Screening for Down Syndrome

Down syndrome is a genetic and chromosomal condition. In every cell of the human body there are 23 pairs of chromosomes. One chromosome in each pair comes from the mother, and the other comes from the father.

Down syndrome occurs at the time of conception and is not under the direct influence of a woman or her partner. When someone has Down syndrome, there is an extra copy of the 21st chromosome in their cells and they have 47 chromosomes instead of 46. Therefore, another name for Down syndrome is trisomy 21.

Down syndrome is the most common chromosome disorder that we know of. Statistically, 1 in 1,100 babies born worldwide will have Down syndrome.

People with Down syndrome have a range of characteristics which vary between individuals. Intellectual, developmental and health challenges are common, as is having certain characteristic physical features.

The combined first trimester screening test can be done between 10 and 13 weeks of pregnancy. This is done via ultrasound and a blood test.

• Advancing maternal age is one clear risk factor for having a baby with Down syndrome. This is because older eggs do not divide as well as younger ones do. After the age of 35, the risk increases. However, most Down syndrome babies are born to women under the age of 35 because women in this category have more babies.
• Being carriers for genetic translocation for Down syndrome can be passed on by both men and women.
• Having one child with Down syndrome previously.

The combined first trimester screening test can be done between weeks 10 and 13 of pregnancy. Results from a blood test are examined for hormonal changes in the mother which can detect issues with the baby's chromosomes. An ultrasound is also done between 10 and 13 weeks and examines the back of the baby's neck to measure how much fluid is present. This is called nuchal translucency. In babies with Down syndrome the nuchal fold is bigger. There are also changes in the formation of the baby's nose.

A computer analysis combines the results of the two tests and the mother's age to give a risk profile of the baby having Down syndrome. If there is found to be an increased risk, a diagnostic test may be advised.

Non-invasive prenatal testing (NIPT) is another option for genetic screening. This is a new, highly sensitive test which analyzes the mother's blood to check for DNA from the baby. The NIPT can be done from 10 weeks of pregnancy and has a 99% accuracy result for detecting Down syndrome.

Down syndrome and other chromosomal abnormalities are tested for in the NIPT. If the result shows an increased likelihood of the baby having Down syndrome, diagnostic testing such as chorionic villus sampling (CVS sampling) and or amniocentesis are recommended.

Different screening tests can provide different results. Sometimes results can indicate a high chance of Down syndrome when the baby does not have it. A low chance result can also be returned when it is present.

Ask your maternity care provider about the specific testing you are being offered.

The second trimester screening is done between weeks 14 and 22 of pregnancy. A maternal blood test, also called a triple test, can be done between weeks 14 and 18. If the combined first trimester screening test hasn’t been done, this test is another option. This is a blood test which looks for hormones which may be present from the baby's blood.

Most pregnant women have an ultrasound between weeks 18 and 20 of pregnancy. This helps to see that the baby is growing and developing normally. It is also to screen for any complications. Down syndrome may be detected at this scan.

The most accurate way of knowing if a baby has Down syndrome is to have diagnostic testing. These are generally not advised unless the preliminary screening results show the baby is at increased risk.

Diagnostic tests for Down syndrome increase the chances of miscarriage, so they need to be carefully considered before giving consent. Women who have had previous children with genetic or chromosomal abnormities and older women are more likely to be advised to have diagnostic testing.

There are two additional diagnostic tests that can be done.

Chorionic villus sampling is also known as CVS. This is when a fine needle is used to collect cells from the placenta. Laboratory testing looks for missing, abnormal, or extra chromosomes which show Down syndrome is likely. CVS is generally done between weeks 11 and 14 of pregnancy.

Amniocentesis is a test where a fine needle collects a small amount of amniotic fluid. It is examined to look for extra or missing chromosomes. Amniocentesis is generally done between weeks 15 and 18 of pregnancy. An ultrasound is done at the same time to minimize risk and make sure the needle is inserted accurately.

Both CVS and amniocentesis involve collecting a sample of the baby's genetic material to be examined in a genetics laboratory. Results from tests are generally back within a week.

All babies are examined and checked by a healthcare professional at birth. Babies with Down syndrome have similar physical characteristics. A blood test taken from the baby can confirm the diagnosis of Down syndrome. This checks for the number of chromosomes in the baby's cells.

It's useful to consider what you may do once you have the results of your Down syndrome testing. This is a highly personal decision, best made after discussion with your maternity care provider, partner and family. There are several options when it comes to counseling and being supported. Speak with your maternity care provider about consulting with a genetic counselor.

How long does it take to get CVS and Amniocentesis tests back?

You will need to speak with your maternity care provider about the results. Depending on the findings, you may be referred to a genetic counselor.

Do I have to have testing for having a baby with Down syndrome?

Like all other prenatal tests, you have a choice about what screening is right for you.