1. Screening for Down’s syndrome
FAQ’s
Dr Sheila Balakrishnan
MD DNB FRCOG FICOG
Prof in OBGYN
Medical College Trivandrum

2. Do you think screening for Down’s is necessary and if so for what age group?

3. What is meant by ‘apriori risk’?

4. 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

5. Age Midtrimester Term 35 1 in 384 1 in 250 40 1 in 106 1 in 70

6. Best method of screening as of today?

7. First trimester screening by the combined test-
NT PAPP A and β hCG-
is added to the apriori risk

8. What is NT?

9. USS term for the physiologic collection of fluid underneath the nuchal skin extending for a variable distance over the fetal head and back at weeks
For unclear reasons this collection is unduly sensitive to fetal problems

10. Who described NT and in which year?

11. Kypros Nicolaides in 1992

12. What are the requirements of a good NT?

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

14. 11 weeks – 13 weeks 6 days
CRL 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

18. What other markers can be added to NT in the scan to increase detection rate?

19. Increases detection rate to 95% and decreases false pos rate to 2%
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

21. 73% Downs absent nasal bone
0.5% normal

22. A 11-14 weeks scan reports an NT of 1.2 mm. What does it imply?

23. 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

24. Are there any absolute cut offs for NT and the other tests?

25. 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

26. What is the detection rate with the combined test?

27. 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%

28. Other methods available?

29. Second trimester screening by triple and quadruple test
Integrated test
Sequential test
Contingent test
In all these amniocentesis rate is higher (about 8%)

30. 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
Detection rate
85%
70%
95%
90%

31. What is the contingent test?

32. 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

33. What is the genetic sonogram?

34. Soft tissue markers of aneuploidy
Nuchal skin fold thickness
Absent or hypoplastic nasal bone (7)
Ventriculomegaly
Aberrant right subclavian artery
Renal pyelectasis
Choroid plexus cysts
Hyperechoic bowel
Short femur
Echigenic focus in heart
Structural anomalies
like duodenal atresia

35. What will you recommend in the foll situations 1 Choroid plexus cysts
2 Increased NSFT in second trimester scan

36. 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

37. Which structural anomalies are definite indications for karyotyping?

38. Duodenal atresia
Omphalocoele
AV canal defects

39. Thick NT and normal KT, what next ?

40. Cardiac anomalies
Structural defects like omphalocoele
Predictor of TTTS in multiple pregnancies
Inspite of everything a fetal loss of 4% is reported- COUNSEL

41. 11-13 week scan Uterine artery PI
FMF 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 weeks scan.

42. 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%

43. Thank you