Ultrasound - system scan
What is ultrasound and what is its purpose?
This is an imaging test carried out for the purpose of diagnosing several types
of defects in organs and in the structure of the body and in cases of intrauterine growth retardation.
It is important to note that the test cannot detect functional problems in the fetus
such as mental retardation, and there are also certain defects that cannot be detected
Two system ultrasound scans are usually performed during pregnancy (health organizations
usually fund only one scan), along with monitoring progress at the beginning of
and towards the end of the pregnancy.
The examination is used in a number of situations:
- First system scan - performed between the 15th and 16th week of gestation.
- Second system scan - usually performed between the 22nd and 24th week of gestation.
The scan can identify certain defects in the structure of the body and the various
organs, and other fetal problems such as abnormal growth rate.
- It monitors the development of the fetus and the progression of the pregnancy. For
example, ectopic pregnancies (outside the uterus), gestational age, number of fetuses,
normal fetal growth, location and structure of the placenta, heart rate and breathing
movements and the amount of amniotic fluid can all be shown on the scan.
- Ultrasound examination is an accessory for the physician that helps him to decide
whether or not to recommend invasive diagnostic tests such as amniocentesis, chorionic villus sampling, etc.
Abnormal findings in the ultrasound examination
The purpose of the test is to identify defects in the fetus, but not all the defects
that appear on the ultrasound have health implications. Some may only have statistical
significance such as an increased risk for other problems. It is therefore important
to differentiate between the following terms:
- ultrasonographic signs (signs visible on ultrasound that are not defects).
Defects in the structure of certain fetal organs that can impair the function of
the organs, such as heart
defects of brain structure,
supernumerary digits, cleft lip, etc., can be detected by ultrasound.
Such findings are of great medical importance, and all aspects should be considered:
- multi-disciplinary counseling, a geneticist and an expert in the field of the defect
will consider the severity of the defect and its implications for the normal functioning
of the affected organ, the prognosis (possibility of repair), the need for treatment/surgery,
and whether there will be any long-term effects after repairing the damage.
- The possibility that the defect may be part of a syndrome (i.e. there are other
abnormal findings in addition to this particular defect) should be considered. It
is especially important to determine whether there are any chromosomal problems,
defects in other systems, mental retardation, etc. Such an evaluation is
performed by the geneticist based on the type of defect, the family history, presence
of other defects, etc.
- Specific tests should be offered in order to confirm or rule out the presence of
a syndrome, such as a more complex problem that can involve functional disorders,
mental retardation, or defects that cannot be detected by ultrasound.
- If the defect is not so severe as to warrant termination of pregnancy, the method
of follow-up should be decided upon through to and after birth.
Causes of different defects
Even though not all problems have a genetic origin, the geneticist takes the whole
picture into account and his/her work includes differentiation and separation between
the different possibilities. Therefore
genetic consultation is important for obtaining information about
the defect itself in order to determine its cause, the risk for its recurrence,
and whether it is genetic or a result of other causes.
- Genetic causes are not necessarily hereditary, but may result from a genetic change
in the ovum or sperm cell of the embryo itself. This condition is called a spontaneous
- It may be the case that everything is normal from a genetic point of view, but there
were problems that interfered with the normal development of one of the organs -
for example, a disruption in the blood flow to a certain area (or to the entire
body) at a critical stage when the structure of the organ was being formed, an intrauterine
infection, or various metabolic disorders such as deficiency of a certain
vitamin required for normal development, etc.
These are changes in the structure of the fetus that do not directly affect its
functioning or esthetics.
This means that they are not important in themselves, but may have statistical implications
involving an increased risk for other problems that require attention.
Fetal organs do not develop at the same rate.
There are structural differences between fetuses in the same way that there are
differences in the build of adults.
Examples include width of the
kidney pelvis, thickness of the neck, length of the femur (thigh bone),
etc., which differ among individuals andpresent a statistical risk for other problems
irrespective of the organ involved.
It is therefore important to consider a number of aspects when giving genetic counseling:
However, for some signs there is disagreement between researchers. They disagree
over the exact extent of the increased risk that is indicative of a syndrome, and
in some cases whether a risk exists at all. Also, the method of precisely calculating
the final weighted risk (in addition to other tests) has not been fully determined.
However, in most cases whether or not there is an increased weighted risk, an assessment
following consultation determines whether or not further tests are required.
- Determining whether the degree of the abnormality (severity of the sign) has or
has not exceeded the borderline, and if it has, assessing whether it is of such
a degree that it changes status from a "sign" to a defect (with functional disorder).
For instance, the widening of the kidney pelvis in excess of a certain limit (depending
on the week of pregnancy) may indicate a kidney defect and functional disorder.
The width of the brain ventricles up to a certain limit is considered a "sign",
after which it becomes a "problem". The presence of increased echoes from the intestines
after the 18th week is a sign that may indicate
cystic fibrosis and chromosomal syndromes, especially if of long duration,
and it may also indicate defects in the digestive tract.
- The rate at which the statistical risk increases and suggests other problems such
as chromosomal syndromes (Down syndrome),
etc., should be evaluated. Each sign has implications for an increased risk of specific
problems arising from it.
- The type and severity of the sign must be evaluated. Evaluation of the weighted
risk (overall picture) is based on the results of other tests (blood biochemistry screening, nuchal translucency, other ultrasound signs).
- If there is a high risk for the presence of a syndrome, further tests that can determine
the extent of the risk are recommended. These tests are performed through the subject's
health organization or the Ministry of Health.
It should be noted that parents' anxiety about ultrasonographic signs revealing
an actual problem is completely disproportionate. In most cases, the additional
risk for an abnormal fetus is negligible because
every pregnancy, even if all tests, including amniocentesis and ultrasound,
are normal, have at least a 2% risk for "nasty surprises", including mental retardation.
Common signs: echogenic focus
in the fetus' heart ventricles ("golf ball"),
slight widening of the kidney pelves,
choroid plexus cysts in the ventricles of the brain, two umbilical blood vessels,
cysts on the sides
of the neck, slight
shortening of the femur,
slightly increased nuchal translucency,
echogenic intestines, etc.
In all such cases, the physician must be consulted and genetic counseling sought
based on his decision. Genetic counseling should certainly be recommended in cases
of more marked increased nuchal translucency, echogenic intestines seen after the
18th week, if a sign is particularly severe, and when there are other coexisting
problems such as an increase in other ultrasound parameters or normal biochemical
blood screening but a demonstrated risk in excess of 1:1000. Counseling is also
recommended when there are several ultrasonographic signs.
Ultrasound can reveal findings that
have two aspects: obstetric problems and genetic problems - for example,
a delay in the fetus' growth in the uterus, an excess or a reduced amount of amniotic
fluid, etc. In such cases, combined counseling by an obstetrician (from a "high
risk pregnancy" department and/or an ultrasound physician) and a geneticist is required.