All pregnant women are offered screening for Down's syndrome. Find out how your chances of having a baby with Down's syndrome can be worked out.

Down's syndrome is caused by an abnormal number of chromosomes. For example, It affects around one in every 1,000 babies. There's no such thing as a typical person with Down's syndrome. Like everyone, people with Down's syndrome vary a lot in appearance, personality and ability. People with Down's syndrome have learning difficulties and some have more serious difficulties than others. It's hard to tell how much Down's syndrome babies will be affected as children or adults. There is currently no cure for the condition, but there are treatments that can help someone with the syndrome lead an active and independent life.

Conditions linked with Down's syndrome include heart problems and reduced hearing and vision. Many of these problems can be treated, and frequent health checks can ensure that any problems are detected as early as possible. Most people with Down's syndrome live to around 60, and some live to be over 70.

What causes Down's syndrome?
Inside the cells of our bodies are tiny structures called chromosomes. Chromosomes carry the genes that determine how we develop. Most people have 23 pairs of chromosomes in each of their cells. When our bodies produce the sperm and egg cells needed to create babies during reproduction, the chromosome pairs divide and rearrange themselves. Sometimes these pairs don't divide correctly, and this causes the baby's cells to have an extra copy of chromosome number 21. This causes Down's syndrome.

The extra chromosome can't be removed from cells, so there's no cure for the condition. The chromosomes divide incorrectly by accident, not because of anything the parents have done.

Some people think that only older women can have a baby with Down's syndrome, but this is not true. Anyone can have a baby with Down's syndrome, but the risk increases with age. The older a mother is, the more likely she is to have a baby with the condition.

Screening
Screening for Down's syndrome and other genetic disorders is offered to all pregnant women. There are different ways of carrying out screening tests.

Combined screening
This involves a blood test and an ultrasound scan. All women should have dating and nuchal translucency scans between 8-14 weeks of pregnancy. These scans may be combined. The blood test measures two blood chemicals associated with pregnancy. At the ultrasound, the radiographer measures the thickness of the nuchal translucency (a pocket of fluid) at the back of your baby's neck. This information is used to calculate your individual chance of having a baby with Down's syndrome. This nuchal translucency scan can be used in multiple pregnancies (if you're having twins or more).

Serum screening
Serum screening is a blood test that screens for Down's syndrome, usually when you are around 16 weeks pregnant. It measures three or four blood chemicals that are associated with pregnancy, and this can be used to work out your individual chance of having a baby with Down's syndrome. Serum screening is not used for pregnancies of twins, triplets or more.

Screening results
Some maternity services give the result as 'lower risk/screen negative' or 'higher risk/screen positive'.

If the screening test shows the risk of the baby having Down's syndrome is lower than the recommended national cut-off level for risk, this is known as having a low-risk result. A low-risk result means that you are at a low risk of having a baby with Down's syndrome, but it doesn't mean that there is no risk at all.

If the result shows the risk of the baby having Down's syndrome is greater than the recommended national cut-off level for risk, this is known as an increased risk, or higher risk, result. An increased risk means you will be offered diagnostic tests but it does not mean that your baby definitely has the condition.

The diagnostic procedure you will be offered is either chorionic villus sampling (CVS) or amniocentesis, which will give you a definite answer about Down's syndrome.

Your doctor will explain the result to you and help you to decide whether you want to have further testing.

Amniocentesis and chorionic villus sampling
These tests will tell you whether your baby definitely has Down's syndrome. Find out how they are carried out and what the risks are.

These are diagnostic tests that will give you a definite diagnosis of Down's syndrome and, sometimes, other abnormalities. Your doctor will explain what's involved, and you'll usually be offered counselling.

Amniocentesis
After counselling, amniocentesis may be offered from the time you are 15 weeks pregnant if:

• you have a positive or higher-risk screening result for Down's syndrome
• an ultrasound detects an abnormality that's associated with a genetic disorder
• your history or your family history suggests that there may be a risk of your baby having a genetic or chromosome disorder such as Down's syndrome, sickle cell disorderor thalassaemia

What happens
Using ultrasound as a guide, a fine needle is passed through the wall of the abdomen into the amniotic fluid that surrounds the baby. The cells within the fluid contain the same chromosomes as the baby. A small sample of this fluid is extracted and sent to a laboratory for testing. Most women feel only mild discomfort.

Usually, the fluid will be tested for Down's syndrome and other serious syndromes, such as spina bifida and sickle cell. The results should be available within three working days. If all the chromosomes have to be looked at, it can take up to three weeks. This test will reveal your baby's sex, so tell your doctor if you want to know at this stage whether your baby is a boy or girl.

The risks
Amniocentesis has a 0.5-1% risk of causing a miscarriage. At most, one test in a hundred will result in pregnancy loss. When deciding whether or not to have this test, try to balance the risk of miscarriage with how important the result will be to you.

Chorionic villus sampling (CVS)
CVS can be carried out earlier than amniocentesis, from when you are around 11 weeks pregnant. It can give you an earlier diagnosis if you're at risk of having a child with an inherited disorder, such as cystic fibrosis, sickle cell, thalassaemia or muscular dystrophy.

What happens?
The test takes 10-20 minutes and may be a little uncomfortable. Using ultrasound as a guide, a fine needle is passed through the abdomen into the womb (uterus). Sometimes a fine tube is passed through the vagina and cervix into the womb instead. A tiny piece of the developing placenta, known as chorionic tissue, is taken. The chromosomes in the cells of this tissue are examined. As with amniocentesis, a rapid result can be obtained, but if all the chromosomes are checked the results can take up to two weeks.

The risks
CVS has a 1-2% risk of miscarriage. This is slightly higher than for amniocentesis.

If a test detects an abnormality
It's always difficult when you're told that something is wrong with your baby, especially if you're faced with a painful decision about the future of your pregnancy. Your doctor will make sure you see the appropriate health professionals to help you get all the information and support you need so you can make the right choices for you and your family.