OBS: Antenatal Care -- Down's syndrome

Question 1

How to advise a patient regarding DSS1 result?

Answer 1

(<1:250) Low risk. Reassure that further invasive testing is not needed
(>1:250) Option of NIPT as secondary screening.
Diagnostic tests: CVS at 11~16w, amniocentesis at 16~20w (risk of miscarriage 0.2~0.3%)
If diagnosed, discuss: ToP before 24w / surgical correction of malformations + caretaking arrangements

Question 2

Sensitivity of Down’s syndrome

Answer 2

0.9

Question 3

FP rate of DSS1

Answer 3

0.05

Question 4

What is MoM and its normal range?

Answer 4

Multiple of Median. A measure of deviation of a test from the median of unaffected pregnancies of the same gestational age. <0.2 is low, >2-2.25 is high.

Question 5

Why do we used MoM instead of actual hormonal levels?

Answer 5

Actual hormonal levels vary with gestational age and weight of the mother. Using MoM can avoid bias and make it more convenient to interpret hormonal levels.

Question 6

Legal basis of TOP

Answer 6

Cap 212.
By 2 registered MOs, believe TOP is necessary because
(i) continuance of pregnancy would involve risk to life / health of mother
(ii) risk that the child would suffer from severe physical/mental abnormality
Must be <24w unless life-threatening

Question 7

Medical procedure of TOP

Answer 7

Mifepristone 200mg PO + Misoprostol 400ug sublingual (1~2 days later).
Inpatient, mini-labour procedure.
SE: GI upset, headache, fever

Question 8

Can Down’s syndrome be inherited?

Answer 8

Recurrence risk:
1. Non-disjunction: 1%
2. Balanced translocation: 10~15% if mother is the translocation carrier
3. Mosaicism: not likely

Question 9

What is the most important factor in determining the background risk of Down’s syndrome?

Answer 9

Advaned maternal age

Question 10

USG Features to ensure accurate measurement of nuchal translucency

Answer 10

1. Good magnification: only fetal head & upper chest
2. Appropriate view: mid-sagittal, with mid-line structures shown
3. Appropriate positioning: neck at neutral position
4. Correct placement of calipers: caliper on-to-on, away from amnion, beware of nuchal cord