13. Prenatal screening Flashcards
Population screening in the UK
UK National Screening Committee: makes independent, evidence based recommendations to ministers
Exists to protect and improve the nations health and wellbeing and reduce health inequalities.
Develops standards, implements and supports screening policy in collaboration with NHSE.
Quality assures screening [Screening Quality Assurance Service]
NHSE implements and runs screening services across England
What is screening?
Screening identifies apparently healthy people who may be at increased risk of a disease or condition, enabling earlier treatment or informed decisions.
NHS foetal anomaly screening programme
Early pregnancy scan
Screening for Down’s Edward’s Patau’s syndromes
- First trimester combined test - Second trimester quad test
18+0 – 20+6 weeks fetal anomaly scan
Why scan at 10-14 weeks?
Viability
- 2-3% of women attending for a scan will have miscarried
Accurate dating
- NICE guidance: use scan dates in
lieu of LMP dates
- crucial for screening tests
- reduces need for post dates induction
of labour
Detecting multiple pregnancy
- determine chorionicity
Diagnosis of structural abnormality
- spina bifida
- anencephaly
- exomphalos & gastroschisis
- bladder outflow obstruction
Screening for chromosomal conditions: Down’s -Trisomy 21
Edward’s- Trisomy 18
Patau’s –Trisomy 13
Chorionicity
T sign (left) are monochorionic and so need closer screening as are further at risk of complications Lambda sign on right are dichorionic Both show single placental mass but in DC there is an extension of the placental tissue into the base of the intertwin membrane
Structural abnormalities
- spina bifida
- anencephaly- exomphalos & gastroschisis
- bladder outflow obstruction
Chromosomal abnormalities
Down’s -Trisomy 21
Edward’s- Trisomy 18
Patau’s –Trisomy 13
Prenatal diagnosis/diagnostic invasive tests
Chorionic Villus Sampling CVS 11+ weeks
Amniocentesis 16+ weeks
Around 1% risk of miscarriage
[higher in twins]
Screening tests
identify individuals at ‘high’ or ‘low’ chance of having a baby with a trisomy
A low chance result does not exclude trisomy in the baby.
A high chance result does not indicate that the baby is
definitely affected.
No risk of miscarriage
Diagnostic tests
give definitive information on the fetal chromosomes by confirming the presence of an extra chromosome or absence of a chromosome
Risk of miscarriage
Maternal/foetal influencing factors for combined screening
Maternal age Gestational age Ethnicity Smoking IVF Multiple pregnancy Weight Diabetes Past history of chromosome abnormality Fetal sex Analytical Imprecision
First trimester combined screening
Maternal age +
nuchal translucency scan +
Blood tests (PAPPA, BhCG)
= chance of baby having trisomy 21 and/or trisomy 13/18
Combined screening result process
Cut off for high / low chance = 1 in 150
Higher chance? Phoned within 3 working days
Lower chance? Letter within 2 weeks
Detection rate: 82%
Screen positive rate: 2.7%
Increased NT > 3.5mm
Increased chance of
Chromosomal anomaly
Cardiac anomaly
‘Syndromes’
Offer: Karyotyping – array CGH
Fetal cardiac scan
Anomaly scan
Second trimester maternal serum screening: quadruple test
14+2 to 20 weeks: late bookers only
Chance of T21 only
Gestational age, maternal age, smoking, weight, ethnicity and … Maternal serum markers: Decrease in: UE3 AFP
Increase in: Inhibin A
BHCG
