Measurements for Down Syndrome

Earlier parameters used included the actual to expected FL ratio, the BPD/FL ratio. More recently the nuchal translucency thickness is used in detecting Down syndrome fetuses.

In Down syndrome both the femur and the humerus tend to be shortened.

The expected FL = -9.645 + 0.9338 x BPD

If the ratio is below 0.84 the likelihood of Down syndrome is higher.

The BPD/FL ratio should be established for each individual population, and itís value varies with gestation. The mean +1.5 SD is usually used as the cutoff level.

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GA (weeks) --- BPD/FL ratio#

15 --------------------- 1.93

16 --------------------- 1.93

17 --------------------- 1.76

18 --------------------- 1.74

19 --------------------- 1.69

20 --------------------- 1.58

21 --------------------- 1.54

22 --------------------- 1.47

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Positive predictive value 1/294 for the general population ( normally 1/1000 )
and 1/112 for maternal age over 35 (normally 1/270).

The BPD/FL ratio and the nuchal skin fold thickness can be used in conjunction with the maternal age incidence to arrive at a new probability for the occurrence of Down syndrome in a particular fetus.

The nuchal translucency thickness is the thickness of the skin fold behind the nape of the neck. In chromosomal abnormalities this may be thickened due to venous or lymphatic engorgement. In the measurement of the nuchal skin fold thickness, critical landmarks should include the cavum septum pellucidum, the cerebral peduncles and the cerebellar hemispheres. Calipers are placed from the outer skull table to the outer skin surface.

Percentage of Down syndrome fetuses with nuchal fold >6 mm after 16 weeks vary from 69% (Benacerraf 1991) to as low as 3.8% (Grandjean 1995) in different reports. A mean value of about 50% can be obtained from different reports.

The Percentage of Down syndrome with Nuchal folds equal or greater than 3mm before 14 weeks ranges from 1.5% (Rodeck 1995) to 18% (Nicolaides 1994) and 45% (Salvesen 1995) in different reports

The Nicolaides group, basing on their findings in 1015 fetuses at 10-13 weeks with nuchal fold greater than 3mm arrived at the following risks estimates:

3mm ------ 3 times

4mm ------ 18 times

5mm ------ 28 times

6mm ------ 36 times
the incidence by maternal age.

The incidence of Down syndrome at the age of 35 is around 1 out of 270. If the fetus has a nuchal skin fold of 6mm the risk of Down syndrome would have become 270/36 or 1 out of 7.5.

In general the combination of beta-HCG and nuchal translucency thickness can increase the detection rate of Down syndrome fetuses by almost 3 folds compared to maternal age alone.


Estimated risks of fetal trisomies at 10-14 weeks gestation on the basis of maternal age (background) alone and age plus nuchal fold thickness of 3mm, 4mm and >4mm. (Pandya et al 1995).


Other markers which may be present in Down syndrome include Cardiac defects, cardiac echogenicities, tricuspid regurgitation, cystic hygroma, duodenal atresia, omphalocele, polyhydramnios, choroid plexus cyst, renal calyceal dilation, and echogenic bowels.


More recently the failure to visualize the fetal nasal bone at around 11-13 weeks is considered as a very reliable marker for Down Syndrome.

Read also: 1. Diagnosing Down Syndrome.

2. Soft Markers - A Guide for Professionals.

3. Ultrasonographic "soft markers" of fetal chromosomal defects.







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