THICKENED NUCHAL TRANSLUCENCY (NT)
Thickened nuchal translucency
By the end of this activity you should be able to:
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

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describe what is meant by thickened nuchal translucency (NT) in a foetus
draw a graph from a given data set with the correct labels, units and scales
select the correct data to carry out calculations
correctly carry out a percentage calculation using data provided.
The NT is formed by a layer of fluid beneath the skin of the neck and the
underlying tissues. This layer of fluid is present in all foetuses but it has been
shown that the greater the amount, the greater the chance of the foetus being
affected by some disorders. However, as is the case with every marker, even
a very significant increase in NT thickness does not mean there is necessarily
a problem. In the context of an increased chance of a chromosomal
abnormality, this can be determined only by the appropriate diagnostic test ,
such as chorionic villus sampling or amniocentesis .
(a)
(b)
© ‘The Mystery of Nuchal Translucency’ by Lindsay D Allan, Cardiol Young 2006 ,
16: 11–17, CUP
Figure 1
(a)
A foetal specimen is seen after interruption of pregnancy with the membranes
intact. The translucent fluid-filled area at the back of the foetal neck can be
clearly seen.
(b)
A 12-week-old foetus is seen on ultrasound. The me asurement of the nuchal
translucency is markedly increased.
ANTENATAL SCREENING (H, HUMAN BIOLOGY)
© Learning and Teaching Scotland 2011
1
THICKENED NUCHAL TRANSLUCENCY (NT)
Questions
The following table shows the incidence of chromosomal defects according to
foetal NT in a screening study involving 96,127 singleton pregnancies.
1.
Complete the table then draw a bar graph of the results and describe the
trend.
NT (mm)
Total number
Number of
chromosomal
defects
<3.4
95,086
315
3.5–4.4
568
120
4.5–5.4
207
69
5.5–6.4
97
49
>6.5
166
107
Percentage of
chromosomal
defects
2.
The following table gives the results of a study on the pregnancy
outcome of foetuses with increased NT. The study measured the NT
thickness in 1930 pregnancies. An enlarged NT thickness was
discovered in 27 foetuses.
2
ANTENATAL SCREENING (H, HUMAN BIOLOGY)
© Learning and Teaching Scotland 2011
THICKENED NUCHAL TRANSLUCENCY (NT)
(a)
Which result from the table best supports the statement that even a very
significant increase in NT thickness does not diagnos e a foetal
problem? Justify your answer.
(b)
What percentage of foetuses had an enlarged NT measurement?
(c)
What was the mean enlarged NT thickness?
(d)
What percentage of foetuses with an enlarged NT measurement had an
adverse outcome?
(e)
What percentage of foetuses with a normal karyotype were found to
have anomalies discovered at ultrasound examination?
ANTENATAL SCREENING (H, HUMAN BIOLOGY)
© Learning and Teaching Scotland 2011
3
THICKENED NUCHAL TRANSLUCENCY (NT)
Answers
2.
4
(a)
The result that shows a thickened NT measurement of 9.3mm.
This is the largest NT measurement yet there was a live birth with
no fetal anomalies.
(b)
Number with enlarged NT = 27, total number studied = 1930 .
Percentage with increased NT thickness = 27/1930 × 100 = 1.4%
(c)
Total NT thickness of 27 results in the table = 136.3.
Mean = 136.3/27 = 5.05mm
(d)
27 foetuses had an enlarged NT measurement. Only 10 were
normal so 17 had an adverse outcome. 17/27 × 100 = 62%
(e)
6 out of the 17 foetuses who had abnormalities discovered by
ultrasound had a normal karyotype. 6/17 × 100 = 35%
ANTENATAL SCREENING (H, HUMAN BIOLOGY)
© Learning and Teaching Scotland 2011