About amniocentesis

The fluid surrounding your baby in your womb (amniotic fluid) contains cells and various substances from your developing baby that can provide information about his or her health and development. Amniocentesis involves taking a small sample of this fluid to check for genetic abnormalities in your baby.

Amniocentesis is usually carried out between 15 and 18 weeks of pregnancy, when you're in your second trimester. However, it can be carried out later in pregnancy if necessary.

Why have an amniocentesis?

Amniocentesis is usually used to identify chromosome disorders, such as Down's syndrome (trisomy 21) and Edwards' syndrome (trisomy 18).

Chromosomes are structures in every cell in your body that contain the genetic instructions for life (genes), inherited from your parents. Normally, our cells contain 46 chromosomes: 23 inherited from each parent. Most people who have Down's syndrome have an extra chromosome 21 and most people with Edwards' 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 your baby has also been infected. The procedure can also detect some other inherited conditions, such as Duchenne muscular dystrophy and sickle cell anaemia.

It’s your choice whether or not to have an amniocentesis. You will usually be offered the test if:

  • a screening test has suggested that there is a high risk of your baby having a genetic problem such as Down's syndrome
  • you have had a previous pregnancy where your baby was affected by a genetic disorder
  • you or your partner have an inherited condition such as sickle cell anaemia, or there is a history of certain genetic disorders in your family

Deciding to have an amniocentesis

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 show there is a problem, you will need to make some decisions, including whether to continue with or terminate the pregnancy (abortion). These decisions may be difficult, and it's a good idea to discuss the issues with your partner, family, doctor, genetic counsellor or midwife before deciding to have an amniocentesis. Some women choose to have an amniocentesis even though they would not terminate the pregnancy, so that if a disorder is identified, it can allow them time to adjust and prepare for their baby’s birth.

Only you can make the final decision about whether to have amniocentesis. You will need to weigh up how important it is to you to have early information about whether your baby has a genetic disorder, against the risks involved in having the procedure. It’s important to take your time before you decide and feel that you have been given a chance to ask any questions you may have.

Preparing for 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.

You may be asked to have a full bladder when you come for your appointment, but your doctor will let you know before you come.

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.

About the procedure

Your doctor will use an ultrasound device to check the position of your baby and the placenta in your womb. This helps to find the best place to take a sample of some amniotic fluid without damaging the placenta, the umbilical cord or your baby. Your doctor will use ultrasound throughout the procedure to watch where the needle goes and monitor your baby.

Your doctor will insert a long, fine 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 (the membranes that surround your baby) but this should only last a few seconds. The doctor will remove a small amount of fluid.

The procedure itself usually takes about 10 minutes.

Sometimes the test will need to be repeated, for example if not enough fluid can be taken at the first attempt. If the second attempt fails, you will be offered to have the procedure again on another day.

If you have a rhesus negative blood group, you will be offered an injection of anti-D after the procedure. This prevents your body reacting to your baby’s blood if his or her blood group is different. See our frequently asked questions for more information about having a rhesus negative blood group and what this means if you’re having an amniocentesis.

Getting your results

The sample of your amniotic fluid will be sent to a laboratory for testing. For certain conditions, (for example, Down's syndrome and Edwards' syndrome) you may receive the initial results within a few days of an amniocentesis. It can take a laboratory two to three weeks to provide a final report with the full results.

You can usually choose whether to receive your results by phone, by letter, or come into the hospital to receive them. Your midwife may also be able to come to your home to give you your results.

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. Sometimes, the results of the test may not be clear, and you may be offered a second amniocentesis.

Rarely, your baby may be born with another disorder despite you having a normal result.

What are the risks?

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, other women decide that the risks outweigh the benefits for them. The final decision is yours, and 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 your vagina) for a few hours afterwards.

If you need them, you can take over-the-counter painkillers such as paracetamol. Follow the instructions in the patient information leaflet that comes with the medicine and ask your pharmacist for advice.

If you have any other side-effects such as feeling shivery and unwell, fluid loss, heavy bleeding or contractions, contact your doctor or midwife, or the hospital where you had your test immediately.


Complications are when problems occur during or after the procedure. Most women are not affected, but the complications of an amniocentesis include the following.

  • Injury to you or your baby from the needle – this can include puncturing of the placenta. Using ultrasound to guide the needle reduces this risk.
  • There is a small risk that the membranes around your baby may rupture as a result of amniocentesis. This normally heals by itself, but it can result in miscarriage.
  • You may develop an infection if the needle punctures your 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 (see frequently asked questions for more information). This risk is reduced when you're given an injection of the appropriate antibodies after your test.

About one woman in every 100 who have an amniocentesis may have a miscarriage as a direct result of the procedure.

The exact risks are specific to you and differ for every person. Ask your doctor to explain how these risks apply to you.


Produced by Pippa Coulter, Bupa Health Information Team, November 2012.

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