Amniocentesis

About amniocentesis

This test is not offered as a matter of routine. It is used to detect Down’s, Edwards’, and Patau’s syndromes and other specific known genetic disorders. There is no evidence that the procedure itself harms the baby as the test is carried out under ultrasound guidance, but the test is most safely performed after 15 weeks of pregnancy. The procedure is done by an obstetric doctor trained to undertake amniocentesis.

Who should consider amniocentesis?

• If you have a received a high-chance result from NIPT, combined screening, or the quadruple test in this pregnancy.
• If there is a potential problem found on an ultrasound scan in this pregnancy, which may suggest a chromosomal abnormality.
• If you had a previous pregnancy or baby/child affected by a genetic condition that can be tested for by an amniocentesis, including Down’s, Edwards’ or Patau’s syndrome.
• If either biological parent has a family history of genetic disorders, including some rare inherited diseases that can be tested for by amniocentesis.
• If you who have missed the screening tests for Down’s, Edwards’ or Patau’s syndrome, or request invasive testing for your own reasons.

The age-related chance of having a baby with Down’s syndrome is as follows:

• 25: 1 in 1,500
• 30: 1 in 910
• 35: 1 in 380
• 38: 1 in 190
• 40: 1 in 110
• 45: 1 in 30

Benefits

It is the only way to know whether a baby’s chromosomes are affected by certain conditions.

Risks

Amniocentesis has been available for many years and in Britain over 30,000 women have the test performed every year. We therefore know a lot about its safety and accuracy. We know that the test can sometimes cause a miscarriage for approximately 1 in 200 (0.5%) who have the test. There is no evidence that the procedure itself harms your baby as the test is carried out under ultrasound guidance.

Alternatives

We will discuss with you whether an alternative procedure such as CVS (chorionic villus sampling) would be appropriate.

Is the amniocentesis test reliable?

No test is absolutely perfect, but the chromosome test for Down’s, Edwards’ and Patau’s syndromes is very reliable. It fails to give a clear result in fewer than one in 1,000 cases. If you are having the amniocentesis for other genetic disorders, you should discuss the accuracy of the test with your genetic doctor or counsellor.

Will the test reveal anything other than Down’s, Edwards’ or Patau’s syndrome?

Although you may be having an amniocentesis to detect one of these conditions, or another specific, known genetic disorder, occasionally the test may detect problems with the other chromosomes. If the results show anything abnormal you will be told what the abnormality is and how this may affect your baby.

How is amniocentesis performed?

The test involves taking a small amount of the water (amniotic fluid) that surrounds the baby in the womb (uterus). This water contains cells from the baby that can then be tested in the laboratory.

Before the test is performed, an ultrasound scan is carried out to check your dates and the position of both the baby and the placenta (afterbirth). You will not need a full bladder for this scan.

The skin over the abdomen is cleaned with antiseptic, and a small area is numbed with local anaesthetic. A fine needle is then passed through the wall of the womb. A sample of the water that surrounds the baby is removed and sent to the laboratory. The position of both the baby and the needle are monitored throughout the procedure by ultrasound scanning.

Is amniocentesis painful?

Most women and birthing people say afterwards that the test is uncomfortable rather than painful and feels similar to period pain. Most women and birthing people say they are aware of a ‘pushing’ feeling and some soreness over the area afterwards, but that generally, the thought of the procedure is worse than the actual test.

What happens after the amniocentesis test?

You will probably be at the hospital for about half an hour, but the test itself just takes a few minutes.

If your blood group is Rhesus negative, you will be given an injection of anti D following the procedure to prevent the formation of antibodies in your blood, which may otherwise affect future pregnancies.

We encourage you to bring a companion with you for support during and after the test. It is a good idea to take things easy for a couple of days, although this will not affect your risk of miscarriage.

You may experience some vaginal blood spotting. A 'period pain' feeling may last for 24 to 48 hours. This is not unusual and should settle after rest and Paracetamol, which is safe to take in pregnancy. If you have excessive pain, are leaking any waters, experience vaginal bleeding or develop a high temperature, please contact 01223 217217 (maternity assessment) for advice – this line is answered by a midwife 24 hours a day, seven days a week. Please call your local maternity unit if you are not planning to have your baby at the Rosie Hospital.

When do you get the results?

The laboratory test involves the extraction of DNA from amniotic fluid cells. The DNA is tested using a rapid method called a QF-PCR (Quantitative Fluorescent Polymerase Chain Reaction) test. These tests will assess three of the possible 23 pairs of chromosomes in the baby. The chromosomes that will be assessed will be chromosome 21, 18 and 13, as too many of these chromosomes in an individual are the most common cause of fetal abnormality in the population, namely trisomy 21 (Down’s syndrome), trisomy 18 (Edwards' syndrome) and trisomy 13 (Patau’s syndrome).

These tests do not detect other chromosomal rearrangements (a structural change in a chromosome) or abnormalities of the sex chromosomes. If clinically indicated, sex chromosome tests and microarray may be undertaken (the microarray test can detect changes that cause extra or missing copies of genes, and a separate information leaflet and consent form are required).

The QF-PCR test usually takes about three working days. In certain situations a further test will check all chromosomes of the baby and this result will take about two weeks. If the chromosomes are normal, we will send you a written report by post.

If you are having an invasive test because of ultrasound anomaly, genetic conditions or history then the rapid QF-PCR and microarray will be performed. The rapid test result will be available within three working days and the microarray result will be available at 10 to 14 working days.

If you are having an invasive test due to an increased chance from Down’s syndrome screening then only the rapid QF-PCR test would be performed.

If you wish to have the microarray test or a full karyotype then an additional charge would be made to cover the laboratory expenses involved in this.

As the full karyotype is not performed, it is anticipated a small number of babies will be affected by clinically important chromosomal abnormalities which will not be detected by QF-PCR.

What if the results are abnormal?

If a chromosome problem is detected, you will usually be contacted by either the Fetal Medicine Unit or your own referring hospital. You will usually be contacted by telephone and given an opportunity to discuss the findings. You will be told what the abnormality is and how this could affect your baby. You will have the chance to discuss the results fully before making any decisions.

You will be contacted by one of:
a) a genetics counsellor by prior arrangement
b) the Fetal Medicine Unit at the Rosie Hospital, or
c) your own referring hospital.

HIV infection

We would encourage women and birthing people who are HIV positive to discuss invasive testing with their specialist midwife or doctor to obtain further information. This is because there is a small risk that the HIV virus can be passed on to the developing baby during the procedure.

Outcome of pregnancy

Outcome information is very important to us as this enables us to audit and improve our service. Following your appointment with us, you may be given an outcome form and an addressed envelope, which we would be very grateful if you would complete and return to us.

Medication

Bring all of your medicines (including inhalers, injections, creams, eye drops or patches) and a current repeat prescription from your GP. Please tell the ward staff about all of the medicines you use and any allergies you have.

MyChart

We would encourage you to sign up for MyChart. This is the electronic patient portal at Cambridge University Hospitals that enables patients to securely access parts of their health record held within the hospital’s electronic patient record system (Epic). It is available via your home computer or mobile device.

Further information

If you have any questions about amniocentesis, please telephone the Fetal Medicine Unit at the Rosie. We can arrange for a fetal medicine midwife to talk to you if you wish. We recognise that everyone will have their own particular questions and concerns to discuss. The midwife or doctor will usually be able to answer any questions you may have before you have your test. Please also call us if you need to change or cancel your appointment for any reason (including illness) so your slot can be used by others. You can call the Fetal Medicine Unit directly on 01223 216185.

Useful resources/ References

Antenatal Choices and Results (ARC) charity (opens in a new tab) – Helpline: 020 7713 7486

Royal College of Obstetricians and Gynaecologists (2021) (opens in a new tab) Amniocentesis and chorionic villus sampling, green-top guideline no.8.