After you've had the amniocentesis procedure, the sample of amniotic fluid that surrounds the unborn baby in the womb will be taken to a laboratory for testing.

There are two different types of tests – a rapid test and a full karotype. These are described below.

Rapid test

A rapid test looks for abnormalities on specific chromosomes (the threadlike structures that carry genetic information in the form of genes).

A rapid test can identify a number of chromosomal conditions that cause physical and mental abnormalities. These are:

• Down's syndrome – caused by an extra chromosome 21
• Edward's syndrome – caused by an extra chromosome 18
• Patau's syndrome – caused by an extra chromosome 13

The results of a rapid test should be ready after three working days. The test is almost 100% accurate, but it only tests for the three syndromes listed above.

Full karyotype

Each cell in the body contains 23 pairs of chromosomes. A full karotype checks all of these.

The cells in the sample of amniotic fluid are grown for up to 10 days in a laboratory. They are then examined under a microscope to check the number of chromosomes and their appearance.

Results from a full karotype will usually be ready in two or three weeks. In about 1 in every 100 tests, the results may not be clear. This could be because the mother's blood has contaminated the sample of amniotic fluid, which may have prevented the cells from growing properly. 

Negative test results

In most cases, the results of amniocentesis are negative. This means that the baby doesn't have any of the disorders tested for.

However, it's possible to have a negative result from amniocentesis, but your baby may still be born with the condition tested for, or they may be born with another chromosomal condition. This is because a normal test result doesn't exclude every chromosomal disorder.

Genetic disorders are caused by mutations (changes) in the genes (units of genetic material). Each chromosome contains thousands of genes. This means it's not possible to test for every possible genetic mutation, so a baby may occasionally be born with a condition that wasn't detected.  

Positive test results

A positive test result means your baby has the disorder that was tested for. The implications of a positive test result will be fully discussed with you.

You should be aware that there's no cure for the majority of chromosomal conditions, so you need to consider your options carefully. These may include:

• continuing with your pregnancy while getting information and advice about the condition so you're prepared for caring for your baby
• ending your pregnancy 

Talk to your GP or midwife if you're considering ending your pregnancy. They'll be able to give you important information and advice.

For example, your options for ending your pregnancy will depend on how many weeks pregnant you are when you make the decision.

If you decide to end your pregnancy, you may wish to talk to a counsellor afterwards. Your GP or midwife will be able to arrange this for you.

Read more about genetic testing and counselling.