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Short Femur


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.
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.

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.

A combination of two signs: short femur and slightly increased head circumference

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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Comments (1)


Tuesday, August 07, 2012 2:12 PM
(1) Snm  says:


Femur

My sons femurs measured two weeks behind.i was at 1:1100 risk for downs and am now 1:220 because of the femurs. What else can cause shorter femurs ? This issue has never been brought up until my 28 week ultrasound.





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