Informed Choices: Diagnostic Prenatal Testing (Amniocentesis & CVS)

I have actually written two posts around genetics recently – so this will be a shorter post, just reviewing the potential risks, benefits and ethical considerations behind genetic testing – specifically with a focus on the diagnostics amniocentesis and chorionic villus sampling (CVS) since I haven’t written about these before.

You can refer to my 2 previous posts here: one is on the NIPT and eFTS blood tests offered in pregnancy for trisomy & other genetic screening; the other is on my experience seeing a genetic counsellor and how to prepare for this type of appointment.

Looking for the rest of the Informed Choices series? Click here or search “informed choice” in the search bar.

Disclaimer: this blog is not medical advice. This blog is for informational purposes only. Please seek medical advice from your trusted healthcare provider.

What is chorionic villus sampling?

Chorionic villus sampling, also known as CVS, is an invasive diagnostic test that is offered early in pregnancy – between 11 and 14 weeks generally.

The test can help to determine the presence of a suspected Trisomy, or other chromosomal differences.

Using an ultrasound-guided technique, the test can be done either transcervically (through the vagina and cervix) or transabdominally (less common), similarly to an amniocentesis (explained below). The tool or needle takes a small piece of placental tissue for testing.

Results are received in about 2-3 weeks time.

What is amniocentesis?

Amniocentesis is considered an invasive diagnostic test. It is invasive because it is not just a blood sample, like the screening test the NIPT. It is a procedure that involves retrieving an amniotic fluid sample with fetal cells by guided ultrasound.

An ultrasound is done first to visualize your baby, placenta and so on. A needle is then inserted into your belly and the ultrasound is used to help guide the needle to a small pocket of the amniotic fluid. It can be uncomfortable, but only a small amount of fluid is removed so it should be short-lived (however sometimes it does need to be re-inserted or dual insertion may be required for twins with separate sacs). Generally this test is offered only between weeks 15 to 20 of pregnancy, but it can be pursued later on if it is considered necessary.

The sample is then sent off for testing – generally, an amniocentesis is indicated to confirm if Trisomy 21 or another syndrome. It may be offered to you after an ultrasound with soft markers for Down Syndrome for example, or after a high risk NIPT result.

What’s the difference between CVS and Amniocentesis?

CVS samples from the placental tissue, while amniocentesis takes a fetal cell sample from the amniotic fluid. With CVS, a result could come back positive for a trisomy, but it could be as a result of confined placental mosaicism. This means that the detected missing or additional chromosome is only present in placental cells, and not the fetus. Sometimes, amniocentesis will be offered as a follow up if this is suspected, since it looks at the baby’s cells.

Confined placental mosaicism can present with placental problems and does often require some follow up.

If you’re considering between the two, there are risks/benefits to both that should be discussed with your provider. Some are explored below, however this is not exhaustive or complete.

Risk/Benefit Analysis

There are some risks to amniocentesis, which is why the decision to pursue it can sometimes be harder for people. In terms of post-procedure issues, fluid may leak from your vagina or you may bleed – this requires a call to your provider. Your provider will also review with you any other reasons to call – such as prolonged cramping that is severe or development of a fever. If you have Rh- blood type, you will likely require Rhogam after the procedure.

Other complications are reported as rarehowever they concern many people when considering the testing. One major complication you’ve almost certainly heard related to amniocentesis is increased risk for miscarriage. Studies do show there may be a slight increase in risk, at about 1 per 3000 cases. Notably, about 10 years ago the risk was reported as about 1 in 200. Notably – the exact risk of miscarriage after an amniocentesis is hard to estimate, which is why you might hear or see such varied numbers. It is therefore something that should be discussed in-depth with your care provider.

I know this was something I considered when we were offered it. I have spoken about my experience and thought process around this in this post but I don’t know if I ever followed up on this, so I will discuss that briefly below.

Another cause for concern is the possibility of causing injury to your baby. Again, studies show a potential small increase in risk of this occurring– but the risk is considered very low.

In regards to benefits, there are several which is why the testing is offered in the first place. It can confirm the presence of certain Trisomy diagnoses and other chromosomal differences – or reassure parents that these issues are not there. This can allow for better preparation, considerations of options, and in some cases peace of mind. Note that not all chromosomal differences can be tested for or confirmed by amniocentesis.

CVS is considered the “alternative” to amniocentesis. Its miscarriage risk appears lower than amniocentesis at <1% (or 1 in 455 women). In rare cases, complications such as bleeding or infection may occur. Bleeding is a particular problem if you are Rh- blood type, so Rhogam may be administered similarly to the amniocentesis. Spotting after the procedure can occur if it is done transcervically. Similarly to amniocentesis, CVS cannot detect all chromosomal differences.

One risk that you may have heard associated with CVS is limb defects or injury. This seems to be associated with the performance of CVS prior to 11 weeks. The risk is otherwise low – estimated at 6 in 10000 cases by the World Health Organization, and this is the same as the risk for those who do not receive CVS.

CVS is diagnostic though (unlike the NIPT for example which is a screening test, with notably good sensitivity and specificity for certain trisomies). An additional benefit is that it can be done in first trimester.

A Note on Ethics

Genetic testing (in general, not just in pregnancy) is a very controversial topic because of ethical and moral considerations. Some people will feel that it is not right, for various reasons. Some will worry about what genetic testing may lead to in the future, with developing technologies. I just wanted to highlight this because it makes genetic testing a very, very personal choice – but also one that should be given great consideration. From a provider perspective, a good allotment of time should be set aside to discuss these decisions. Even something as “simple” or “non-invasive” as a blood test like the NIPT comes with its own ethical concerns and deserves time to be discussed in detail.

I recently read an article for a research paper in my Masters course that examines this topic in the Ontario population – I’ve linked it in the above paragraph several times, but especially if you are a healthcare provider, it is worth a more thorough review (click here to read the study by Vanstone et al., 2018 open access). My recent research project made me realize (as well as my own experience) that the testing is not often discussed in-depth, when it should be. For various reasons – not just from an ethical standpoint, but so that we as healthcare providers make sure we are answering people’s questions and making sure they are making an informed choice. I’ve seen reports of many women, for example, not understanding the difference between the screening vs diagnostic options; not understanding the meaning of results (which they absolutely should); and feeling as if they weren’t given enough information about the options and their differences. We need to do better than this.

As always, thanks for reading and I’ll be back soon with another Informed Choices topic!

Additional Resources

U of Michigan Medicine – Should I Have Amniocentesis?

Up to Date – Amniocentesis

Prenatal Screening Ontario – CVS

My Health Albert – Should I Have CVS?

StatPearls – CVS


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