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Amniocentesis

Amniocentesis is a test carried out during pregnancy to detect a number of abnormalities in a developing baby (fetus).

A small sample of the amniotic fluid that surrounds the fetus in the womb is taken and this is tested in a laboratory. Amniotic fluid contains cells and waste products from the fetus that can provide information about the health and development of your growing baby.

Why have amniocentesis?

Amniocentesis can identify certain disorders in the fetus including:

• chromosome conditions, such as Down's syndrome (trisomy 21) and Edward's syndrome (trisomy 18)
• musculoskeletal disorders, such as Duchenne muscular dystrophy, cystic fibrosis or spina bifida (where the spinal cord doesn't develop correctly)
• blood disorders, such as haemophilia, thalassaemia or sickle cell anaemia
• metabolic disorders, such as phenylketonuria or antitrypsin deficiency

When is amniocentesis offered?

Amniocentesis is usually offered:

• if a previous test or screening result, such as the triple test for alpha-fetoprotein, estriol and human chorionic gonadotropin, or a nuchal translucency scan has suggested a high risk of a birth defect or of Down's syndrome
• for women who are over 35 years of age, as the risk of the baby having a genetic abnormality becomes greater after this age
• for women who have a history of abnormalities at birth in their family
• if there is a possibility of a genetic abnormality, for example, women who have, or whose partner has, an inherited condition such as sickle cell anaemia
• to find out the baby's sex if a parent 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)
• to check on the baby if the mother is rhesus negative (see below)

Rhesus disease

Amniocentesis is sometimes used to check whether a baby is at risk of rhesus disease. The rhesus (Rh) system is a way of grouping blood. If the mother's blood is rhesus-negative, and the fetus's rhesus-positive (which can only happen if the father is rhesus-positive), the mother's immune system (her body's defence mechanism) may attack the fetus's red blood cells with antibodies. This can cause anaemia or jaundice in the baby, or even death soon after the birth.

Amniocentesis will monitor whether there has been an immune reaction against the fetus's blood and evaluate its health.

To help prevent the mother from developing antibodies against the fetus's blood, an injection of another antibody is given to the mother in her arm or leg, at 28 and 34 weeks of pregnancy, and after the delivery.

When is amniocentesis performed?

Amniocentesis is normally performed between 15 and 20 weeks of pregnancy. It is done slightly later to check for the risk of rhesus disease, at about 18 to 22 weeks, and repeated testing may be necessary to monitor the fetus.

Before the test, you will be asked to sign a consent form confirming that you have been fully informed of the benefits and risks of amniocentesis, as well as the benefits and risks of any alternative tests. By signing the consent form you are giving permission for the procedure to go ahead.

The doctor will move an ultrasound device over your tummy, which creates an image of your womb on a video monitor. This helps to locate the best place to remove some amniotic fluid. For more information on ultrasound, please see the separate BUPA factsheet, Ultrasound.

The sample of amniotic fluid is then taken using a needle. Some women choose to have the area where the needle will be inserted numbed with an anaesthetic. This involves a small injection into the tummy, which stings a little.

Your doctor will then insert a long, thin, needle through your abdominal wall and into the sac of fluid around the fetus. 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 analysis.

The test can take from 10 minutes up to half an hour and can be slightly uncomfortable. You will be advised to rest for 24 hours when you get home.

Results

Certain conditions (eg Down's syndrome and Edward's syndrome) can be identified within a couple of days using the FISH laboratory analysis (fluorescence in-situ hybridisation).

For an analysis of chromosomal conditions, it takes longer. The amniotic fluid contains cells shed from the skin of the baby and various substances secreted by the baby. Every cell from the baby contains a complete set of its DNA. If you want to do a chromosomal analysis you need to grow the cells first. It can take a laboratory up to four weeks to provide a final result. This is an anxious time for everyone involved and it's a good idea to try to keep occupied until the results come through.

The results are usually presented as a chance of the condition occurring. For example, a risk of 5 in 100 means that out of 100 women with this result, five will have a baby with the diagnosed disorder, and 95 will not. This is the same as a 5 percent chance that the baby has the condition and a 95 percent chance that the baby does not. This will help you to see how certain it is that your baby will have the condition.

Deciding to have amniocentesis

Amniocentesis is a commonly performed and generally safe procedure. For most women offered the test, the benefits in terms of a clear diagnosis of any problem with the fetus are greater than the potential risks. However, in order to make an informed decision, you need to be aware of the possible side-effects and the risk of complications.

Side-effects are the unwanted, but usually mild and temporary effects of a successful procedure. The side-effects of an amniocentesis can include some mild pain or cramps in the tummy.

Complications are the unexpected problems that can occur during or after a procedure. Most women are not affected. The risks of amniocentesis include those listed below.

• Injury to you or the fetus from the needle - using ultrasound to guide the needle reduces the risk. Puncture of the placenta is the most common potential injury but this usually heals without any further problems.
• You may develop an infection because the procedure has let bacteria into the amniotic sac but this is very rare. This can result in a high temperature, contractions of the womb and/or tenderness in the tummy.
• There is a slight risk that your blood will be exposed to the fetus's blood. This is only an issue if your blood is rhesus-negative and your baby's blood is rhesus-positive (see Rhesus disease above). If this is a risk you will be given an injection of the appropriate antibodies after your test.
• The test may produce club foot in the baby but the chance of this happening is higher if you have amniocentesis before reaching 15 weeks of pregnancy.

In approximately 1 in 100 cases, one of these complications of amniocentesis leads to miscarriage.

Before deciding to have an amniocentesis it is also worth thinking about what the results will mean to you.

If the results show that the fetus has normal chromosomes and no signs of a neural tube defect, this is reassuring. However, amniocentesis can't test for every disease or guarantee that your baby will be born completely healthy.

If, on the other hand, the amniocentesis results are abnormal, you need to make some informed decisions. These decisions may depend on various factors, such as the specific defect identified and your feelings. There are a number of things to consider.

• Whether to continue with or terminate the pregnancy. If the fetus has a severe defect some women opt to end the pregnancy (therapeutic abortion). Others want time to prepare themselves and their family for the birth of a baby with health problems.
• Whether to have a caesarean section or a vaginal birth, depending on the health of the baby.
• Where to have the baby. For example, if the baby has a problem that may need surgery, it would be best to give birth at a hospital where surgeons are experienced in surgery for newborns.

These decisions may be difficult and it's a good idea for you to discuss the issues with health professionals before deciding to have an amniocentesis.

Further information

• The Royal College of Obstetricians and Gynaecologists
  020 7772 6200
  www.rcog.org.uk
• Midwives Information and Resource Service
  0800 581 009
  www.midirs.org

References

• NICE October 2003, Clinical Guideline, CG6 Antenatal care: Routine care for the healthy pregnant woman.
• NICE Guidance, Technology appraisals, May 2002, Pregnancy - routine anti-D prophylaxis for rhesus negative women (No. 41): Assessment report of the clinical effectiveness and cost effectiveness of routine anti-D prophylaxis for pregnant women who are rhesus (RHD) negative.
• MedlinePlus, Encyclopaedia, Amniocentesis.
  www.nlm.nih.gov/medlineplus
  viewed on 22 March 2004

Published by BUPA's health information team, healthinfo@bupa.com, April 2005

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