Marker Scan in Detail
What is a marker scan?
A marker scan is a detailed ultrasound scan done with the specific purpose of looking for features which can change the risk of
Chromosomal problems (usually Down syndrome, but also sometimes Edward’s and Patau syndromes). There are certain features on
scan which are more common in these conditions compared to normal pregnancies. The scan can act as a screening test which will
make the chances of Down syndrome more or less likely compared to the risk before the scan was done.
We are also now experimenting with looking at the nasal bone length and prenasal oedema for detecting Downs syndrome after 14 weeks
of pregnancy. We have included a picture of this as a slide on this page taken from a recent meeting in Croatia – The World Congress
for Fetal Medicine, organised by Professor Nicholaides.
Which markers are important?
In large studies of thousands of pregnancies the following features were found to be the most important in changing the risk of
Down syndrome:
- 1. Nuchal pad: This is a thickening of the skin at the back of the fetal neck. It is different to nuchal translucency (which is fluid within the skin at 11 to 13 weeks). The cut-off value for saying the skin is too thick is 6mm from the bone at the back of the skull to the surface.
- 2. Echogenic bowel: If the fetal bowel appears bright (as the fetal bones) this is called “echogenic”. This means that the bowel looks bright and white on scan. It can be caused by other things, but the presence of echogenic bowel would increase the risks of chromosomal trisomy.
- 3. Short femur (and short humerus): If the femur and/or humerus are short this will increase the risk of chromosomal trisomy. The bones grow and it is very important that measurements are compared to a standard. This can either be done with look-up tables and graphs, or more accurately using a computer program designed for the job.
- 4. Echogenic foci in the heart: Bright spots within the fetal heart (sometimes confusingly called “golf balls”) can increase the risk of Down syndrome.
- 5. Dilated renal pelvis: (Also called pyelectasis) The fetal kidneys produce dilute urine which is collected in the middle of the kidney (renal pelvis) before going down into the bladder. The fluid can normally be seen on scan, but if there is more than 5mm depth of fluid within the kidney this would be more than is usually seen. This is usually not a problem for the baby, but should be followed up with a further scan later in the pregnancy.
- 6. Choroid plexus cyst: This is a cyst within the blood vessels in the developing fetal brain. The cysts disappear by 28 weeks of pregnancy, but if present, particularly with other markers can increase the risks of chromosomal trisomy (especially Edward’s syndrome -trisomy 18)
Other features: The following features would also be looked for but are not so important as those listed above, and have be reported by some studies to give an increased risk of Chromosomal problems. The shape of the fetal head tends to be more round than oval; the fetal nasal bone is shorter; the hands tend to be broader with shorter fingers; the little finger can be shorter and bent (clinodactyly) with an absent middle bone; the hands in Edwards syndrome are clenched with overlapping fingers; the feet can be abnormal with so called “rocker bottom” appearance, the heel can be very prominent or there could be a wide gap between the big toe and the other toes (Sandal gap).
Other problems may be seen on scan which would increase the risk of Chromosomal trisomy. Ventriculomegaly (excess of fluid within the fetal brain), Large mid-line or bilateral cleft lip and palate, heart problems (especially large holes in the heart and a complex condition called tetralogy of Fallot), diaphragmatic hernia (hole in the muscle separating the abdomen from the chest allowing stomach, bowel and other abdominal organs to go into the chest, exomphalos (a defect in the abdominal wall where the umbilical cord enters the body), bladder outflow obstruction, bowel blockages, 2 vessels in the umbilical cord (there are usually 3 with 2 arteries and 1 vein).
Will the scan tell me whether my baby has Down syndrome?
The short answer to this question is no, but it does alter the risk of there being a problem, and some people find it helpful.
If you want to know for certain you will need an amniocentesis.
Who might be helped by having a marker scan?
The most common reason for doing a marker scan is in someone who has a slightly high risk from their age, or from another
screening test (often the triple test). If the risk from the screening test is very high many people will want to go for a definite
(diagnostic) test usually amniocentesis, but if the risk is between say 1 in 125 and 1 in 250 this would also be classified as “high”
risk, but the actual risk is quite small, and about the same as having a miscarriage caused by amniocentesis. The people who want an
amniocentesis, or are waiting for their results will not be helped by a marker scan, but for people who want to avoid amniocentesis,
and are willing to accept a small risk of Down syndrome will remain, can find it reassuring enough to avoid amniocentesis. For some
people amniocentesis is not an option, they have already made that decision, but they want to prepare themselves if there might be a
problem. These people can find it helpful if the scan is normal, but worrying if markers are found.
When can it be done?
A marker scan can be done from 16 to 22 weeks of pregnancy. The details of the anatomy within the heart can be difficult before 18 weeks,
and some scans need to be repeated to ensure that all the features have been seen clearly. If the marker scan is done between 16
and 18 weeks we may be happy regarding all the markers, but still recommend a further scan at 20 to 22 weeks to ensure the heart is
seen clearly.
How long will it take?
The scan will take about 10 to 20 minutes and it will then take another 10 to 45 minutes to explain the findings. Most scans are
completely normal, and these are easy to explain, but when markers are seen this needs much more time. We try hard to complete the
scan on the day you come, but you may be with us for several hours if the baby doesn’t allow the correct views to be obtained.
Please leave plenty of time, as we do not want to be rushing through a scan and consultation like this. We will give you as much time
as you need, and can see you again if you have further questions.
Do you need special equipment to do a marker scan?
It is vital to use good ultrasound equipment if a marker scan is being done because many of the things we are looking for are quite
subtle and difficult to see. 3d and 4d scanning may be important in some cases where problems are suspected in the hands, feet or face,
but the vast majority of the information will come from 2d and colour scanning.
How accurate is the marker scan?
Marker scanning in isolation is not a very accurate way of screening for Down syndrome, but it can be useful in combination with other
screening tests. One problem with any screening test is the false positive results, in other words many people are given a high risk
result when their baby is fine. In order to reduce the false positive rate marker scanning can be helpful. If a marker scan is entirely
normal (all markers are absent) then the risk will be reduced by between 50% and 75%. We prefer to use the 50% figure (halving the risk
we started with) when making our calculations, but the Fetal Medicine Foundation use a factor of 3 times reduction in risk. All risk
calculations are approximate figures, such that if a blood test is repeated in the same person it will give generally the same results,
but not exactly the same. If we were to rely on marker scanning alone, without anything else, the pick-up rate for Down syndrome would
be about 50 to 60%, which is worse than any of the other recognised tests. Between 5 and 10% of marker scans will identify some feature
which is potentially a marker, so this has a high false positive rate when compared with the Triple test. The most common things are
dilated renal pelvis, choroid plexus cysts and a borderline short femur. If a single marker is seen on scan the risk is increased slightly,
but when multiple markers are seen the risks increase sharply. Some findings (such as a nuchal pad) will increase the risks significantly,
while others make very little difference if found in isolation (choroid plexus cyst & dilated renal pelvis).
Is marker scanning available in the NHS?
No it is not available
What is actually done?
The scan is done abdominally with scan gel on the abdominal wall. The whole pregnancy will be assessed. The amount of
liquor (amniotic fluid), the position and appearance of the placenta and a detailed scan of the baby itself. The baby will be measured,
and the anatomy examined in detail. The 6 markers detailed above will be looked for and a full anatomy survey undertaken. Views of the
hands / feet and heart may be difficult at 16 to 18 weeks, but the basic 4 chamber view and the presence of hands and feet will be
included in all marker scans.
Is it safe?
A marker scan, like all ultrasound examinations, is a very safe test, and if it avoids some people having an amniocentesis should
actually reduce the chances of miscarriage overall by reducing the numbers of amniocentesis. The main risk is that we might reassure
you that there are no markers, and that the risk of Down syndrome has gone down, but there will always be some cases of Down syndrome
that will be missed.
Does it ever need to be repeated?
About 1 scan in 50 requires a repeat visit to complete the search for markers and some scans done at 16 to 18 weeks may not allow
all the anatomy of the baby to be clearly seen, especially the cardiac views.
Who should I bring with me?
This scan is going to be emotionally charged. We may be telling you there is an increased risk of problems, we may find things
wrong with your baby. It is definitely best to bring someone with you, either your partner or a close relative for support.
You can bring other people if you wish, but young children get quickly bored and are not as interested in the scans as their parents.
We would generally recommend that young children do not come, as you need all your attention focussed on decision making.
When will the results be ready?
The person doing the scan will talk you through what is being searched for and what is being seen. The full interpretation needs all
the measurements to be compared with previous scans and dates, and this requires a full report which will take about 5 minutes to prepare
and to print off from our fetal medicine database. The results are therefore ready straight away.
Who will talk to me about the results?
The results will be discussed with you, either by one of the consultants, or by an experienced midwife counsellor. The whole purpose
of marker scanning is to give people more information, and as such you will need to make the choice about what to do with the results
when you get them. We will do our best to explain everything clearly to you, but ultimately you will have to balance up the risks and
benefits or having an amniocentesis.
What if a marker is found?
If a marker is found it will be discussed with you. Some markers need follow-up scanning and further actions (dilated renal pelvis &
echogenic bowel), but most are of little significance on their own and will disappear without causing any problems for the baby.
What if something else is wrong?
Most scans are normal, but if something else is found this will be discussed and a recommendation made to you and your medical team
regarding further investigation or treatment. We can go ahead with further management for you on a Private Patient basis, but would
always ask you to consult with your own GP, midwife and Consultant Obstetrician.
Can you tell the sex of the baby?
We can only reliably tell you the sex of the baby from 18 weeks onwards, so if a major reason for coming for the scan is to know the
sex of the baby you should come after 18 weeks.
Will I get any pictures or other recording?
We routinely video all of our scans from beginning to end, and can make a DVD recording if you wish for an additional charge.
This record is yours, and can be used for review by yourselves or anyone else you choose. We also take still pictures which are
sometimes to record a finding such as a marker, but sometimes just for you and your family.
Will I get a written report?
All patients will have a written report which details the findings of the scan and the interpretation.
Will my GP get a report?
We routinely fax your GP with a copy of the report. If you DO NOT want us to tell your GP about the scan that is your right,
but we would strongly encourage you to let us tell your GP as the scan is then part of your overall medical record and can
help other doctors and midwives trying to advise you.
Will my midwife get a report?
All abnormal scan findings are faxed to your hospital screening midwife co-ordinator. We can give you as many additional copies of
the written report as you wish.