Antenatal screening helps health professionals to estimate the likelihood of your baby having a serious condition or abnormality. If the tests show a high risk of your baby developing a serious condition or abnormality then a diagnostic test is usually offered. This is called prenatal diagnosis and gives a definite answer as to whether your baby has the abnormality or not
You will only be offered a diagnostic test if there is a significant risk of your baby being born with a certain condition or abnormality. The tests are invasive (involve going into the body) and carry a small risk of miscarriage. Your midwife or doctor will explain the significance of the risks involved and give details of what the tests involve.
It is important for both parents to carefully consider any implications of the results of a diagnostic test. You do not have to have the test.
The diagnostic tests usually offered are amniocentesis, chorionic villus sampling (CVS), or ultrasound scanning.
Amniocentesis can be offered to you from 15 weeks of pregnancy. It will be offered to you if:
- you have a positive screening result for Down's syndrome,
- an ultrasound scan detects an abnormality associated with a genetic disorder, or
- your health or family history suggests you may be at risk of having a baby with a disorder such as Down's syndrome.
Amniocentesis involves having a fine needle passed into the amniotic fluid that surrounds your baby in the womb. An ultrasound scan is used throughout the test as a guide to check the position of the baby and placenta. A small sample of the amniotic fluid is taken and sent for testing. This fluid contains the same chromosomes as your baby.
The fluid sample is tested for Down's syndrome or other serious syndromes. It can detect some other rare genetic conditions too if your midwife or doctor has asked for you to be tested for them. This test will reveal to your midwife or doctor the sex of your baby, so tell them in advance if you do not wish to know.
Amniocentesis has a 0.5-1% risk of miscarriage. At most, one in a hundred tests will result in a pregnancy loss.
Chorionic villus sampling (CVS)
The test involves having a fine needle passed through the abdomen into the womb. Sometimes a fine tube is passed through the vagina into the cervix instead. A tiny piece of the developing placenta, known as chorionic tissue, is taken. The chromosomes in the cells of the tissue are examined.
CVS can be carried out from 10 weeks of pregnancy and can let you know early on if your baby is at risk of having an inherited condition such as cystic fibrosis, sickle cell disease, thalassaemia or muscular dystrophy.
CVS has a 1-2% risk of miscarriage. This is slightly higher than the risk for amniocentesis.