What is 'Invasive Testing'?

Chorionic villi sampling (CVS) and Amniocentesis.

Invasive testing is undertaken when you wish to be 100% certain as to whether the fetus has Down syndrome or any other chromosomal abnormality. A CVS is performed between 12- 14 weeks of pregnancy. An amniocentesis is usually performed between 14 -16 weeks, but may be done any time later in the pregnancy if required.

When is invasive testing needed?

Invasive testing would be recommended if you were to fall in to a high risk group for Down syndrome following the 12 week screening test (nuchal translucency scan). It may also been undertaken in cases of advanced maternal age (over 34 years) where no screening test has been performed. Sometimes there is a family history of a particular genetic problem, for example cystic fibrosis. In these cases a CVS would need to be performed to exclude these genetic variations.

Which test is better, a CVS or amniocentesis?

A CVS would be performed if, the test needed to be done before 14 weeks gestation and or a specific genetic information was required due to the patient having a family history of a specific problem, for example, Dwarfism, Fragile X, muscular dystrophy.

An amniocentesis would be performed if, the test was required after 14 weeks and  Down syndrome or other chromosomal problems needs to be excluded. An amniocentesis or CVS will not exclude specific genetic problems as a routine. Each individual genetic problem neeeds to be tested individually, so a CVS that is tested for cystic fibrosis, for example, will not tell us if the fetus has muscular distrophy.

Are there any risks involved?

There is a risk of miscarriage with both tests. Up until recently, it was thought that the miscarriage rate for CVS was about 1%, but a recent study has shown that when the CVS is performed in a tertiary centre, the chance of miscarriage is considerably less being 0.22% or about 1 in 500.


This study also found the rate of miscarriage for Amniocentesis was also much lower when performed at a tertiary centre, being 0.11% or about 1 in 1000 (Ann Tabor, World Conference in Fetal Medicine 2016)

How are they performed?

In the case of a CVS a scan is first performed by a sonographer and the placental location established. The obstetrician will then give the patient a local anaesthetic and place a needle into the chorionic villi of the placenta. Small samples are then taken and sent to the laboratory for testing. In most cases the needle would be in place for 3 – 5 minutes.

In the case of an amniocentesis the sonographer will first scan the fetus. The obstetrician will then give the patient some local anaesthetic and a needle will be placed within the amniotic fluid. 20 mls of amniotic fluid is taken and the sample sent to the laboratory for testing. In most cases the needle would be in place for about 2 minutes.

Can these tests hurt the baby?

When the needles are being introduced into the placenta or amniotic fluid the obstetrician is viewing the proceedings on ultrasound at all times. This ensures that the needle does not come in contact with the fetus.


How accurate are the results?

The amniocentesis is virtually 100% as the laboratory are looking directly at the chromosomes. The most commonly reported cause for incorrect results is by writing the wrong name on the test tubes. The doctor will ask you to check this before you leave our rooms.

It is possible when doing a CVS to get an ambiguous result (mosaicism). This happens in 4% of cases. When this occurs, an amniocentesis may also need to be performed at 15 weeks to confirm the results one way or another

How long does it take to get the result?

We normally get an interim result (FISH) the evening after the amnio or CVS and we will telephone you with these results as soon as we receive them. This interim result usually tells us about four pairs of chromosomes (13, 18, 21 and sex chromosomes) which together account for 90% of chromosomal abnormalities found. The final results come to us 10 to 14 working days later and tell us about all 23 pairs of chromosomes. This will also confirm the results of the interim report.

Will I be able to find out the baby’s sex?

As long as we have the results of the sex chromosomes we will be able to tell you the baby’s sex with 100% accuracy. This information will only be given to you if requested.

What happens afterwards? 

Most women are able to continue their normal activities. Extreme activities should be avoided for a day or two. A few women experience a period-type pain almost immediately after the procedure. This is not related to miscarriage. It is the muscle of the uterus reacting to the needle. It should settle fairly quickly.

What should I look for?

A 1% of women who have had an amniocentesis or CVS will experience a small amniotic fluid loss (leak) or bleeding. This is usually of no concern and will normally cease within 48 hours. If this happens to you please:
• Rest in bed for 48 hours
• Do not have a bath (showering is fine)
• Do not have sex
• Call Dr Carmody’s rooms on 3371 4933. We will book an appointment for a scan 48-72 hours after the procedure.
If you experience severe pain, blood loss or large gush of fluid out of our office hours, please proceed to your closest hospital – they will contact Dr Carmody. If there is no leak or bleeding within one hour after the procedure, you can resume life as normal. Paracetamol is helpful for mild uterine cramping.