Diagnostic Centre and Well Women Clinic
Medical Centre Singapore
Ultrasound Subcommittee (AOFOG).
Nuchal Translucency (NT Scan)
Nuchal Translucency (NT Scan) is the area
just under the skin at the back of the fetal neck, which can be measured
at 10-14weeks gestation. It is the black area lying between the subcutaneous
fascia and skin. It increases with gestational age, and great care must
be taken to measure it correctly to avoid measurement errors. The optimal
time to perform an (NT Scan), if High -End Machines with expertise to
per cent of internal organs including the heart is available, would be
around 13.0-14.0 weeks.If this scan at 13-14weeks is normal, then the
risk of Chromosomal abnormalities (Eg.Downs Syndrome), would be decreased
by 85 percent. Hence even at 40 year of age if the Nuchal translucency
scan together with Genetic scan at 14weeks are both normal the risk of
the fetus to be chromosomally abnormal (Downs syndrome) would be less
than 0.1 percent. With such low risk, it would not warrant, performing
amniocentesis routinely, if a women is above 35years of age, as, the fetal
loss rate with amniocentesis in best of hands is around 0.2-0.4percent.High-End
machines and the expertise are not available to assess internal organs
accurately, then just perform the NT Scan at around 11-12wks.
NT Thickness (Greater than 2.5mm) is associated with;
Chromosomal aneuploidy, including trisomies21 (Downs Syndrome), 18,13
and triploidy and Turners syndrome (45XO). Identifies 77- 82 percent
of Downs fetuses with a false positive rate of 5-8 percent.
Karyotypically normal fetuses with thickened NT,
there is a greater risk for;
Birth defects, including cardiovascular (ASD, VSD), pulmonary (Diaphragmatic
hernia), renal and anterior abdominal wall (Omphalocoele) defects.
Amnion distruption sequence
Translucency and Early Genetic scan at 13.0-14.0 weeks is crucial for
the following reasons;
Accurately date the pregnancy.
Even in women who are sure of the first day of the Last Menstrual Period,
the gestational age of the fetus can be wrong by 3-4weeks in 15 percent
of cases. With ultrasound at this early stage we can accurately age
the pregnancy, with an accuracy of plus or minus 5-10days.
diagnose multiple pregnancy and there chorionicity early.
2 percent of natural conceptions and 10 percent of assisted conceptions
results in a multiple pregnancy.
To diagnose early pregnancy failure.
This occurs in 3 percent of pregnancies at early stages.
This occurs unfortunately in 3-4 percent of pregnancies.40-50 percent
of Perinatal mortality and infant mortality is due to congenital malformations
and 85-90 percent of these malformations are structural malformations.85-90
percent of these lethal structural malformations can be excluded with
confidence between 13.0-14.0 weeks.
By doing so, the risk of chromosomal (Downs syndrome) abnormalities
can also be decreased by 85 percent at this early stage, as majority
of them have structural defects.
Ultrasound at 22-23weeks.
women should be subjected to a second scan between 22-23weeks,as
all organs of the fetus are formed and can be clearly visualized
by ultrasound. The pickup rate of lethal structural malformations
at this stage is well above 95 percent in expert's hands, with the
ideal ultrasound machines. If the 14 and 22 weeks scans are both
normal, the chances of the women
loosing that pregnancy in a developed country would a
4.9 per thousand.
to visualizing the fetus, the uterine blood flow should be assessed with
the Doppler waveform analysis and the cervical length and the internal
os of the cervix should be measured. If the uterine blood flow and the
cervical assessment are both normal, then it is very unlikely for women
to have any major complications in pregnancy (Egg; EarlyPreterm labour,
Pre-Eclempsia, Growth Restricted fetus).
Assessment of Fetal well-being
of fetal demise prior to birth is due to Chronic Hypoxia (lack of Oxygen);
this is always preceded by growth restriction. The ultrasound is the most
accurate modality to assess the fetal growth. The single most accurate
parameter to measure is the fetal abdominal circumference, which is done
once every 3-4wks after 28weeks, in high-risk pregnancies.
Once the fetus, which is not growing well, is identified, its well being
is assessed, with ultrasound by performing Liquor, Biophysical Profile
and Doppler assessments.
40 percent of Perinatal and Infant Mortality is due to major cardiac malformations.
Unfortunately only 11 percent of major cardiac malformations are diagnosed
prenataly. With the introduction of routine Fetal Echocardiography Cardiac
malformations have become the commonest structural malformation diagnosed
prior to birth.
gestation to visualize the fetal heart clearly, is between 13.0-14.0 weeks
and 95 percent of the time can be done by performing an abdominal ultrasound
with the high-end machines.85 percent of Major cardiac defects can be
excluded by 14weeks.The optimal time to perform fetal echocardiography
is around 23wks and at this stage over 95 percent of major cardiac defects
can be excluded. The most important prerequisite is highly skilled operator.
involves in addition to looking at internal organs of the fetus, it also
at the surface structures, looking for markers of chromosomal abnormalities
(Downs syndrome). Majority of chromosomal abnormalities are reflected
on the Eyes, Ears,Nasal Bone, Tongue and the Fingers. The Genetic Scan
should be done is between 13-14weeks and repeated at 22- 23weeks.If both
scans are normal, then Genetic Amniocentesis is not warranted except in
women over the age of 45,as only then the risk of having a Downs syndrome
baby inspite of a normal Genetic scan is equal to the risk of Fetal Loss
following Amniocenetsis. (Vintzileos etal 1995).
The procedure has existed from 1929,but for prenatal diagnosis of chromosomal
abnormalities (Downs syndrome) it was first used in 1968.Since early 1980
the extraction of the Amniotic fluid around the fetus is done by introducing
a needle through the maternal abdomen into the uterus under ultrasound
guidance. This is a simple outpatient procedure and takes only around
two to three minutes and does not require any local anesthesia. It can
be from14weeks up to 22wks. Well-established laboratories can release
the results within 10to 12days of the procedure. The common chromosomal
aberrations such as Trisomies 21,13,18 and 45XO can be excluded within
24hrs by performing the FISH or PCR Techniques. All chromosomal abnormalities
can be excluded with Amniocentesis with a false negative rate of 1 in
400,000 and a false positive rate of 0.06 percent The complication rate
(Fetal Loss) is around 0.2-0.4 percent as long the Canadian MRC recommendation
for the operator is adhered to, otherwise the fetal loss rate is around
Chorionic villus Sampling (CVS)
CVS involves the examination of chorionic Villi (Placental Tissue).
The chorionic villus is sampled either via the vagina, or through the
maternal abdomen, both under ultrasound guidance. For the abdominal route
local anesthesia is used as the needle used is thicker than that used
for amniocentesis. The procedure like the Amniocentesis lasts about 2-3minutes.The
preliminary result is usually ready by 3days and the final results within
10days.The false negative rate is around 1 in 100,000 and the false positive
rate is around 2 percent for karyotyping. The complication rate (Fetal
Loss) is highly dependent on the expertise, and in experts hands rate
it is 1.2 percent higher than that of Amniocentesis (1.5%). Amniocentesis
should be the invasive test of choice for prenatal diagnosis of chromosomal
abnormalities (Downs syndrome) rather than CVS, as the complication rates
are lower and it is a more reliable test.
As for prenatal diagnosis of Genetic diseases (Eg Thalasaemia, Duchene's
Muscular Dystrophy), CVS is superior to Amniocentesis, hence CVS as a
procedure should reserve for prenatal diagnosis of Genetic diseases rather
than for chromosomal abnormalities.
Fetal Blood Sampling
This procedure involves obtaining fetal blood, either from the umbilical
cord or directly from one the fetal vessels (Left portal vein). It is
also done under ultrasound guidance and requires special skills. The complication
rate (Fetal loss is around 1.2 percent. The results are ready within 48hours.
Fetal Tissue Biopsy
This procedure is also done under ultrasound guidance for prenatal diagnosis
of certain lethal congenital skin conditions.
Intrauterine Fetal therapy
With the availability of the ultrasound, certain conditions can be treated
successfully, prior to birth inutero (Hydrops Fetalis, Hydronephrosis).
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