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Short Femur |
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The length of the femur (thigh bone) usually corresponds with the week of pregnancy plus or minus
one week.
A reduction in the length of the femur of greater than one week can arise from a number of conditions:
- Inaccurate determination of the week of pregnancy.
If the gestational age is accurately known:
- Delayed growth as a result of various non-genetic causes that are associated with a
decrease in the intrauterine blood flow to the fetus.
- Delayed growth of the femur associated with genetic disorders of bone growth
(congenital bone diseases, other syndromes, etc.).
Usually, precise calculation of the week of pregnancy, comparison of the length of the femur
with other growth parameters (head circumference, abdominal circumference and the
length of other bones), and monitoring the rate of growth will allow differentiation
between the various conditions.
If necessary, and depending on the advice of the genetic counselor, additional tests may
sometimes be recommended.
See list of the different situations in the accompanying information sheet.
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The vast majority of the cases of slight shortening of the femur, where its
length is still within the normal range, are not associated with medical problems in
the fetus.
However, a number of studies have indicated an increased statistical risk for
Down syndrome in fetuses with a slightly shortened femur -
some have suggested that there may be a fivefold risk.
On the other hand, other studies have not demonstrated a significant increase in
the risk for Down syndrome.
The "gold-standard" threshold for recommendation of amniocentesis
is a risk of Down syndrome greater than 1:386 based on the results of the
biochemical marker screening tests, and when the
weighted risk is equal to or higher than this, amniocentesis is generally recommended.
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However, since the presence of a short femur in the fetus may statistically increase the risk
of Down syndrome, some physicians suggest that this should be integrated with the results
of the biochemical screening tests and the new threshold for recommending amniocentesis
is a risk for Down syndrome of greater than 1:1000 in the biochemical marker screening tests.
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The anomaly discussed in this section progresses throughout the pregnancy.
If on ultrasound examination during the first or second trimester the head circumference is
found to be greater and the length of the femur less than the number of the weeks of
gestation, and the difference between them is one and a half weeks or more,
then this requires further investigation to ensure that the difference does not increase,
as if it does, this could indicate a problem of bone development.
The vast majority of the cases where a difference has been found are not associated with
significant medical problems in the fetus, although such a combination may indicate that the
fetus has a bone disorder known as achondroplasia.
Achondroplasia is the most common form of very short stature (the term "dwarfism" is not used
nowadays).
In this disorder, the shortening of the femur and the enlargement of the head
circumference become obvious only in the late stages of pregnancy (week 34 and later),
which is relatively late to undertake investigations and to act in accordance with the results.
A molecular test can be performed on amniotic fluid cells in order to identify the mutation in
the gene responsible for achondroplasia as soon as the combination described above is found,
immediately after the system ultrasound scan, while the deviation from the normal is still
small and non-diagnostic (in approximately 99% of cases this test is found to be normal).
These cases should be monitored by performing regular ultrasound examinations, and if the
difference increases, amniocentesis can be performed.
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