Chorionic Villus Sampling (CVS)
CVS can be performed early in pregnancy, usually from 11 to 14 weeks. Using an ultrasound scan as a guide, a very thin needle is used to take a tiny sample of tissue from the placenta. This fine needle is usually put through your abdomen or very occasionally through your vagina.
The cells from the tissue can be tested for Down’s syndrome and other chromosomal and inherited disorders.
The sample is then sent to a laboratory to be analysed. Very occasionally the laboratory is unable to get an accurate result from the sample. If this is the case, you will be offered an amniocentesis.
Some women say the procedure feels uncomfortable and it is not unusual to feel some period-like cramping pains afterwards. It is a good idea to take it easy for 48 hours after the procedure and avoid strenuous physical activity such as heavy lifting.
QF-PCR and FISH are molecular tests that can be performed on a CVS sample to provide a rapid but accurate diagnosis of Down’s syndrome and two other rare but serious chromosomal syndromes called Patau’s and Edwards’ syndromes. Results take on average three working days. All hospitals can offer this service through the laboratories they use, but some will ask you to pay a fee.
A full karyotype means the laboratory use cells from the sample to look at all the baby’s chromosomes under a microscope. They check for any major changes in the chromosomes and can tell the baby’s sex. It is a longer process than the rapid tests and results can take up to two weeks.
Genomic microarray (Array CGH) is an advanced method of genetic testing of a sample from CVS or amnio. It can detect copy number changes in a baby’s chromosomes. This means it looks for where there are deletions (bits missing) or duplications (where there are extra bits) in the baby’s DNA that would not be identified through the full karyotype. It is now sometimes used instead of or as well as karyotyping.
Why do some women miscarry after CVS?
Approximately one out of every 200 women who has a CVS will miscarry. It is difficult to give a definite reason why this happens, especially because at this stage in pregnancy, some women miscarry naturally.
Most of those few women who have a miscarriage due to the procedure have it fairly soon afterwards, within around 72 hours. There is still a risk of miscarriage after this, and for up to two weeks after the CVS, but it is extremely unlikely to happen.
If the healthcare professional performing the procedure is skilled at doing it, this can help to minimise the risk. This skill comes with experience so you can ask how often the doctor who is going to do your CVS does the procedure. The Royal College of Obstetricians and Gynaecologists recommends that a healthcare professional should do at least thirty procedures a year to retain his or her competency.