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:
If the gestational age is accurately
- Inaccurate determination of the week of pregnancy.
- 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
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
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
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
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.
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.