a powerpoint with the 11-13 week scan lecture of the FMF as its source of info

1. PRENATAL
DIAGNOSIS : DOWN’S
Dr Ajay S Dhawle
JNMC, Sawangi Wardha

2.  Screening
 I st Trimester
 II nd Trimester
 Diagnosis
 Chorionic Villus sampling
 Amniocentesis

3. Screening
 Every woman has a risk that her fetus/baby has a chromosomal
defect
 The background or a priori risk depends on MA and GA
 The individual patient-specific risk : multiplying the a priori risk with a
series of likelihood ratios, which depend on the results of a series of
screening tests
 The likelihood ratio for a given sonographic or biochemical
measurement is calculated by dividing the percentage of
chromosomally abnormal fetuses by the percentage of normal fetuses
with that measurement
 Every time a test is carried out the a priori risk is multiplied by the
likelihood ratio of the test to calculate a new risk, which then becomes

4. The risk for trisomy 21:
 Increases with MA
 Decreases with GA
(because about 30% of
affected fetuses die between
the 12th and 40th week of
pregnancy)

5.  In the 1970’s and 1980’s
screening for trisomy 21 was
based on maternal age and
amniocentesis or CVS was
offered to those aged 35
years or older.
 About 5% of pregnant women
were ≥35 years and a policy
of screening based on
maternal age would result in:
 Invasive testing rate : 5%
 Detection rate of trisomy
21 :30%

6.  In the last 30 years the
maternal age of pregnant
women has increased and
now
 about 20% of pregnancies,
including 50% of fetuses with
trisomy 21, are in women
aged ≥35 years

7. What is NT?
 Nuchal translucency is the
sonographic appearance of
subcutaneous
accumulation of fluid
behind the fetal neck in the
first trimester of pregnancy.

8. When should NT be measured?
 GA : 11 – 13+6
weeks
 Fetal CRL : 45–84
mm.

9. Why 11 wks?
 CVS before this gestation is associated with transverse limb reduction
defects.
 After 11 weeks, many major fetal defects can be diagnosed at the NT
scan(eg. Anencephaly, abdominal wall defects, FB, 4 chamber cardiac
view).
Why 13+6 wks?
 Allow women the ease and safety of first trimester termination of
pregnancy.
 incidence of abnormal accumulation of nuchal fluid in chromosomally
abnormal fetuses is lower at 14–18 weeks than before 14 weeks.
 success rate for taking a measurement at 10–13 weeks is 98–100%,
falling to 90% at 14 weeks because the fetus becomes vertical

10. Who should measure NT?
 A sonologist
 Accredited by the FMF
(UK)
 Accredited by the NTQR
programme (USA)
 Pretest and a Post test
counseling MUST

11. How should NT be measured?
 Mid Sagittal Plane
 Neutral fetal position
 Proper magnification
 Proper placement of Calipers
 Adjustment of Gain
 Distinguishing fetal skin from the amnion
 Proper Reporting

12. Mid sagittal plane
Para Sagittal view True sagittal view.
Frontal process of the
Maxilla in view

14. Fetal position
Extended head
 Neutral position.
Neutral position
Hyper extended
Hyperflexed
Hyperextension can NT by upto 0.6mm
Hyperflexion can NT by upto 0.4mm

15. Magnification
Inadequate magnification
 Include only the fetal
head and upper thorax in
the image.
 The magnification should
be as large as possible Adequate magnification
(such that each slight
movement of the
callipers produces only a
0.1 mm change in the
measurement).

16. Callipers
 The crossbar of the calliper should be hardly visible
as it merges with the white line of the border and
not in the nuchal fluid.

17. Amnion
 Both fetal skin and amnion appear as thin membranes at this
gestation

Wait for spontaneous fetal movement away from
the amniotic membrane alternatively,

Ask the mother to cough
and/or by tapping the
maternal abdomen
the fetus is bounced off
the amnion

18. Gain
 In magnifying the image, either pre or post freeze zoom,
it
is important to turn the gain down to avoid fuzzy edges &
underestimation of NT

19.  The NT thickness in euploid
fetuses increases with fetal
CRL
 In 75-80% of trisomy 21 fetuses
the NT thickness is above the
95th centile of the normal range
 In trisomy 21 fetuses there is no
relationship between NT
thickness and maternal age
 Maternal age can be combined
with fetal NT to provide effective
first-trimester screening for
chromosomal abnormalities

20.  In a fetus with a given CRL, every
NT measurement represents a
likelihood ratio which is multiplied by
the a priori maternal and gestational
age-related risk to calculate a new risk
 NT : LR : Risk of Downs
 NT : LR : Risk of Downs
 The risk is greater in a woman of 20
years when the fetus has a high NT
than in a woman of 40 years when the
fetus has low NT

21. First Trimester Biochemical
screening
 Trisomic pregnancies :altered
maternal serum concentrations of
various feto-placental products
 Screening in the second trimester by
maternal age and various
combinations of total or free ß-hCG,
AFP, uE3 and Inhibin A can identify
56-71% of trisomy 21 pregnancies
for a false positive rate of 5%
 Screening in the first trimester by a
combination of maternal age, fetal
NT, FHR and serum free ß-hCG and
PAPP-A identifies about 90% of
trisomy 21 pregnancies for a false
positive rate of 3%

22.  In trisomy 21
pregnancies maternal serum
free ß-hCG is about twice as high
and PAPP-A is reduced to about
half compared to chromosomally
normal pregnancies
 Performance of screening for
trisomy 21 by maternal age and
serum free ß-hCG and PAPP-A:
 Detection rate 65%
 False positive rate 5%

23.  The measured concentration of free ß-
hCG and PAPP-A is influenced by the
machine and reagents used, gestational
age, maternal weight, ethnicity, smoking
status and method of conception
 In Black women the PAPP-A level is
about 60% higher than in White women.
Failure to take into account ethnic origin
would result in substantial underestimate
of the true risk of trisomy 21 in Black
women
 In women who smoke and those
conceiving by IVF serum PAPP-A is
decreased and this could be
misinterpreted for increased risk for

24.  Free ß-hCG is higher than in
euploid pregnancies and the
difference between the two is
higher at 13 than at 11 weeks
 Serum PAPP-A is lower than in
euploid pregnancies and the
difference between the two is
higher at 11 than at 13 weeks
 The difference from euploid
pregnancies in PAPP-A at 11
weeks is greater than the difference
in ß-hCG at 13 weeks and
therefore the overall performance
of biochemical screening is better
at 11 than at 13 weeks

25. Combined screening
 In trisomy 21 compared to euploid
pregnancies:
 The difference in biochemical markers
is greater at 11 than at 13 weeks
 The difference in fetal NT is greater at
11 than at 13 weeks
 Therefore the overall performance of
screening is better at 11 than at 13
weeks


26.  The overall performance of combined
screening is better at 11 than at 13 weeks
and may be best at 10 weeks
 Ultrasound scanning for fetal abnormalities
is better at 12 than at 11 weeks and much
better than at 10 weeks
 A good way of achieving a high
performance of screening for trisomy 21
and diagnosing major fetal defects by
ultrasound is to carry out the blood test at
10 or 11 weeks and the ultrasound scan at
12 weeks

27. Other defects
 In euploid
pregnancies the
average free ß-hCG is
1.0 MoM and PAPP-A is
1.0 MoM
free ß-hCG PAPP-A

Trisomy 21 2.0 0.5
Trisomy 18 0.2 0.2
Trisomy 13 0.3 0.4
Turner 1.2 0.5

28. Differences
 Fetal NT is higher in trisomies 18 and 13
than in trisomy 21
 Serum PAPP-A is lower in trisomies 18
and 13 than in trisomy 21
 Serum free ß-hCG in trisomy 21 is high
whereas in trisomies 18 and 13 this is
low
 Fetal heart rate in trisomy 13, unlike
trisomies 21 and 18, is high


29. New Ultrasound Markers
 Assessment of the new  Nasal bone
markers improves the performance of 
combined screening by increasing the
Facial angle
detection rate and reducing the false  Ductus venosus flow
positive rate  Chromosomal defects
 Examination of the new markers  Major cardiac defects
requires appropriate training of
 Fetal death
sonographers and Certification of their
competence in carrying out these  Tricuspid flow
scans  Chromosomal defects
 The new markers can be assessed  Major cardiac defects
in all patients or only in the 15% of the
total with an intermediate risk (1 in 51
to 1 in 1000) after combined screening

30. New Ultrasound Markers
 There are two strategies for
assessment of the new markers in
screening for trisomy 21 with
similar detection and false positive
rates:
 One, some, or all markers are
examined in all cases
 The markers are examined only in
the subgroup of pregnancies with
an intermediate-risk (between 1 in
51 and 1 in 1000) after combined
fetal NT, FHR, free ß-hCG and
PAPP-A screening, which
constitutes only one sixth (15%) of
the total population

31. Reporting
 NEVER as an isolated measurement, NEVER
without images
 Always mentions ‘a priory’ risk and ‘estimated
‘risk
 MOMs for the Free Sr B HCG & PAPP-A : MUST
in the report

33. Increased NT+ Normal Karyotype: what
next?
 Cardiac defects
 Neural tube defects
 Diaphragmatic hernia
 Skeletal dysplasias
 Genetic syndromes
 Higher risk of stillbirth
 Midtrimester ANOMALY scan + Fetal
ECHO
 Does NOT warrant termination, still a good
chance of having a healthy baby

34. Thank
you