After you have had the amniocentesis procedure, the sample of amniotic fluid (the 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
- a full karotype
These are described in more detail below.
Rapid test
A rapid test looks for abnormalities on specific chromosomes (the parts of the body’s cells that carry 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. This test is almost 100% accurate, but it only tests for the three conditions listed above.
Full karotype
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 before being examined under a microscope to check for:
- the number of chromosomes
- the appearance of the chromosomes
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 due to the mother’s blood contaminating the sample of amniotic fluid, which may have prevented the cells from growing properly.
Negative test results
For the majority of women who have amniocentesis, the results of the procedure will be ‘negative’. That is, their baby will not have any of the disorders that were tested for.
However, it is possible to have a negative result from amniocentesis but your baby is born with the condition that was tested for or with another chromosomal condition. This is because a normal test result does not exclude every chromosomal disorder.
Genetic (inherited) disorders are caused by mutations (changes) in the genes (units of genetic material). Each chromosome contains thousands of genes. This means it is not possible to test for every possible genetic mutation, so a baby may occasionally be born with a condition that was not detected.
Positive test results
If your test result is ‘positive’ it means that your baby has the disorder that was being tested for. If you receive a positive test result, the implications will be fully discussed with you. Be aware that there is no cure for the majority of chromosomal conditions, so you need to consider your options carefully.
Your options may include:
- continuing with your pregnancy while getting information and advice about the condition so that you are prepared for caring for your baby
- ending your pregnancy
If you are considering ending your pregnancy, talk to your GP or midwife. They will be able to provide you with 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.
