Screening Tests in Pregnancy – Screening for Trisomies
Down’s Syndrome, Patau’s Syndrome and Edward’s Syndrome.
Combined Screening, Quadruple Test and Non Invasive Prenatal Testing (NIPT)
These syndromes are also called trisomies. We have 23 pairs of chromosomes as humans. The above syndromes arise when a mistake occurs very early on in conception and instead of just making 2 copies of a chromosome, a third copy is made. Chromosomes contain genes.
The way I describe genes to my daughter is this – when you go to ikea for furniture, you get a package with a set of instructions. If the instructions are correct you build a lovely new piece of functional furniture. If the instructions are wrong then the piece of furniture is built wrong and may or may not work properly. Well in our bodies, our instructions are our genes. A chromosome is a whole package of genes/instructions. Extra information is not always a good thing, it can confuse the body on how it is made. In the above trisomies, the extra instructions mean different things.
These syndromes can affect anyone. It is a random mistake. Nothing you can do or not do can influence this mistake. The risk does however increase with the age of the mother.
Down’s Syndrome – This is an extra chromosome 21. It affects 1 in every 1000 births.
This syndrome is compatible with life. Babies born with Down’s syndrome will have a degree of learning difficulties and health problems. These problems lie on a spectrum from mildly affected to badly affected. More is now understood about Down’s syndrome. These children can have a good quality of life with support from family. For more information on Down’s Syndrome take a look at these websites.
Patau’s Syndrome – This is an extra chromosome 13. It affects 2 in every 10,000 births.
Edward’s Syndrome – This is an extra chromosome 18. It affects 3 in every 10,000 births.
Unfortunately having an extra chromosome 13 or 18 is not compatible with life. Babies with these syndromes sometimes do not survive pregnancy. If born alive they die shortly after.
Why are these tests offered to pregnant women?
Due to the detrimental impact on the health of the baby, if a test shows that an unborn baby has one of these syndromes then the mother is offered a termination. This can be done at any stage of pregnancy. Although never an easy decision, earlier detection allows earlier decisions.
How is the screening test carried out?
You can opt for having the screening test for all 3 syndromes, Down’s Syndrome only, Patau’s and Edward’s only or to decline all. You are usually asked to decide this at your booking appointment.
Initially the first blood test (taken with consent) will only tell you the risk of your baby having one of these syndromes. It is not a diagnostic test. It does not put your pregnancy at risk. This is called Combined Screening if carried out between 10 and 14 weeks. Combined because the risk is calculated using a measurement taken from the back of the baby’s neck during your dating ultrasound as well as the results of the blood test. Combined Screening will detect about 85 – 90% of babies with Down’s Syndrome. If you are too late for this then the risk is calculated from the blood test alone between 14 and 20 weeks. This is called the Quadruple test. This test picks up around 80% of babies with Down’s Syndrome.
If you are having a twin pregnancy, you should be offered a conversation with the midwifery screening coordinator or with an obstetrician. You can still have Combined Screening or the Quadruple test but the results are complex.
Could I not just wait until the 20 week scan?
An ultrasound scan is also a screening test. In many cases a baby with Down’s Syndrome will appear physically normal on a scan. Some babies with Down’s syndrome will have a heart condition which may or may not be seen on scan. Therefore ultrasound is not a reliable method for detecting Down’s syndrome. Babies with Edward’s or Patau’s Syndrome usually have multiple abnormalities which are often seen on an ultrasound scan. These can sometimes be seen on the dating scan and almost definitely on the anomaly scan.
The blood screening test allows earlier detection. If a serious abnormality is diagnosed an earlier termination can be carried out.
How will I get the results?
A low risk result from the initial blood test (screening test) results are given usually via letter sent to your home address within 2 weeks of the test date.
A high risk result from the initial screening test is usually given to you via a telephone call, unless you have specified any other arrangements). This should be within 3 working days from the test date. A further appointment will be offered to discuss further options.
What happens next?
If you get a low risk result, you will carry on with your planned antenatal care. A low risk result does not mean that the baby does not have the condition it means that it is very unlikely.
If the screening test comes back with a high risk result then you are offered a diagnostic test which is either a CVS – chorionic villus sampling 11 – 15 weeks pregnancy or amniocentesis after 15 weeks. These are invasive tests that do carry a small risk of miscarriage which is about a 1% risk.
Everybody has different opinions on these tests and the consequences. It is a personal choice that you have to make. Please do not let anybody else influence your decision. It is yours to make. If you are struggling with the decision on whether to have these tests then ask yourself these questions. What do the above syndromes mean to you? How do you feel about them? If you knew your unborn baby had one of these syndromes, what do you think you would do? Is termination something you would consider? Would you want a screening test or diagnosis in order to gain more information/preparation?
Information on Combined screening and the Quadruple Test from
Non Invasive Prenatal Testing (NIPT)
You may have been told or heard about NIPT. This is another screening test. It is performed via a maternal blood test. There is no risk to the baby. It is not currently available on the NHS. However much research is being carried out to see if it has a place in the NHS. The test is currently available via private clinics at a cost. It can be performed form 10 weeks of pregnancy and results are usually available 10 days to 2 weeks after the test.
NIPT can pick up other conditions too. Whilst at the private clinic this should be discussed with you. You can usually opt in or out of this.
So how is does NIPT work?
Within mums blood you can find cell free DNA, whilst pregnant some of this will be cell free Fetal DNA. NIPT will detect this cell free Fetal DNA, once analysed a result can be given on the risk of that baby having one of the above trisomies.
How is it different to the tests offered on the NHS? Although still classed as a screening test it is over 98% accurate. NIPT tests DNA. Combined Screening/Quadruple test analyses maternal hormones.
What results can we get from NIPT for Down syndrome?
Positive: Predicted to be affected by Down syndrome. An invasive test should be offered to confirm the result.
Negative: Highly unlikely to be affected by Down syndrome.
Inconclusive: Inconclusive results happen in up to 4% of cases. This is usually because the proportion of fetal DNA present in the sample is not high enough to give an accurate result. NIPT may be repeated with the hope that the cffDNA levels will have increased due to the increased gestation.
NIPT detects around 98% of all babies with Down, Edwards and Patau syndromes.
Why isn’t NIPT 100% accurate?
False positives are when a test result is positive for the problem but actually the problem isn’t there.
These occur in around 0.3% of cases. Possible causes include:
Cell free Fetal DNA comes form the placenta. Most of the DNA in the placenta is identical to the baby’s DNA. Occasionally an anomaly occurs and some of the in the DNA in the placenta is different to that of the baby. The baby is completely unaffected. This is called placental mosaicism. Sometimes a problem is detected in the mothers DNA that isn’t present in the baby.
False negatives are when the test result is negative for the problem but actually the problem is there.
A possible causes of false negative results could be the proportion of fetal DNA in the maternal blood is too low. This may be due to early gestation or an increased maternal BMI. A scan is needed to confirm gestation prior to the test.