Obstetric Ultrasound Markers
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- Obstetric Ultrasound Markers
What are ultrasound markers?
When we look at all the structures of the baby during the ultrasound exam, often slight deviations from normal anatomy are detected. These deviations are called "markers" because they usually do not mean that there is anything wrong with the baby, but indicate that the risk of the baby having a problem may be slightly increased. However, it is very important to realize that many normal babies also have markers and most often if a marker is found the baby will still be completely normal.
However, despite the fact that these "markers" usually do not mean there is any problem with the baby, their discovery can cause a great deal of anxiety for the parents. Parents should realize that these markers are not considered an abnormality and most have no significance whatsoever. They usually disappear in a matter of weeks. However, the reason they are not ignored is that occasionally, ultrasound markers may be a sign of an underlying chromosomal abnormality.
What are chromosomal abnormalities?
Chromosomes carry all our genetic material: having the right number in every cell is crucial to normal development. We should all have 23 pairs of chromosomes. At the moment of conception, 23 from the father’s sperm and 23 from the mother’s ovum (egg) come together to form a new life. Mistakes can happen during this process and when they do, most will miscarry. In fact, half of all miscarriages are due to chromosomal abnormalities. Sometimes though, the pregnancy continues with a baby that has either too many or too few chromosomes, or sometimes with pieces of chromosomes mixed up or missing.
A baby with Down’s syndrome has three of chromosome number 21 instead of two. This is why the condition is also known as ‘trisomy 21’. The next two most common chromosomal abnormalities are Edward’s syndrome (‘trisomy 18’) and Patau’s syndrome (‘trisomy 13’).
Many babies with chromosomal abnormalities will have ultrasound markers that may be seen during the 18 week anatomy ultrasound exam, but most will also have structural abnormalities. So if the baby looks normal in every other way, it is usually always normal and there shouldn’t be any need to be concerned about these results.
The most common markers are discussed below.
These probably cause the greatest anxiety. To be told your baby has a cyst or cysts in its brain is scary, to say the least. In fact, a choroid plexus cyst is just a collection of fluid in the part of the brain which makes the fluid that cushions the brain and spinal cord. The ‘cyst’ is just a small build-up of fluid as it moves through the connecting tubes. As the baby grows, the tubes get bigger and the fluid moves on. CPCs are almost always gone by 24 weeks and do no harm to the baby’s brain.
CPCs look like black spots in the hemispheres of the brain and were first seen in the 1980s. They caused a scare initially because many babies with Trisomy 18 were found to have CPCs. However, research has shown that these babies almost always have other abnormalities, such as strawberry-shaped heads, overlapping fingers and abnormal feet, heart, kidney or abdominal problems. Babies can have these cysts on one or both sides of the brain and there can be one or more cysts on each side. However, the number or size of the cysts do not matter as these facts have no bearing on the overall risk for a chromosomal abnormality. The risk is the same if there are one or multiple cysts.
It is now known that one per cent of all babies have one or more CPCs in early pregnancy. In the absence of other markers (isolated CPCs) they are usually insignificant and do not require any further tests or investigation in almost all cases. Usually, the only circumstance where a mother may be referred to see a specialist about these markers are when her risk for having a abnormal child is higher than normal. This is usually only when she has had a previous child with a chromosomal abnormality, a close relative has had a child with a chromosomal abnormality, or she is 35 years of age or older.
Most mothers will still be considered low risk even if a cyst is found and in this case there is no need for any more tests. There is also no need to perform an ultrasound later in the pregnancy to check if the cyst is still there because they cysts always go away on their own whether the baby is normal or not.
What are echogenic foci of the heart?
These appear as small white balls attached to a muscle in the heart, most commonly in the left side. Though occasionally associated with heart defects or chromosomal abnormalities, they are very common (about four per cent of babies have them) and most are a variation of normal.
In the absence of other markers, they are usually insignificant and do not require any further tests or investigation in almost all cases. Usually, the only circumstances where a mother may be referred to see a specialist about these markers are when her risk for having a abnormal child is higher than normal. This is usually only when she has had a previous child with a chromosomal abnormality, a close relative has had a child with a chromosomal abnormality, or she is 35 years of age or older.
Most mothers will still be considered low risk even if an echogenic foci found and in this case there is no need for any more tests. There is also no need to perform an ultrasound later in the pregnancy to check if the echogenic foci is still there because they usually go away on their own whether the baby is normal or not.
What is renal pelvic dilatation (RPD)?
Also known as mild hydronephrosis or excess fluid in the centre of the kidneys, renal pelvic dilatation (RPD) is seen on scan as black spots in one or both kidneys. The fluid is urine that is produced by the baby’s kidneys and collects in the middle before going down tubes called ureters to the bladder. The build-up of fluid is usually due to a slight obstruction in the flow of the urine and, as with choroid plexus cysts (CPCs), the tubes get bigger as the baby grows and the urine flows more easily.
Some babies with Down’s syndrome have RPD but it is a very poor marker. Many normal babies have RPD, too. If no other markers or abnormalities are seen, your baby will be scanned later in the pregnancy and usually the kidneys will look normal. If there is still increased fluid, your baby will usually have an ultrasound after birth to see if there is a true problem. Even if there is a problem with the flow of urine, it is usually not associated with a chromosomal abnormality and can be cured with medications or surgery. Finding out about the RPD in pregnancy is good because it allows your baby to get prompt treatment for the condition at birth.
Are there other markers?
There are other markers that are less common such as short limbs, echogenic (white) bowel in the belly, thickening of skin of the back of the neck, and a gap between the big toe and the second toe. These are markers for Down’s syndrome but as with the other markers, they do not mean there is a problem with the baby as they can be seen in babies who are normal as well.
What will happen if a marker is found?
A single marker is almost certainly of no significance but may increase the risk of chromosomal abnormalities slightly. If two or more markers are found, the risk of the baby having a chromosomal abnormality is increased further. But however many markers or abnormalities are seen on a scan, a diagnosis can only be made by genetic testing via Chorionic villus sampling (CVS) or amniocentesis. This is because the only way of knowing if a baby’s chromosomes are normal is to examine them and to do this, some of the baby’s cells must be obtained.
Your overall risk of having a child with a chromosomal abnormality will depend on a multitude of factors. The ultrasound results, certain blood screening test results, your age, your previous pregnancies and your family history all play a role. Based on this, your doctor may refer you to see a specialist (perinatolgist) if he or she feels that your risk is high enough. The perinatologist will then make an evaluation and discuss whether genetic testing of your baby could be an option to consider.
This information has been prepared by Dr. Jay A. Zelazo, MD FRCPC (Diagnostic Radiologist) at Canada Diagnostic Centres.
