The 11-14 Week Ultrasound Examination
Why have a scan at 11-14 weeks?
Nuchal Translucency:
Currently the most accurate non invasive test for detecting
Down syndrome during pregnancy is the measurement of the nuchal
translucency with ultrasound between 11-14 weeks of pregnancy.
This is normally less than 2.5mm and when increased (ie>2.5mm)
may indicate the baby has Down syndrome or another chromosomal
abnormality.
What is the Nuchal Translucency?
The nuchal translucency is a collection of fluid beneath the
fetal skin in the region of the fetal neck and is present
in all fetuses in early pregnancy. The fluid collection is
however increased in many fetuses with Down syndrome and many
other chromosomal abnormalities. It is called a "translucency"
because on ultrasound it appears as a black space beneath
the fetal skin. It is this black space that you will see measured
during the ultrasound scan.
If the Nuchal Translucency is increased what happens next?
If the nuchal translucency is increased then you will be offered
a needle test which is called chorionic villus sampling or
C.V.S. (ref CVS information leaflet). This test involves taking
a biopsy from the placenta and examining the baby's chromosomes.
In these circumstances an urgent report is requested (short
term culture) and if sufficient cells are present in the sample
the results may be processed as early as 2-3 days. The complete
analysis (long term culture) takes 2-3 weeks.The normal short
term culture excludes major chromosomal problems such as Down
Syndrome and the long term looks at the chromosomes in greater
detail thus excluding more subtle chromosomal abnormalities.
How is the scan performed?
We generally recommend that the scan be performed through
the vagina (ref. transvaginal information leaflet). In these
circumstances a very narrow ultrasound probe is gently placed
a short distance into the vagina and the ultrasound beam directed
towards the baby. With this approach greater detail is obtained
of the fetal anatomy. However if you wish to have an ultrasound
examination through the lower abdomen this can be arranged.
What happens if the chromosome test
(CVS) is normal?
For patients where there was an increased nuchal translucency
and later a normal CVS result, there is still a slight increased
risk of a structural abnormality in the baby (6%, compared
to the background risk of 2-3%); eg a congenital heart defect.
For this reason a detailed examination of the fetal anatomy
is required around 18-20 weeks.For most patients the anatomy
is normal and the pregnancy progresses uneventfully.
What happens to the increased nuchal
translucency with advancing pregnancy?
In the majority of patients this disappears and is not visible
at the routine 18-20 week ultrasound. This explains why it
is important that the scan be performed around 11-14 weeks.
ie there is only a narrow window of opportunity when this
ultrasound marker of chromosomal abnormality is evident.
What is the risk of the scan?
A transvaginal and transabdominal ultrasound examination is
a safe investigation at all stages of pregnancy.
What else can the 11-14 week ultrasound
scan show?
A scan at 11-14 weeks provides an opportunity to assess the
anatomy of the baby. At this early stage of pregnancy it is
now possible to determine many structural abnormalities previously
only visible at 18-20 weeks. During the examination many parts
the fetal anatomy will be pointed out to you. It is also an
opportunity to determine the number of babies present and
the baby's heart rate.
Normal Nuchal Translucency
This image shows a longitudinal section through a fetus
with the head to the left and the legs to the right. The arrows
point to the tiny collection of fluid beneath the fetal skin,
ie. The Nuchal Translucency which in this baby is normal.
Increased Nuchal Translucency
This image shows a longitudinal section through a fetus
at 11 weeks. The head is the left and the legs to the right.
The arrows point to the increased Nuchal Translucency. Placental
biopsy (CVS) in this fetus revealed Down Syndrome.
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