These are diagnostic tests that will give you a definite diagnosis of Down's syndrome and, sometimes, other abnormalities. Your midwife or 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 disorder or 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 midwife or 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 midwife or 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.
The organisation Antenatal Results and Choices helps parents with all the issues around antenatal testing and its implications. They can give you more information or put you in touch with parents who decided to continue with a pregnancy in which an abnormality had been detected, or those who decided to have a termination.
