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
Quad test: detection rates
SINGLETONS: DR = 75% SPR = 5.5%
TWINS: Monochorionic twins: DR 80% SPR 3% Dichorionic twins: DR 40-50% SPR 3%
Higher chance of T13/18/21 on combined or quad screening test? What next?
Explain result – reframe the chance: 1 in 50 = 2%]
Three main options:
1- Do nothing
2- Diagnostic invasive testing [CVS / Amnio]
3- NIPT – non invasive prenatal testing
[not available on NHS currently]
Use scenarios:
Could you cope throughout pregnancy not knowing?
How would it be to find out your baby has Downs at birth?
What would be worse:miscarrying a normal baby or finding out at birth that your baby had Downs?
What would you do if youknew for sure that your baby had Downs right now?
Would you consider termination?
NIPT: non-invasive prenatal testing
Screening for aneuploidy
Cell free fetal DNA [cff DNA] in maternal blood from 5 weeks
Pregnancy specific
Test maternal blood from 10 weeks
Aneuploidy: Screening for T21 sensitivity and specificity over 99%
Marketed as Harmony, SAFE, Panorama, NIFTY test
CFF DNA is from the placenta so still have risk of placentally confined mosaicism. May also detect a maternal mosaicism or maternal chromosome rearrangement or maternal malignancy.
Current use in NHS:
Fetal Rh D typing [11 weeks]
Fetal sexing [from 7 weeks] – 99.5% accurate
Some monogenic disorders
NIPT on the NHS
UK National Screening Committee recommended a 3 year evaluative roll out [was due to start Oct 2018]
To be offered to women with high chance result on screening [combined or quad] with a singleton pregnancy
Absolute exclusions [Risk of false positive results]
Maternal malignancy
Multiple pregnancy
Blood transfusion within 4 months
Organ transplant
Vanished twin / demised twin
Known chromosome or genetic anomaly in the mother
Considerations:
stem cell therapy, egg donation, IVF
NIPT advantages and disadvantages
Advantages
High detection rates, low screen positive rates
Reduction in invasive diagnostic testing [cost effective]
A further option for women
Disadvantages
Screening test: Not diagnostic [false positives / false negatives]
Confirm screen positive results with invasive test
Conditions diagnosed?
Options - Continue - Continue and adoption - Termination [medical / surgical] [Trisomy 21: 90%] Support Antenatal Screening Co-ordinator National support groups: Antenatal Results and Choices [ARC] Downs syndrome association [DSA] Soft U.K. [Tri 13 and 18], Unique, Contact-A- Family Meet other parents [Specialist Health Visitor / local groups] Obstetric / neonatal / paediatric teams Genetic counselling
Pre-screening test information
Prior to entering a screening programme Tests are optional What are we screening for? What the test will not tell you Timing of tests Communicating results – higher / lower chance Invasive tests [miscarriage rate] Options if abnormality diagnosed Contact for further advice
