Down's Syndrome: The facts
From the cause to the tests, we help you understand what you need to know about Down's Syndrome
What is Down's Syndrome?
A flashback to school science classes may remind you that the body’s genetic structure is made up of cells, which each contain 23 pairs of chromosomes. Down's syndrome is a naturally occurring genetic difference caused by an extra copy of the 21st chromosome. Babies with Down's syndrome are born in every country to parents of all ages, races, religions and backgrounds.
People with Down's syndrome have some level of learning disability. With the right support, the majority will meet the usual childhood milestones and make good progress at mainstream school.
Babies, children and adults with Down's syndrome inherit their families’ characteristics and have more in common with their families than each other. This includes personality, sense of humour, interests and hobbies as well as physical characteristics.
Down's syndrome can make some people more prone to some common health conditions, but like the rest of society, people with Down's syndrome are living longer and healthier lives. Improvements in understanding of the condition and education means people with Down's syndrome are able to live full, rewarding and increasingly independent lives.
Like any baby, nobody can predict the strengths and weaknesses that a person with Down's syndrome will have. A small percentage of people with Down's syndrome (estimated at c15%) may have more complex needs that require extra support. Whatever the level of assistance required, there is excellent support available from many health professionals, teachers, peers and support groups. Families with a child with Down's syndrome are eligible to financial support and adults with Down's syndrome are also entitled to financial benefits.
Screening at 10-14 weeks pregnant
An optional screening test for Down's syndrome is offered to all women in England. This will usually be done with a combined test (ultrasound and blood test) in weeks 10-14 of pregnancy. The ultrasound test/nuchal translucency (NT) scan checks the amount of fluid behind your baby’s neck, while the blood test samples the proteins human chorionic gonadotrophin (hCG) and pregnancy-associated plasma protein (PAPP-A) in your blood.
Screening at 14-20 weeks pregnant
If your pregnancy is too far along for the combined test you will be offered other options. For Down's syndrome the quadruple test (a blood test which samples four proteins) is available in weeks 14 to 20.
Is screening essential?
Antenatal screening is optional and should be carefully considered together with your partner. It is advisable to research Down's syndrome before screening so that you understand what it is and what living with the condition means, so that you understand what you are screening for, which can help you determine whether to screen or not and to be prepared for your scan. There are lots of excellent resources available from www.dsa.org.uk and www.positiveaboutdownsyndrome.co.uk features stories written by parents and young people with Down's syndrome.
None of the above tests carry any risk of hurting your baby or miscarriage. If you choose not to be tested you can still have your dating ultrasound scan, but you should be aware that abnormalities may be picked up during the scan, and you will be told if they’re discovered.
Understanding your test results
Once you’ve been screened, the results of your test combined with your age will be used to determine the likelihood that your baby could be affected by Down's syndrome.
This test is not conclusive but will give you the likelihood of your baby being affected. The results will be characterised as ‘high chance’ or ‘low chance’, with high chance being a 1 in 20 likelihood, and low chance as less than 1 in 150 likelihood.
If your results are high chance you will usually be notified within 3 working days, while low chance results should be returned within 2 weeks. However, this can depend on your local area, so check with your midwife for details.
What happens if the result is ‘high chance’?
If your results are categorised as high chance, this doesn’t mean your baby definitely has the condition. You will be offered the opportunity to have further diagnostic tests if you want confirmation as to whether the syndrome is present or not.
A few health trusts offer a non-invasive test (NIPT) which takes a further blood sample and provides a result with 98% accuracy, so this means it is not diagnostic as it is not definitive. The test offers no risk of miscarriage, but you may have to pay to have this done privately.
Alternatively, the NHS offers two types of test; amniocentesis, where a fine needle is used to take a sample of amniotic fluid, or chorionic villus sampling (CSV) where a fine needle is used to take a sample of the placenta. These tests are conclusive but carry a one in 100 risk of miscarriage, so should be considered carefully before going ahead.
If you have a high chance of baby having Down's syndrome you should be offered counselling with a trained midwife who can help you understand more about the condition and who can help you source additional information, including contact with local support groups and families who have a child with Down's syndrome, if you would find this helpful.
What is Nuchal Translucency?
The nuchal translucency is a name for the collection of fluid behind the neck of a foetus that can be seen on ultrasounds in early pregnancy. The fluid is present in all foetuses; however, an increased collection indicates chromosome differences present in conditions such as Down's syndrome. It is called a 'translucency' because on an ultrasound this appears as a black space beneath the foetal skin.
What is a Nuchal Translucency scan?
A nuchal translucency scan is an optional ultrasound offered to all pregnant women during their first trimester. The scan is carried out between weeks 11 - 13 of pregnancy and is used to screen for Down's syndrome, or other chromosomal conditions by detecting and measuring the amount of nuchal translucency in the foetus.
An increased amount of nuchal translucency on the scan indicates chromosomal difference, however this is just an indication rather than a confirmed diagnosis.
The NT scan can be combined with a blood test, known as the first trimester combined screening, to get a more accurate prediction. The blood test will measure the presence of 2 hormones hCG and PAPP-A, that are produced by the foetus and passed into the mother’s bloodstream. Low levels of PAPP-A in early pregnancy also indicate an increased chance of genetic differences in the foetus.
The blood test, combined with the results of NT screening and the mother's age, provide a more accurate assessment - without the blood test, screening is 75% accurate for predicting Down's syndrome, with the blood test accuracy increases to 85%.
What happens during a Nuchal Translucency scan?
You’ll be referred by your doctor for your NT scan, which will normally take place at a hospital and be carried out by a sonographer.
The scan usually takes about half an hour. You’ll be asked to lie down on your back, the sonographer will then apply gel across your lower abdomen and move a handheld device called a transducer across the gel. The scan shouldn’t be painful but you may feel some pressure as the sonographer presses on your abdomen to get a clear picture.
Occasionally it can be difficult for the sonographer to get a clear scan from an abdominal ultrasound, in which case a vaginal ultrasound will be offered. This is when a small transducer will be inserted into the vagina to get a closer look at the foetus and clearer images. This method can be a little uncomfortable but is safe for the baby.
What are the risks of a Nuchal Translucency scan?
There are no known risks to mother or foetus when having a nuchal translucency scan and no aftereffects of an abdominal or transvaginal ultrasound.
However, it is important to be aware of the implications of taking the test and the decisions to consider based on results. If the result of your scan is high chance you could be offered further diagnostic tests that can tell you for certain whether your baby has a genetic condition, however these more invasive tests do have a risk of miscarriage.
Getting the results of my NT scan
The time it takes for you to receive the results of your NT scan can depend on various factors including the complexity or urgency of results, but you should usually receive them within 2 weeks.
When you receive the results, it is important to understand what a normal NT measurement is. For a baby that is between 45 mm and 84 mm in size, an NT of less than 3.5mm is considered normal. An NT less than 1.3 mm is considered to be low chance and an NT of 6 is considered high chance for Down's syndrome and other potential chromosomal differences.
What if the results show my baby has a high chance of Down's syndrome?
If the results show a high chance of baby having Down's syndrome it’s important to remember the test is not 100% accurate and it’s not known for certain whether your baby has the condition.
You’ll have the choice to undergo further testing such as Chorionic villus sampling (CVS) or Amniocentesis, to find out for certain if your baby has a genetic condition. These tests do carry the risk of miscarriage so it can be a difficult decision to make and one not to be done in a hurry.
Watch this NHS video on the Down's syndrome test.