Antenatal screening and testing can detect the likelihood of a baby being born with Down syndrome. However, the condition is often diagnosed after birth when physical characteristics of Down syndrome are recognised.
Pregnant women of any age should be offered screening for genetic conditions such as Down syndrome.
Antenatal screening is a way of assessing the likelihood of your baby developing or already having developed an abnormality during your pregnancy. It cannot diagnose conditions such as Down syndrome but may help with the decision to have pre-natal diagnostic tests.
You should ideally have antenatal screening for Down syndrome by the end of your first trimester (13 weeks and six days). Most women are screened between 11 to 13 weeks. However, if necessary, it is possible to have screening up to 20 weeks into your pregnancy (this may involve more blood tests).
Read more information about antenatal appointments and the pregnancy care guide.
Screening tests can:
- reassure you that your baby has no detected structural abnormalities
- give you time to prepare for the arrival of a baby with additional needs
- enable you to consider termination
Tests can also provide valuable information for your care during the pregnancy. However, no test can guarantee your baby will be born without an abnormality. No test is 100% accurate and some abnormalities may remain undetected.
If the chance of Down syndrome (or any other condition) is shown to be high, you may be offered pre-natal diagnostic tests (see below). These are tests to find out the likelihood of your baby being born with the suspected condition.
The screening test used for Down syndrome is known as the ‘combined test’. It includes a blood test and ultrasound scan.
A sample of your blood is 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 chance of having a baby with Down syndrome.
A special type of ultrasound scan, known as nuchal translucency, measures the pocket of fluid behind the baby’s neck. Babies with Down syndrome usually have more fluid in their neck than normal. Measuring the thickness of fluid will help determine whether your baby is likely to have Down syndrome.
Pre-natal diagnostic tests
If the results of your antenatal screening or ultrasound scan suggest your baby may have an increased chance of a condition, such as Down syndrome, you may be advised to have further tests.
Chorionic villus sampling (CVS) or amniocentesis both test for health problems while the baby is still in the womb.
It is important to be aware that both of these tests can result in complications, although this is uncommon. Your GP or midwife can discuss your options with you in detail.
Chorionic villus sampling (CVS)
Chorionic villus sampling (CVS) involves taking a small sample of the placenta for closer examination. The placenta is the organ in which the foetus develops.
CVS can be performed after 10 weeks of pregnancy. The sample is taken either by passing a thin needle through the wall of your abdomen (tummy), or by passing a small tube through your vagina and the neck of your womb (cervix).
The needle or tube is guided into the correct position using an ultrasound scan. After the sample has been taken, it will be sent to a laboratory for testing. The results will show whether your baby has Down syndrome.
Possible complications of CVS include infection, heavy bleeding and miscarriage. It is estimated that approximately 1 in every 100 women will have a miscarriage following CVS.
Amniocentesis involves taking a small sample of amniotic fluid (the fluid that surrounds the foetus in the womb) for examination.
Amniocentesis is usually carried out after week 15 of pregnancy, and can be carried out up until week 22. The sample of amniotic fluid is taken by passing a needle through your abdomen (tummy) and womb. The sample is 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 will be sent to a laboratory for testing.
Possible complications of amniocentesis can include infection and injury to you or your baby.
As with CVS, there is also a 1 in a 100 chance of having a miscarriage after the procedure.
Being told that your baby may have Down syndrome can be difficult news to deal with. You should be offered counselling to enable you and your partner to discuss the impact of the diagnosis upon you and the available options. This will allow you to make an informed decision about how to proceed with the pregnancy.
Read more about genetic testing and counselling.
Diagnosis after birth
Once your baby is born, the initial diagnosis of Down syndrome is usually based on your baby’s physical appearance.
Read more about the characteristics of Down syndrome.
If your doctor needs to definitively diagnose Down 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 chromosome 21, your baby will be diagnosed with Down syndrome.