampling for prenatal diagnosis. Alfirevic Z. The Cochrane Library, 4, 2001.

Antenatal screening for Down's syndrome. Nick Wald et al. Health Technology Assessment 1998, volume 2, number 1. Chorion villus sampling versus amniocentesis for prenatal diagnosis. Alfirevic Z et al. The Cochrane Library, 4, 2001.

Early amniocentesis versus transabdominal chorion villus sampling for prenatal diagnosis. Alfirevic Z. The Cochrane Library, 4, 2001.

Antenatal screening for Down's syndrome. Nick Wald et al. Health Technology Assessment 1998, volume 2, number 1.

After you have had the amniocentesis procedure, and a sample of amniotic fluid has been taken, it usually takes a couple of days to test it for conditions such as Down's syndrome and Edward's syndrome.

A laboratory process known as fluorescence in-situ hybridisation (FISH) is usually used to test for these conditions.

Other chromosomal conditions may take longer (up to four weeks) to identify because cells may need to be grown from the sample for chromosome analysis.

Test results

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

However, occasionally, it possible to have a ‘normal’ result but the baby is born either with the condition that was tested for, or another chromosomal condition. This is because a ‘normal’ test result does not exclude every chromosomal disorder.

If your test is ‘positive’ it means that 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. You should be aware that there is no cure for the majority of chromosomal conditions. Therefore, you need to consider your options carefully. These might include:

• continuing with your pregnancy while acquiring information and advice about the condition, so that you are prepared for caring for your baby, or
• ending your pregnancy.

If you are considering ending your pregnancy, you should talk to your GP, or midwife, as they will be able to 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 help arrange this.

Amniocentesis is usually performed between 15-20 weeks of pregnancy. It is sometimes also carried out between 18-22 weeks to determine the risk of the foetus developing rhesus disease. It may be necessary to carry out further testing in order to monitor the foetus.

Amniocentesis can be performed earlier than 15 weeks but, if possible, this is avoided due to an increased risk of causing miscarriage, or of producing club foot in the baby.

Another diagnostic test, called chorionic villus sampling (CVS), can be carried out slightly earlier than amniocentesis (from about 10 weeks). Rather than testing the amniotic fluid, it tests tissue from around the site of the placenta. With CVS, the risk of miscarriage is 1-2%, which is slightly higher than the risk of miscarriage for amniocentesis.

Amniotic fluid contains cells that have been shed from the skin of the developing baby, as well as the waste products from the foetus.

Every cell from the baby contains a complete set of its DNA. Therefore, the samples that are obtained from amniocentesis are very useful for assessing the developing baby's health, and for diagnosing any potential problems.

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

Detecting chromosomal conditions

Amniocentesis is used to identify chromosomal conditions, such as Down's syndrome - a condition that affects a person's physical appearance, and their mental development and learning ability. It is caused by a faulty chromosome which the baby inherits from its parents.

The amniocentesis procedure can also detect a number of other, similar chromosomal conditions, such as Edward's syndrome, which is a condition that causes severe physical and mental abnormalities and is the result of an extra chromosome, known as trisomy-18.

Other conditions

A number of other, non-chromosomal conditions can be detected either through antenatal screening tests, or blood spot screening tests, which are carried on newborn babies.

Blood spot screening tests involve taking a small blood sample from the heel of a baby, and then testing it for a number of different conditions.

For example, sickle cell disorders, which adversely affect the formation of red blood cells and their ability to carry oxygen, and cystic fibrosis which is a genetic (inherited) disorder in which internal bodily secretions become thick and sticky and hinder the function of organs, such as the lungs and digestive system.

Amniocentesis is a method of detecting serious, or potentially serious, disorders in the unborn child.

If amniocentesis highlights a problem with the development of the foetus, there may be an opportunity for early treatment of the disorder while the baby is still in the womb.

If a serious abnormality is detected, amniocentesis gives parents the choice of whether they want to have a child with the abnormality, or whether they would prefer the pregnancy to be terminated at an early stage.

Amniocentesis may be offered in the following situations:

• if the mother has had a child in the past with a birth defect,
• if there is family history that suggests that a disorder may be likely,
• if a previous test has shown that alpha-fetoprotein (AFP) levels in the blood are higher or lower than normal (a higher level suggests neural tube defects, such as spina bifida, while a lower level suggests a chromosome abnormality, such as Down's syndrome),
• to determine the baby's sex if a parent is a carrier of a disease which has a greater likelihood of affecting a boy or a girl. For example, Duchenne muscular dystrophy is a condition that is more likely to occur in boys than in girls, or
• to check on the foetus's blood group if the mother's blood group is rhesus-negative if the foetus's blood group is rhesus-positive, there is an increased chance that baby will get rhesus disease, which can cause anaemia, jaundice, or even death, soon after birth.