Down syndrome is a genetic disorder that occurs very early in your pregnancy when your baby is first forming. While most people have 23 pairs of chromosomes, people with Down syndrome have an extra copy of chromosome 21.There are 2 types of screening tests that are available that tell you if your baby might be at risk of having Down syndrome. Further diagnostic tests can then be done to confirm the results.
This involves a blood test and an ultrasound scan.
Women may be offered 2 ultrasounds at the beginning of their pregnancy:
dating scan between weeks 8 and 14 of pregnancy
nuchal translucency scan between 11 weeks and 13 weeks and 6 days of pregnancy (sometimes called the ‘12 week scan’).
If you choose to be screened for Down syndrome, the dating scan and the nuchal translucency scan can be carried out at the same time, between 11 weeks and 13 weeks.
At the ultrasound scan appointment, the sonographer measures the thickness of the nuchal translucency (a pocket of fluid) at the back of your baby’s neck.
You have the blood test between 9 and 12 weeks. The blood test measures the levels of two different hormones that occur naturally in your blood during pregnancy. When there’s a genetic condition the levels of these hormones are usually abnormal.
The information from the blood test is combined with your age and the nuchal translucency measurement and used to work out your individual chance of having a baby with Down syndrome.
If it has not been possible for you to have the combined screening test in early pregnancy, you will be offered a different blood test between 14 and 20 weeks.
This test is sometimes called the 'triple test' or 'maternal serum screening' test (MSS) and measures proteins associated with pregnancy. This information is combined with your age and used to work out your individual chance of having a baby with Down syndrome.
If the screening tests come back indicating that there is a risk that your baby may have Down syndrome, your midwife or doctor will explain the results to you and help you to decide whether you want to have further testing. One of your options will be to have a further diagnostic test which provides a definite answer.
The diagnostic test will either be by chorionic villus sampling (CVS) or amniocentesis. Your midwife or doctor will explain the result to you and help you to decide whether you want to have further testing.