What we may suspect in NT Scan:-For any foetal problem be it anatomic be it genetic, such abnormalities are usually but not always
it is reflected on NT changes. Why
fluid collects in nape of the neck of foetus is unclear. As gestation
changes - NT
varies as per CRL. When to
counsel for CVS /amnio?? If the risk calculation (statistical risk
probability) is above 1: 50 then only CVS procedure for confirmation.
1.
Risk stratification: We always are concerned about
probability of chromosomal abnormality in any foetus. This is also termed as likelihood ratio. If mother has one child with Down’s syndrome,
then recurrent risk will be 2%. . NT :--Normally NT
steadily increase for the period
foetal CRL from 44 mm to 85 mm .What is normal NT(subcutaneous fluid
collection behind the nape of the neck) ??
( one of the 21 soft markers) .The fluid thickness is for
a CRL of 45 mm ( means 11 weeks ) will be 1.9
mm in 85 percentile and 50 percentile NT will be
& NT will in 85 mm ( CRL at 13.6 ) but it will be 2.8 mm
in 95 percentile
What is normal
NT(subcutaneous fluid collection behind the nape of the neck) ?? ( one of the 21 soft markers) .The fluid
thickness is for a CRL
of 45 mm ( means 11 weeks ) will be 1.9 mm
in 85 percentile and 50
percentile NT will be & NT
will in 85 mm ( CRL at 13.6 ) but it will be 2.8 mm in 95
percentile The NT is only reliable from
45 - 84 mm caliper
of USG should be
able to read upto 0.1 mm. Crown Rump length (CRL) Inaccurately
measuring the CRL can have a serious effect on the risk calculation for
Down syndrome screening. The CRL is used to standardise both biochemistry and nuchal
translucency. ... For the NT scan / combined first trimester screen,
the CRL must be between 45mm and 84mm. Crown Rump
length (CRL)
Inaccurately measuring the CRL can have a serious effect on the risk
calculation for Down syndrome screening.
The CRL is used to standardise both biochemistry and nuchal translucency.
If the CRL is over measured the biochemistry will appear to be immature, with
the potential to increase the false positive rate. Under measurement of the CRL
will potentially have the reverse effect on the biochemistry. Similarly
inaccurate measurement of the CRL can adversely affect the accuracy of the
nuchal translucency component of the risk calculation.
For the NT scan / combined first trimester screen, the CRL must be between 45mm
and 84mm.
It is important to date the pregnancy using the CRL measurement for the
combined screening test. An NT scan
must take place at a particular time in your pregnancy. This is between 11
weeks and 13 weeks plus six days, to be exact , or
when your baby's crown rump length (CRL) is between 45mm (1.8in) and 84mm
(3.3in). So the NT scan will usually happen alongside your
routine dating scan.
How to measure the CRL accurately: At 12 weeks of
gestational age, an "average" nuchal thickness
of 2.18mm has been observed; however, up to 13% of chromosomally normal fetuses
present with a nuchal translucency of greater than 2.5mm.
1.
MAGNIFY the image. Make the fetus as large as possible before freezing the
image, ensuring the whole fetus is clearly demonstrated.
MIDSAGITTAL Obtain a midline sagittal section of the whole fetus. The
image of the head should demonstrate the echogenic tip of the nose, the
rectangular shape of the palate, and the translucent diencephalon. The head
should be in line with the full length of the body, which is displayed along
with the full length of the spine.
NEUTRAL POSITION The fetus should be neither hyperextended nor flexed.
There should be fluid visible between the chin and the chest. Ensure the
cervical spine is not extended. If the fetus is prone it is helpful to invert
the image to ensure there is not hyperextension.
HORIZONTAL The fetus needs to be at 90 degrees to the axis of the
ultrasound beam. ie a line drawn between the crown and rump is at 90 degrees to
the ultrasound beam.
CROWN AND RUMP CLEARLY DEFINED The crown is the skin above the parietal bone.
The rump is the skin inferior to the tip of the sacrum. There should be a
pocket of fluid between the ends of the fetus and the uterine wall.
Use the + calipers.
CALIPER PLACEMENT Place the intersection of the + calipers at the outer
margin of the skin borders
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