All pregnant women, no matter what their age, should be given the choice to have their baby screened for Down's syndrome.
Antenatal screening is a way of assessing whether your unborn baby could develop, or has developed, an abnormality or condition during your pregnancy.
Antenatal screening cannot diagnose conditions such as Down's syndrome. However, what it does show is how likely it is that your baby will develop the syndrome. If the risk of Down's syndrome (or any other condition) is shown to be high, further testing can then be arranged to help confirm whether your baby has the condition.
Antenatal screening for Down's syndrome is carried out through a blood test and an ultrasound scan. This is known as a 'combined test'.
Blood test
During a blood test, a sample of your blood will be taken and tested to check the levels of certain proteins and hormones. If your blood contains abnormal levels of these substances, you may have an increased risk of having a baby with Down's syndrome.
Nuchal translucency ultrasound scan
During your pregnancy, you will have a number of ultrasound scans to check the development of your baby. An ultrasound scan is a painless procedure which uses high frequency sound waves to produce a picture of the inside of your body - in this case, the womb.
To screen for Down's syndrome, you will need to undergo a special type of ultrasound, known as a nuchal translucency. This type of ultrasound works in the same way as a normal ultrasound scan, but it focuses on measuring the space between the spine and the nape of the baby's neck.
All babies tend to collect fluid behind the neck. However, babies with Down's syndrome normally have more fluid in the neck than normal. By measuring the thickness of fluid, your doctor can help to determine how likely it is that you will have a baby with Down's syndrome.
Diagnostic tests
If the results of your antenatal screening or ultrasound scan suggest that your baby may be at an increased risk of a particular health problem or condition, such as Down's syndrome, you may be advised to undergo further testing.
There are two types of test that can help to diagnose potential health problems or conditions while your baby is still in the womb. These are chorionic villus sampling (CVS) and amniocentesis.
It is important to be aware that both of these diagnostic tests can, in rare cases, result in complications. Your GP, or midwife, will be able to discuss all of your options with you in detail.
Chorionic villus sampling (CVS)
Chorionic villus sampling (CVS) involves having a small sample of the placenta (the organ in which the foetus grows and is protected and nourished) taken for examination. It can be performed after 10 weeks of pregnancy. The sample is taken either by passing a thin needle through the wall of your abdomen, or by passing a small tube through your vagina and the neck of your womb (cervix).
The needle, or tube, is put into the correct position using an ultrasound scan for guidance. The sample is taken and is later sent to a laboratory for testing. The results will be able to show whether your baby has Down's syndrome.
Possible complications of CVS can include infection and heavy bleeding. Approximately 1% of women who have CVS also go on to experience a miscarriage. However, it is important to remember that the risk of miscarriage is very small, and most procedures cause no complications.
Amniocentesis
Amniocentesis involves taking a small sample of amniotic fluid (the fluid that surrounds the foetus in the womb) for examination. It is usually performed after 15 weeks and can be carried out until 22 weeks. The sample is taken by passing a needle through your abdomen and womb. The amniotic fluid can then be drawn out through a syringe.
As with CVS, the needle is guided into the correct position using an ultrasound scan. After the sample of amniotic fluid has been taken, it is sent to a laboratory for testing.
Possible complications of amniocentesis can include infection, or injury, to you, or your baby. As with amniocentesis, there is also a 1% chance of miscarriage. Amniocentesis is a fairly common procedure, and the risk of complications is low.
Counselling
Being told that your baby may have Down's syndrome can be difficult news to deal with. You will therefore be offered counselling to give you, and your partner, the chance to express your feelings and to ask questions about how this diagnosis may affect you both.
A counsellor will be able to make you aware of the different options available to you, allowing you to make a more informed decision about how to proceed with the pregnancy.
Diagnosis after birth
Once your baby is born, the initial diagnosis of Down's syndrome is normally based on your baby's physical appearance. Your doctor will assess your baby and look for the typical physical features associated with Down's syndrome, such as eyes which slant upwards, or a flat back to the head.
If your doctor needs to definitively diagnose Down's syndrome, they will arrange for a blood test known as a chromosomal karyotype. A sample of your baby's blood will be taken and sent to a laboratory so that the chromosomes in the blood can be analysed. If the blood test finds that there is an extra 21 chromosome, your baby will be diagnosed with Down's syndrome.
