Chorionic villus sampling (CVS) is only offered if you are pregnant and have an increased chance of having a baby with a genetic or chromosomal condition. It can diagnose a range of conditions.

You'll be offered CVS if your test results or medical or family history suggest you have a higher chance of having a baby with a genetic or chromosomal condition.

You do not have to have the test if it's offered. It's up to you to decide whether you want it.

CVS can be used to diagnose a number of conditions, including:

• Down's syndrome – all children born with Down's syndrome have some degree of learning disability and delayed development, but this varies widely between children
• Edwards' syndrome and Patau's syndrome – conditions that can result in miscarriage, stillbirth or severe physical problems and learning disabilities
• cystic fibrosis – a condition where the lungs and digestive system become clogged with thick, sticky mucus
• Duchenne muscular dystrophy – a condition that causes progressive muscle weakness and disability
• thalassaemia – a condition that affects the red blood cells, which can cause anaemia, restricted growth and organ damage
• sickle cell disease – where the red blood cells develop abnormally and are unable to carry oxygen around the body properly
• phenylketonuria – where your body cannot break down a substance called phenylalanine, which can build up to dangerous levels in the brain

CVS cannot detect neural tube defects. These are birth defects affecting the brain and the spinal cord, such as spina bifida, which can usually be detected with an ultrasound scan.

If you're offered CVS, ask your doctor or midwife what the procedure involves before deciding whether to have it.

You may also find it helpful to contact a support group, such as Antenatal Results and Choices (ARC).

ARC is a charity that provides information, advice and support on all issues related to screening during pregnancy.

Reasons to have CVS

CVS will usually tell you for certain if your baby will or will not be born with any of the conditions that were tested for.

You might find that your baby does not have the condition screening tests said they might have, which can be reassuring.

But if the test confirms that your baby does have the condition they were tested for, you can decide how you'd like to proceed.

Read more about the results of CVS for more information.

Reasons not to have CVS

Miscarriage can happen. Up to 1 out of every 100 people who have CVS will miscarry.

Find out more about the complications of CVS

You may choose to have an alternative test called amniocentesis later in your pregnancy instead.

You may decide you'd rather find out if your baby has a genetic condition when they are born.