Amniocentesis involves taking a small sample of amniotic fluid so the cells it contains can be tested. Amniotic fluid surrounds the unborn baby (foetus) in the womb (uterus).
You will not usually need to do anything special to prepare for amniocentesis. 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 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.
You'll have an ultrasound scan before and during amniocentesis.
An ultrasound scan uses high-frequency sound waves to produce an image of your womb that's relayed to a monitor.
The ultrasound scan allows healthcare professionals to:
Before the needle is inserted into your abdomen, the area may be numbed with anaesthetic.
This involves having a small injection into your belly and may sting slightly.
But anaesthetic is not usually necessary because research suggests it does not have much effect in most cases.
An antiseptic solution will first be used to clean your tummy (abdomen) to minimise the risk of infection.
A long, thin needle is then inserted through your abdominal wall. It may cause a sharp stinging sensation.
Using the ultrasound image as a guide, the needle is passed into the amniotic sac that surrounds the baby.
A syringe is then used to remove a small sample of the amniotic fluid, which will be sent to a laboratory for analysis.
In around 6 in every 100 women having amniocentesis, there's a problem drawing out enough fluid, so the needle has to be reinserted a second time. This is usually because of where the baby is positioned.
If the second attempt is also unsuccessful, you'll be offered another appointment.
Amniocentesis is not usually painful, but you may feel uncomfortable during the procedure.
Some women describe experiencing a pain similar to period pain or feeling pressure when the needle is taken out.
The procedure 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 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 amniocentesis, it's normal to have cramps similar to period pain and light vaginal bleeding called spotting for a few hours.
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: