Amniocentesis and Chorionic Villus Sampling (CVS)

Amniocentesis and CVS are diagnostic tests that are performed prenatally to detect whether a fetus has a chromosome abnormality such as Down Syndrome (Trisomy 21).  Less commonly they are performed to detect inherited genetic disorders in couples known to have an increased risk of a particular condition.  These procedures enable laboratory testing of fetal chromosomes from fetal cells in the amniotic fluid (amniocentesis) or placental cells (CVS).

These are diagnostic tests with very high accuracy for the detection of chromosomal anomalies in particular and in some cases of heritable genetic disorders.

Although considered quite safe, these procedures are not without risk. In general terms the risk of the procedure needs to be balanced against the risk of the condition targeted.

Patients with a specific past history of a genetic or chromosomally abnormal pregnancy may elect to have a diagnostic test without any preliminary screening procedures and of course no patient is compelled to have a diagnostic test regardless of an assessment of increased risk.

It is important to be aware of your blood group when booking either of these procedures.  If your blood group is rhesus negative you will need to have an injection of anti D following the procedure to prevent the development of rhesus disease in future pregnancies.

Chorionic Villus Sampling

Chorionic villi are small moss like projections that are part of the early placenta.  The cells present in the chorionic villi almost always have the same chromosomes as the fetus hence abnormalities in the chromosomes of the chorionic villi are also found in the fetus.

During a CVS procedure a fine needle is passed through the mother’s abdomen and a sample of the small fragments of chorionic villi are obtained which can then be examined for chromosome anomalies.

A CVS is usually performed from 11 weeks of gestation with ultrasound guidance.

It is not without risk.  The main risk is of pregnancy loss, which for CVS in most reported studies is between 1:100 and 1:200.

The preliminary results for Down syndrome and other major chromosomal defects are usually available within 3 – 4 days.    The full chromosome analysis takes between 2 and 2 1/2 weeks.

Common Reasons for Amniocentesis and CVS

  • High risk First Trimester Ultrasound Screen results
  • Increased risk by maternal age, without a prior screening test.
  • Previous pregnancy with chromosomal abnormalities or genetic disorder.
  • Family history of chromosomal anomaly or genetic disorders.  Most chromosomal anomalies are random events and do not have a familial predisposition.
  • Major abnormality. Amniocentesis may be indicated if a major abnormality is found at the time of a fetal anatomy scan irrespective of a low risk First Trimester Screening result.
Who makes the decision to have a CVS or Amniocentesis?

The patient: in conjunction with her partner after discussing the pros and cons with their medical practitioner and being satisfied with the level of information provided.

What to do on the day of your procedure?

You only need to prepare yourself for the ultrasound examination.  You do not need to fast.  It is advisable to have your partner or support person drive you to and from your appointment.  Some patients experience some cramping discomfort after the procedure.


It is best to rest for 24 hours after the procedure.

Coronavirus (COVID-19) Health Alert

The World Health Organization (WHO) declared COVID-19 a worldwide pandemic on 11 March 2020. The Government of Western Australia has declared an end to the COVID-19 Pandemic State of Emergency on 4 Nov 2022. Please inquire with our medical receptionists about the relevant steps taken to manage bookings with COVID-19 still in the community.

The following guidelines provided from St John of God Hospital apply within our clinics:

    • Face masks are not mandated but they should be worn in crowded indoor settings and where physical distancing is not possible; patients are free to wear them at their discretion.
    • We can confirm vaccination status of our employees and contractors meet WA Health regulations.
    • Hand hygiene is to be practiced at all times.
    • Please do not enter the clinic if you are unwell, tested positive for COVID-19 in the last seven days, been in close contact with a COVID-19 positive person in the last seven days, or if you have any symptoms of infectious illness such as: fever, cough, shortness of breath, sore throat, loss of smell or taste, runny nose, diarrhoea, nausea or vomiting.

Please attempt to confirm your appointment before coming.

Healthdirect Hotline: 1800 020 080 | Department of Health Health Warnings

Updated 05 Jan 2023