L13 Prenatal Screening

Question 1

Population screening in UK

Answer 1

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

NHSE implements and runs screening services across England

Question 2

What is screening?

Answer 2

Screening identifies apparently healthy people who may be at increased risk of a disease or condition, enabling earlier treatment or informed decisions

Question 3

Why scan at 10-14 weeks?

Answer 3

Viability
-2-3% of women attending for 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

Detect multiple pregnancy
-determine chorionicity

Diagnosis of structural abnormality

• spina bifida
• anencephaly
• exomphalos & gastroschisis
• bladder outflow obstruction

Screening for chromosomal conditions:

• Down’s -trisomy 21
• Edwards’s -trisomy 18
• Patau’s -trisomoy 13

Question 4

Prenatal diagnosis/ diagnostic invasive test

Answer 4

Chronic Villus Sampling CVS 11+ weeks

Amniocentesis 16+ weeks

Around 1% risk of miscarriage (higher in twins)

Question 5

What are diagnostic tests?

Answer 5

Give definitive information on the foetal chromosomes by confirming the presence of an extra chromosome or absence of a chromosome

Risk of miscarriage

Question 6

What are screening tests?

Answer 6

Identify individuals at ‘high’ or ‘low’ chance of having a baby with a trisomy

A low chance result does not exclude trisomy in the bay

A high chance result does not indicate that the baby is definitely affected

No risk of miscarriage

Question 7

First trimester combined screening

Answer 7

Maternal age + ultrasound + PAPPA BhCG = Chance of baby having trisomy 21 and/or trisomy 13/18

Question 8

Maternal age related risk for chromosomal abnormalities

Answer 8

Increases trisomy, 21, 18, 13

same no matter what age in turner’s syndrome and triploidy

Question 9

Nuchal Translucency Scan

Answer 9

CRL 45-84mm

[11+2 to 14+1 weeks]

Question 10

Combined screening: Maternal serum biochemical markers

Answer 10

free beta-hCG

PAPP-A

Question 11

Maternal /fetal influencing factors for combined screening

Answer 11

Maternal age
Gestational age
Ethnicity
Smoking
IVF
Multiple pregnancy
Weight
Diabetes
Past history of chromosome abnormality
Fetal sex
Analytical Imprecision

Weight = 3kg error = 5% error in risk calculation

Smoking inc BHG by 20% and reduces PAPPA by up t 15%

Ethnicity: afro caribbean inc PAPPA by up to 60%

Question 12

Combined screening result process

Answer 12

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%

Question 13

Increased NT > 3.5mm

Answer 13

Increases chance of

• Chromosomal anomaly
• Cardiac anomaly
• ‘Syndromes’

Offer:
Karyotyping - array CGH
Fetal cardiac scan
Anomaly scan

Question 14

Second trimester maternal serum screening: quadruple test

Answer 14

14+2 to 20 weeks: late bookers only

Chance of T21 only

Gestational age, maternal age, smoking, weight, ethnicity and…

Maternal serum markers:
Decreased UE3 and AFP
Increased Inhibit A BHCG

Question 15

QUAD test: Detection rates

Answer 15

Singletons: DR=75% SPR=5.5%

Twins:
Monochorionic twins:
-DR 80% SPR 3%
Dichorionic twins:
-DR 40-50% SPR 3%

Question 16

Higher chance of T13/18/21 on combined or quad screening test? What next?

Answer 16

Explain result - reframe the chance: 1 in 50 =2%

3 main options:
1- do nothing
2- diagnostic invasive testing (cvs/amnio)
3- NIPT - non invasive prenatal testing (not available on NHS currently)

Question 17

NIPT: non-invasive prenatal testing: screening for aneuploidy

Answer 17

Cell free foetal 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

Question 18

NIPT on the NHS

Answer 18

UK national screening committee recommended a 3 year evaluative roll out

To be offered to women with high chance result on screening (combined or quad) with a singleton pregnancy

Question 19

Absolute exclusions (risk of false positive results) of NIPT on NHS

Answer 19

Maternal malignancy

Multiple pregnancy

Blood transfusion within 4 months

Organ transplant

Vanished twin/demised twin

Known chromosome or genetic anomaly in the mother