Screening For Down Syndrome
Dear Parents to be,
Congratulations! You are stepping into the one of the most rewarding journey of your life, the destination of which will be blissful parenthood. It is indeed our pleasure to support you through this journey as the Fetal Medicine Unit. The endeavour of Fetal Medicine is to safeguard the health and wellbeing of your unborn baby. We specifically follow the growth and development of the fetus and look for any problems and suggest the corrective treatment. Another important aspect in fetal wellbeing is to look for chromosomal or genetic problems that may be inherent and that can profoundly affect the quality of life after birth. In this context, we want to educate you about screening for Down Syndrome.
What is Down Syndrome?
Down Syndrome (DS) is a chromosomal disorder in which the affected individual has one extra chromosome in addition to what is the normally allocated number of chromosomes. Every human being has 46 chromosomes arranged in 23 pairs. In people with Down Syndrome, the chromosome pair number 21 has three copies instead of two so that the total number of chromosomes is 47 instead of the normal 46. Each man’s sperm has 23 chromosomes and so does each woman’s egg, so that when conception occurs and the sperm fertilises the egg, a new human being with a full complement of chromosomes is formed. Sometimes mistakes occur and lead to what are described as chromosomal abnormalities. Down Syndrome is one of the commonest of these.
At conception, instead of one number 21 chromosome from the father and one from the mother coming together, a third chromosome creeps in and is then duplicated in every cell of the baby’s body. Down Syndrome, also called Trisomy 21, is the most common chromosomal abnormality, occurring approximately once in every 600 births.
DS is a condition that some babies are born with. It affects about one in every 800 babies. Children can have a wide range of learning difficulties and some medical problems, including heart defects, bowel problems, chest problems and sinus problems. Some adults with DS are able to get jobs and live fairly independent lives. However, most people with DS need long-term support and ongoing care. DS is caused by the presence of an extra chromosome (number 21) in the cells of a developing baby. Usually it is not inherited and so a baby can be affected even if there is no history of Down’s syndrome in the family. We do not know exactly why babies get DS. Although older women are more likely to have a baby with the condition, any woman can have a baby with DS. It is not caused by anything the parents have done or not done.
Why should we test for Down’s syndrome?
Down Syndrome is the most common chromosomal abnormality and can have a major social, financial and emotional impact on a family. It is the commonest cause of global developmental delay in children, which is NOT inherited.
What are the characteristics of Down’s syndrome?
People with Down Syndrome will all have some degree of learning difficulty. Many people with Down’s will go to ordinary schools and lead semi-independent lives, but others will need full-time care.
Certain medical conditions are more likely to occur in people with Down Syndrome.These include:
- Heart defects, which occur in one in three children with Down’s
- Digestive tract defects
- Problems with sight
- Problems with hearing
All people with Down’s will have certain physical characteristics, such as eyes that slant upwards and outwards, a single crease running across the palm of the hand, low-set ears and small hands.
Babies born today with Down Syndrome can expect to live between 40 and 60 years on average.
Are you at risk of having a baby with Down’s syndrome?
Every mother is at a risk of having a child with Down Syndrome and this increases with the mother’s age, especially over the age of 35. However, majority of babies with Down’s syndrome are born to the relatively younger mothers because there are more number of pregnancies in women below the age of 35.
What can you do to find out whether or not you baby has Down’s syndrome?
The mother can undergo certain tests in pregnancy. There are two types of test available – screening tests and diagnostic tests.
A screening test estimates the risk of your baby having Down Syndrome. It only tells you whether or not you are at particularly high risk of having a baby with Down Syndrome. This does not confirm the chromosome status of your baby.
A diagnostic test can give you a definite diagnosis, but also carries a small risk of miscarriage.
What screening tests are available?
There are many different screening tests available.
There are ultrasound tests, blood tests and a combination of the two.
The ultrasound tests include the nuchal transluscency scan between 11- 14 weeks of pregnancy and the anomaly scan between 18-20 weeks. In these scans, apart from the general growth and development, we look for markers for Down Syndrome in the fetus and depending on the presence or absence of these markers, we assess the risk for Down Syndrome
The blood tests include the first trimester test
which measures two hormones in the mother – beta HCG and PAPP-A between 10-14 weeks of pregnancy or the second trimester triple serum screening in which three hormones (beta HCG, AFP and uE3) are measured in the maternal blood between 15 – 20 weeks.
If both ultrasound and blood tests are done, then the risk assessment obtained from both can be combined to get a more accurate final risk assessment.
These are the screening tests. They can’t tell you for certain that your baby has or has not got Down syndrome. A screening test can help you decide whether or not to have a diagnostic test.
The diagnostic test is either a CVS (Chorionic Villus Sampling) or Amniocentesis which will confirm the chromosomal status of the fetus and tell you definitely whether or not your baby has Down Syndrome. However, these are invasive tests and they carry a risk of miscarriage. Hence they are generally offered only if the screening tests are suggestive of a problem.
A cut-off point of 1 in 300 is usually used at BFMC. If your risk is less than that, you will be described as “screen negative” which means that you are unlikely to be carrying a baby with Down’s (but it’s not impossible). We will discuss all results with the parents. However, if any couple consider their ‘personalized risk assessment’ high enough to warrant an invasive test, this will be discussed and their opinion respected.
These tests will detect many pregnancies affected by Down syndrome, but they will also identify some pregnancies as being "at risk" of Down Syndrome when, in fact, they are not; these are known as "false-positives".
What is spina bifida?
Babies with spina bifida have an opening in the bones of the spine, which can result in damage to the nerves controlling the lower part of the body. This causes weakness and paralysis of the legs and sometimes bowel and bladder problems.
Serum screening to test for DS and spina bifida (Quadruple test)
The Quadruple test is a blood test that screens for DS and spina bifida. We offer this test to every woman who did not have a combined test (done at 11 – 14 weeks) earlier in pregnancy. We only offer you the Quadruple test if it is too late for you to have the combined test or in the context of a “Sequential screen”. This is when you have an ‘intermediate risk” for DS at the combined test or you did not wish to do an invasive test at the 11 – 14w scan but you are keen to screen the pregnancy further. In both these situations, the Quadruple test is combined with the Anomaly scan (also called Genetic Sonogram) at 18 – 20 weeks which confirms the presence or absence of markers for a complete and thorough risk assessment.
The Quadruple test is a screening test, which means it cannot tell us whether your baby does or does not have DS or spina bifida. However, it can tell us which women are at a greater risk of their baby having either of these conditions.
The test that we offer is called the ‘Quadruple test’ because it measures the amount of four hormones in your blood: AFP beta HCG inhibin-A oestriol.
The amounts of these substances are combined with details of your age, weight and the gestational age of your pregnancy (how far into your pregnancy you are – calculated from a dating scan). From this information, a computer programme calculates the risk of your baby having DS. The level of AFP is also used to calculate if there is an increased risk of your baby having spina bifida.
What are the benefits of having the Quadruple test?
The Quadruple test is a screening test. It will give you a more accurate idea of the risk of your baby having DS than an estimate based on your age alone. However, it cannot tell you whether your baby definitely does have or does not have DS. The Quadruple test picks up around 80% of babies with DS. This means that around one in five (20%) of Down’s babies are not picked up by this screening test. The false positive rate (the likelihood of predicting Down’s syndrome in babies without DS) is about 3%. The test also picks up about 80% of babies with spina bifida. This means that about one in five (20%) of babies with spina bifida will not be picked up by the Quadruple test. However, the best screening test for spina bifida is a detailed ultrasound scan that is offered to all women in mid pregnancy i.e. at 18 – 24 weeks.
What are the limitations of the Quadruple test?
The test can only be performed between 15 weeks and 20 weeks and 6 days of pregnancy (based on ultrasound dates). If you are pregnant with twins, we recommend that you have the combined test in early pregnancy (between 11 weeks and 2 days and 13 weeks and 6 days), as this test is able to give a risk for each baby. The Quadruple test is not recommended for multiple pregnancies.
Are there any risks?
The blood test is not dangerous in any way and will not harm you or your baby.
Are there any alternative tests?
An alternative is a detailed scan in mid-pregnancy. Around half (50%) of babies with DS will have features that can be picked up at this scan. This means that around half or 3 out of 5 DS babies will not have these features and will therefore have completely normal scan findings. Almost all babies (95%) with spina bifida will have features that can be picked up at by the midpregnancy scan. This means that 1 in 20 (5%) of babies with spina bifida will not have any features on scan.
The dating scan
Before you can have the Quadruple test, you need to have a scan to accurately date your pregnancy. This may be early in pregnancy or it may be at the detailed mid-pregnancy scan. This scan can also exclude or confirm a twin pregnancy.
When will I get the results?
The result for spina bifida is reported as ‘screen negative’ or ‘screen positive’. This means that the results are assumed to be either positive or negative based on the results of the test. The result for DS is given as a number, and gives an estimate of your baby’s risk of being born with DS. If your result is screen negative or low risk, we will send your results by email. If your result is screen positive or high risk, you will be contacted by telephone to return to the clinic to discuss your options in detail with one of our Specialists. If you have not received your results 10 days after your Quadruple test, please contact the fetal medicine unit.
What does a ‘low risk’ result mean?
If you have a ‘low risk’ result, it means that the chance of your baby having DS is less than one in 300. The chance of your baby having spina bifida is reported in the same way. A ‘low risk’ result does not rule out the possibility that your baby might have either of these abnormalities – it simply means that the risk is low. What does a ‘higher risk’ result mean? This indicates that the chance of your baby having Down’s syndrome is greater than 1 in 300/ your age related risks, or that the risk of spina bifida is increased. This does not necessarily mean that your baby has DS or spina bifida. Most women who have a ‘higher risk’ result will not have a baby with either of these conditions.
You will be invited to discuss the results, and further possible tests, with one of the specialist doctors at BFMC.
Eligibility and detection rates:
|Test||Test type||Time Frame||Detection rate|
|Non Invasive Cell Free Fetal DNA||Blood Test||99.6%|
|SEQUENTIAL SCREENING COMBINES ALL THE BELOW||Blood + Scan||97-98%|
|Combined / Oscar test / First Trimester Screening Test||Blood + Scan||10 to 13+ 6 Weeks||90%|
|NT Scan||Scan||11 - 13 + 6 weeks||85%|
|Second Trimester Quad||Blood Test||15 - 20 + 6 weeks||75%|
|First Trimester Double Marker||Blood test||10 - 13 weeks||67%|
|Second Trimester Triple||Blood test||15 - 20 + 6 weeks||65%|
|Genetic Sonogram||Scan||18 - 23 weeks||60-70%|
WE HAVE ADDITIONAL INFORMATION LEAFLETS DETAILING FIRST AND SECOND TRIMESTER SCREENING TESTS.