Screening for Down's syndrome Flashcards
What is the purpose of the combined test?
Purpose of estimating the risk that the foetus in any given pregnancy may have Down’s syndrome.
What does the combined test involve?
Measurement of nuchal translucency on USS with serum levels of 2 proteins- Beta-hcg and PAPPA-A.
How will beta hog levels compare in a fetus with down syndrome?
They will be elevated- (produced by the placenta)
How will PAPPA-A levels compare in a fetus with down syndrome?
Also produced by the placenta, will be reduced in Down’s syndrome.
When specifically can nuchal translucency be measured?
11+2 weeks and 14+1 weeks. (beyond this point the significance of NT is unknown)
What is the quadruple test, when is it done and what does it measure?
- 14+2 weeks until 20 weeks exactly, and it measures 4 markers. Detection rate is 80%.
PAPP-A: produced by placental syncytiotrophoblasts; levels reduced in pregnancies affected by Down’s syndrome.
Beta-hCG: produced by placental syncytiotrophoblasts; raised levels in pregnancies affected by Down’s syndrome.
AFP: produced by fetal yolk sac and liver; reduced levels in pregnancies affected by Down’s syndrome.
uE3: produced by placenta and fetal adrenals; reduced levels in pregnancies affected by Down’s syndrome.
Inhibin-A: produced by placenta; raised levels in pregnancies affected by Down’s syndrome.
As well as the blood results, what else is taken into account to determine a mothers risk of having a downs baby?
Maternal age, weight, gestation, ethnicity
What does a risk of 1:790 mean?
In 790 women carrying a baby with this result, only one will be affected by Downs and the other with the same hormone levels and nuchal translucency will not.
What happens to women with a high risk from the screening tests?
These women will be offered a definitive diagnostic procedure, CVS from 11 weeks and Amnio from 15.
What happens in CVS?
A small placental tissue sample is taken and results are back in about 48 hours. There is a higher risk of miscarriage but benefit is that it can be performed earlier than amnio.
What happens in amnio?
A needle is used to sample the amniotic fluid within the sac that the fetus develops- rapid chromosomal analysis can be performed rapidly within 48 hours with a 99.5% accuracy rate. 0.8% risk of miscarriage.
