Chorionic villus sampling (CVS) involves taking a sample of cells from the tissue of the placenta (the chorionic villi).
You will not usually need to do anything special to prepare for CVS. You can eat and drink as normal beforehand.
In some cases, you may be asked to avoid going to the toilet for a few hours before having CVS because it's sometimes easier to do the test when your bladder is full.
Your doctor or midwife will tell you about this before you attend your appointment.
You can 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 (the protective sac that cushions the baby) or touches the baby.
The test can be carried out using 2 different methods: transabdominal CVS and transcervical CVS.
Your tummy is cleaned with antiseptic before a local anaesthetic injection is used to numb it.
A needle is inserted through your skin into the womb and 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 the neck of your womb (the cervix).
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 10 women who have this procedure.
But there's no difference in the rate of miscarriages between the 2 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, but you may have a sore tummy afterwards.
Transcervical CVS feels similar to a cervical screening test.
CVS usually takes around 10 minutes, although the whole consultation may take about 30 minutes.
Afterwards, you'll be monitored for up to an hour in case you have 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 having CVS, it's normal to have cramps similar to period pain and light vaginal bleeding called spotting for a few hours.
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 afterwards:
The first results should be available within a few days. This will tell you whether a genetic or chromosomal condition has been discovered.
If rarer conditions are also being tested for, it can take 2 to 3 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.