73 Prenatal testing Flashcards
Features of screening tests offered during pregnancy?
- Offered to all pregnant women to assess the chance of mother or baby having a particular health problem or disability
- Simple tests (e.g. blood test, ultrasound scan or questionnaire)
- They don’t provide a definite diagnosis
Features of diagnostic tests used during pregnancy?
- Follow-on tests to find out whether the baby does have a particular condition
- Offered to women who have had a ‘higher-chance’ result from screening
- E.g. chorionic villus sampling or amniocentesis (risk of miscarriage), detailed ultrasound scan
Why carry out screening and testing?
- To REASSURE parents
- To inform and PREPARE parents for the birth of an affected infant
- To allow in utero TREATMENT
- To allow TERMINATION of an affected fetus
- To provide information so that parents may choose between 2, 3 or 4: key issue here = choice
Examples of screening and diagnostic testing?
- Natural conception
• Non-invasive screening, e.g. ultrasound, serum test
• Invasive prenatal diagnostic (PND) testing, e.g.
chorionic villus sampling, amniocentesis
• NIPT, sometimes ‘NIPD - In vitro fertilisation
• Preimplantation genetic diagnosis (PGD)
Ultrasound in screening: • What is it? • When is it? • Why is it? • Ethical issues arising?
• What is it? – Uses sound waves; painless; no risk • When is it? – Dating scan: 12 weeks (8 – 14) – Anomaly scan: 20 weeks (18 – 20 weeks + 6 days) • Why is it? – Anomaly: physical abnormalities e.g. spina bifida • Ethical issues arising – Everyone is offered anomaly scan, not everyone chooses to take it
What is Down’s syndrome?
- Trisomy 21
- Commonest cause of identifiable learning disability
- Usually due to non- disjunction at chromosome 21 at meiosis
- 50 % will have a congenital abnormality
- 80% profound or severe intellectual impairment
- Natural prevalence 1:600 live births but incidence now 6:10,000
Down's Syndrome screen: • What is it? • When is it? • Why is it? • Ethical issues arising
• What is it?
– Combination of ultrasound + serum test
• When is it?
– 10 – 13 (+6) weeks
• Why is it?
– Measures the chance of DS; not a diagnostic test (can also detect other
trisomies - Edwards, Patau)
• Ethical issues arising
– Risk (low v high); if greater than 1:150 then option to take diagnostic test (amniocentesis or CVS)
What are the analyse markers in blood in the serum test for Down’s Syndrome?
- Pregnancy associated plasma protein-A (PAPP-A)
* Free ß- human chorionic gonadotrophin (free ß-hCG)
What does ultrasound in Down’s Syndrome screen look for?
Nuchal translucency (NT)
How good is the ‘combined’ screening test for DS?
• False positives
– Test indicates DS, but fetus not affected – 2.2%
• False negatives (related to “detection rate”)
– Test does not indicated DS, but fetus affected – 16%
• Combined test: FN 16%, FP 2.2%
• Recommended screening test
How is the quadruple test used during DS screening?
- Used if women presents later (14 weeks 2 days +)
- Blood test: alpha-fetoprotein (AFP); total human chorionic gonadotrophin (hCG); unconjugated oestriol; inhibin-A
- FN: 20% FP: 3.5%
Name screening tests
- Ultrasound
2. Down’s Syndrome screen/ Quadruple test
Name diagnostic tests
- Amniocentesis
2. Chorionic villus sampling
Amniocentesis: • What is it? • When is it? • Why is it? • Ethical issues arising
• What is it?
– Needle inserted through the abdomen and into amniotic fluid
• When is it?
– 15+ weeks (15 – 18)
• Why is it?
– For karotyping if screening tests suggest aneuiploidy
– DNA anaylsis if parents carrier of an identifiable gene
– Enzyme assays for inborn errors of metabolism
– Diagnosis of foetal infections
• Ethical issues arising
– 0.5-1% risk of miscarriage (also, delay in getting results); infection, injury
How good is amniocentesis?
- False-positive rate of 0.1-0.6%
* False-negative rate of 0.6%
