Screening for Down’s

syndrome FAQ’s

Dr Sheila Balakrishnan

MD DNB FRCOG FICOG

Prof in OBGYN

Medical College Trivandrum

Do you think screening for Down’s is necessary and if

so for what age group?

What is meant by ‘apriori risk’?

A woman’s risk of having a fetus with trisomy 21 depends on her age, gestational age and her history of chromosomal defects

In general we mean the age related risk at term

35 1 in 250 1 in 38440 1 in 70 1 in 10645 1 in 20 1 in 30

35 1 in 384 1 in 250

40 1 in 106 1 in 70

45 1 in 30 1 in 20

Age Midtrimester Term

Best method of screening as of today?

First trimester screening by the combined test-

NT PAPP A and β hCG-

is added to the apriori risk

What is NT?

USS term for the physiologic collection of fluid underneath the

nuchal skin extending for a variable distance over the fetal head and back at 11-14 weeks

For unclear reasons this collection is unduly sensitive to

fetal problems

Who described NT and in which year?

Kypros Nicolaides in 1992

What are the requirements of a good NT?

The maximum thickness of the subcutaneous translucent area between the skin and soft tissue that overlies the fetal spine in the sagittal plane

11 weeks – 13 weeks 6 days

CRL 45- 84 mm

Mid sagittal view

Neutral position of head

Magnification (>70%)

Widest part of translucency to be measured

Frontal process of maxillary bone should not be seen

Amnion must be seen separate

Biggest of 3 measurements

What other markers can be added to NT in the scan to increase

detection rate?

Nasal bone (fallacy – nasal skin)

Ductus venosus (absent or reversed flow)

Tricuspid regurgitation

Increases detection rate to 95% and decreases false pos rate to 2% FMF angle >85°

(b/w the upper surface of upper palate and frontal bone in midsagittal view)

The more the markers the more error can occur

Nasal bone screeningNormal fetuses = 603

Down's = 59

27%

99.5%

0

20

40

60

80

100

120

Normal T21Presence of nasal bone

Fre

qu

en

cy

(%

)

73% Downs absent nasal bone

0.5% normal

A 11-14 weeks scan reports an NT of 1.2 mm.

What does it imply?

Not useful and action should not be taken on basis of absolute values usually

All values are to be expressed as MOM

Special software available

Standardisation is by the FMF which provides certification

Are there any absolute cut offs for NT and the

other tests?

NT >3.5 mm falls above 95th centile for any week bw 11-14 vweeks

PAPP A <0.5 MOM(think of IUGR)

β hCG >2.5 MOM

These values justify invasive testing in themselves

What is the detection rate with the combined

test?

85-90% for a false pos rate of 5%

Amniocentesis rate is about 5%

Adding nasal bone may increase the detection rate to 90%

If NT alone – 77%

Other methods available?

Second trimester screening by triple and quadruple test

Integrated test

Sequential test

Contingent test

In all these amniocentesis rate is higher (about 8%)

Method

• First trimester screening

• Triple test

• Quadruple test

• Integrated test

• Sequential test

Components

• NT, β hCG and MSAFP

• MSAFP, β hCG and uE3

• MSAFP, β hCG, uE3 & Inhibin

• First trimester screen & Quadruple test

• First trimester screen & Quadruple test

Detection rate

• 85%

• 70%

• 70%

• 95%

• 90%

What is the contingent test?

FTS is offered and depending upon results women are divided into three groups

High risk- invasive procedure

Low risk- reassurance

Intermediate risk- offer second trimester serum screening and invasive testing

What is the genetic sonogram?

Soft tissue markers of aneuploidyNuchal skin fold thickness Absent or hypoplastic nasal bone (7)VentriculomegalyAberrant right subclavian artery

Renal pyelectasisChoroid plexus cystsHyperechoic bowelShort femurEchigenic focus in heart

Structural anomalies like duodenal atresia

What will you recommend in the foll situations

1 Choroid plexus cysts

2 Increased NSFT in second trimester scan

Presence of one such marker should promote a detailed search for other markers and if more than one is present, invasive testing can be recommended

One marker like NSFT, nasal bone or a structural anomaly may be an indication for amniocentesis

Which structural anomalies are definite

indications for karyotyping?

Duodenal atresia

Omphalocoele

AV canal defects

Thick NT and normal KT, what next ?

Cardiac anomalies

Structural defects like omphalocoele

Predictor of TTTS in multiple pregnancies

Inspite of everything a fetal loss of 4% is reported- COUNSEL

11-13 week scanUterine artery PIFMF Certificate of competence in measurement of uterine artery PI

The Fetal Medicine Foundation has now established software for the calculation of risk for preeclampsia. The software is provided free of charge to those who comply with the FMF regulation of NT screening and have demonstrated competence in the Doppler assessment of the uterine arteries at the 11-13 weeks scan.

FMF screening for PE

Established software for prediction of PE

Uterine artery PI + maternal history, BP, Papp A and PlGF

Risk calculated for early and late PE

Detects 90% of early onset PE and 45% late PE for a false pos rate of 10%