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Chorionic villus sampling (CVS) involves taking a sample of cells from the tissue of the placenta (the "chorionic villi").
You won't usually need to do anything special to prepare for CVS. You can eat and drink as normal beforehand.
In some cases, you may be advised to avoid going to the toilet for a few hours before the test, because it's sometimes easier to perform when your bladder is full. Your doctor or midwife will tell you about this before you attend your appointment.
You may want to bring a partner, friend or family member for support when you have the test.
CVS is carried out under the continuous guidance of an ultrasound scan. This is to make sure nothing enters the amniotic sac (protective sac that cushions the baby) or touches the baby.
The test can be carried out using two different methods: transabdominal CVS and transcervical CVS.
Your tummy is first cleaned with antiseptic before a local anaesthetic injection is used to numb it.
A needle is then inserted through your skin into the womb. The needle is guided to the placenta using the image on the ultrasound scan.
A syringe is attached to the needle, which is used to take a small sample of cells from the chorionic villi. After the sample is removed, the needle is removed.
A sample of cells from the chorionic villi is collected through your cervix (the neck of the womb).
A thin tube attached to a syringe or small forceps are inserted through your vagina and cervix, and guided towards the placenta using the ultrasound scan.
The transabdominal method is preferred in most cases, because it's often easier to carry out.
Transcervical CVS is also more likely to cause vaginal bleeding immediately after the procedure, which occurs in about 1 in every 10 women who have this procedure. There is, however, no difference in the rate of miscarriages between the two methods.
Transcervical CVS may be preferred to transabdominal CVS if it's easier to reach your placenta this way.
CVS is usually described as being uncomfortable, rather than painful.
In most cases, an injection of local anaesthetic will be given before transabdominal CVS to numb the area where the needle is inserted, although you may have a sore tummy afterwards.
Transcervical CVS feels similar to a cervical screening test.
The procedure usually takes around 10 minutes to perform.
Afterwards, you will be monitored for up to an hour, in case the test causes any side effects, such as heavy bleeding. You can then go home to rest.
It's a good idea to arrange for someone to drive you home, as you might not feel up to it yourself.
After CVS, it's normal to have cramps similar to period pain and light vaginal bleeding called "spotting" for a day or two. You can take over-the-counter painkillers such as paracetamol (but not ibuprofen or aspirin) if you experience any discomfort.
You may wish to avoid any strenuous activity for the rest of the day.
Contact your midwife or the hospital where the procedure was carried out for advice as soon as possible if you develop any of the following symptoms after the procedure:
The first results should be available within a few days, and this will tell you whether a genetic or chromosomal condition has been discovered.
If rarer conditions are also being tested for, it can take two to three weeks or more for the results to come back.
You can usually choose whether to get the results over the phone or during a face-to-face meeting at the hospital or at home.
Read more about the results of CVS.