Antenatal Screening Flashcards
What is the purpose of screening?
to identify individuals at a higher risk of a disease or condition
What is the purpose of screening for chromosomal abnormalities during pregnancy?
to establish which women should receive more invasive tests to establish a definitive diagnosis
What is involved in the combined test?
When is it performed?
- this is the most accurate screening test for Down’s syndrome
- it is offered to all women between 11 and 14 weeks gestation
- it involves USS and maternal blood tests
this is an OPT IN test
What is the role of USS in the combined test?
- it measures the nuchal translucency
- this is the thickness of the back of the neck of the foetus
- Down’s syndrome causes nuchal thickness > 6mm
What maternal blood tests are included in the combined test?
beta-HCG:
- higher result = greater risk
PAPP-A:
- lower result = greater risk
beta-HCG = beta-human chorionic gonadotrophin
PAPP-A = pregnancy-associated plasma protein-A
What is done to screen for Down’s syndrome if the woman misses combined screening?
- the triple or quadruple test can be performed between 14 to 20 weeks gestation
- these involve blood tests ONLY
- they are not as accurate as combined screening
What blood tests form part of the triple test?
beta-HCG:
- higher result = greater risk
serum oestriol:
- lower result = greater risk
alpha-fetoprotein (AFP):
- lower result = greater risk
How is the quadruple test different to the triple test?
- it is the same, but has an additional blood test
- it also includes inhibin-A
- higher result = greater risk
When is someone deemed high risk of Down’s syndrome after screening?
What additional test are they offered?
- the screening tests provide a risk score for the foetus having the condition
- if the risk is > 1 in 150, they are high risk
- high risk women are offered amniocentesis or chronionic villus sampling (CVS)
How can CVS / amniocentesis give a definitive answer about Down’s syndrome?
- they involve taking a sample of fetal cells
- karyotyping can then be performed
- this provides a definitive diagnosis
What is chorionic villus sampling (CVS) and when is it performed?
- it involves an ultrasound-guided biopsy of placental tissue
- performed when testing is earlier in the pregancy (before 15 weeks)
What is amniocentesis and when is is performed?
- it involves ultrasound-guided aspiration of amniotic fluid using a needle & syringe
- it is performed later in pregnancy (after 15 weeks) than CVS
- there needs to be enough amniotic fluid present in order for it to be safe to sample
What is a new alternative that is being introduced in place of invasive testing for high risk mothers?
non-invasive prenatal testing (NIPT)
- this is NOT diagnostic, but gives a very good indication as to if the fetus is affected
What is involved in NIPT?
- a blood test is taken from the mother
- this contains fragments of DNA
- some DNA fragments come from placental tissue and represent fetal DNA
- these can be analysed