Prenatal Diagnosis

Question 1

Describe Antenatal Care in Lothian

Answer 1

-GP appointment following positive pregnancy
test
-Booking appointment with midwife 8 - 10 weeks
including dating ultrasound scan and
haemoglobinopathy screening (Thal +/- Sickle)
-First trimester screening offered (nuchal
translucency; hCG; PAPP-A) 11-14 weeks
-If late booker second trimester screening offered
for T21 and NTD 14-18 weeks (hCG; AFP)
-Detailed second trimester ultrasound scan
-Monitoring in primary care throughout

Question 2

What changes if there is a history of miscarriages (in terms of screening)?

Answer 2

• Early scan to check viability

- Ultrasounds at different weeks

Question 3

What are the possible diagnosis’s that can be made when presented with polydactyly, encephalocele, and echogenic kidneys?

Answer 3

• Trisomy 13

- Meckel Gruber Syndrome

Question 4

What is Trisomy 13/ Patau’s syndrome associated with (features)?

Answer 4

• Polydactyly
• Microcephaly (small head size)
• Holoprosencephaly (failure of the forebrain to divide properly)
• Heart defects
• Structural eye defects, including microphthalmia, cataract, retinal dysplasia
• Cleft palate
• Abnormal genitalia
• Kidney defects
• Rocker-bottom feet

Question 5

Describe Meckel Gruber Syndrome

Answer 5

• Autosomal recessive
• Due to genes that encode for cilia
• Usually lethal in new-born period
• Relatively rare
• Affect kidneys, polydactyly, affects brain and eyes

Question 6

What is Duchenne Muscular Dystrophy?

Answer 6

• Severe progressive muscle wasting condition
• Gowers manoeuvre (weak in hip)
• Muscle replaced by fatty tissue, creatine kinase raised

Question 7

What is the inheritance of DMD?

Answer 7

• X linked
• 2/3 mothers of boys with DMD are carriers
• This is reduced to 1/2 if they have an unaffected son

Question 8

What is CVS?

Answer 8

• Chorionic villus sampling
• Out patient procedure
• Sample from placenta
• Performed at 10.5-12 weeks gestation
• Miscarriage risk 0.5%
• Transabdominal under ultrasound guidance

Question 9

What is foetal sexing on maternal blood?

Answer 9

-Cell free foetal DNA (cffDNA)
-5-10% of total cell free DNA in maternal plasma
-Originates from trophoblast
-Detectable from 4-5 weeks gestation
-Levels increase with gestation period
-Fragmented DNA
-Cleared from circulation rapidly after delivery
-Non-invasive
=SRY amplified then male, if fails to amplify= female

Question 10

What are the challenges of non-invasive foetal sexing?

Answer 10

-Technically difficult
=Very low DNA concentration
=High levels of maternal DNA background
-Reliable detection of cffDNA only possible from 9 weeks gestation, confirm by scan
-Not applicable in twin pregnancies, including vanishing twins
-Samples need to be extracted quickly

Question 11

What is PGD?

Answer 11

• Preimplantation Genetic Diagnosis
  1. In vitro fertilisation
  2. Embryo culture
  3. Embryo biopsy
• Cell screening
  5. Transfer of desired embryo
• Genetic testing by haplotyping= genetic fingerprint

Question 12

What is the criteria to receive NHS funded PGD in Edinburgh?

Answer 12

-Known genetic condition which conveys a “significant risk of a serious genetic condition being present in the embryo” (HFEA Code of Practice)
-No living unaffected child* as a couple
-Female age < 39 yrs
-Anti Mullerian hormone (AMH) ≥ 6 or antral
follicle count > 8
-Female BMI < 30
-Both partners non-smokers for > 3 mths
-Couple living at same address for > 2 yrs
-Both partners must be eligible for NHS treatment

Question 13

What are the features of Spina Bifida?

Answer 13

• Lemon shaped head
• Banana shaped cerebellum
• Abnormal spine with sac protruding

Question 14

Describe neural tube defects

Answer 14

-Neural tube closure occurs early in pregnancy; from cervical spine distally day
18-28
-Outcome varies from spina bifida to
anencephaly
-Incidence 1/300 N.Ireland – 1/1000 USA
-Polymorphisms in MTHFR
-Mostly multifactorial but can be syndromic, chromosomal or teratogen-induced
-If couple has one affected
child, increased chance of
another (5%)
-Recurrence risk can be
decreased by high dose
Folic acid (5mg)
-Pre-conceptual Folic acid
important for all women –
400 micrograms, 2months
prior to conception and up
to 12 weeks of pregnancy

Question 15

What does First trimester screening involve?

Answer 15

-Nuchal translucency; hCG;
PAPP-A)
-Offered 11 – 14 weeks
-Positive predictive value 13%, negative predictive value 99.95%
=First trimester screening combines maternal age, nuchal translucency measurement (ultrasound measurement of the thickness of the fold at the back of the fetal neck) and maternal serum markers Free Human Chorionic Gonadotrophin (HCG) and Pregnancy-associated plasma protein A (PAPP-A). It is carried out between 11 weeks and 13+6 weeks gestation.

Question 16

What does Second trimester screening involve?

Answer 16

-Offered to late bookers for
T21 between 14-20 weeks (hCG; AFP, Inhibin A, Unconjugated Estriol)
-Positive predictive value 2.3%, negative predictive value 99.8%
=Women who missed first trimester screening can be offered second trimester screening. For this maternal blood is taken between 15 and 20+6 weeks gestation to measure Alpha-fetoprotein (AFP), HCG, unconjugated oestradiol and inhibin A.A high hCG:AFP ratio increases the chance that the fetus is affected by Down syndrome.

Question 17

What does Down syndrome present as at second trimester screening?

Answer 17

-Low AFP / PAPP-A MoM (multiples of the mean)
=Parity
=Maternal age
=Previous affected pregnanies...
If chance higher than 150= high risk
-Raised hCG
-Increased nuchal translucency

Question 18

What does Neural Tube defects present as at second trimester screening?

Answer 18

Raised AFP > 2MoM
-An AFP of more than 2 multiples of the median (MoM) corrected for maternal weight indicates an increased risk of neural tube defect. Detailed second trimester ultrasound scanning should identify over 90% neural tube defects.

Question 19

Describe NIPT as second line screening test

Answer 19

-Non invasive= no risk of miscarriage
-Uses cffDNA
-In higher chance (>1:150) population has positive predictive value
-Not diagnostic
=91.3% for Trisomy 21
=84% for Trisomy 18
=87% for Trisomy 13

Question 20

What are the potential problems with NIPT?

Answer 20

• Confined placental mosaicism
• Maternal malignancy
• Maternal chromosome anomalies
• Vanished or demised twin
• Blood transfusion within last 4 months or transplant

Question 21

Describe amniocentesis

Answer 21

-Performed between 14 and 16 weeks
gestation
-15-20mls amniotic fluid aspirated
-Small risk of membrane rupture
-Associated miscarriage rate 0.5-1%
-Rapid aneuploidy screen for Trisomy 21,
13 and 18

Question 22

Describe QF-PCR

Answer 22

-Quantitative fluorescence
polymerase chain reaction
=Amplification of chromosome specific short tandem repeats
(STRs)
=Sample DNA is amplified by PCR using fluorescent primers
=Products can be visualised and quantified as peak areas of the respective repeat lengths
-For chromosomes 21, 13, 18,
sex chromosomes

Question 23

Describe Trisomy 21/ Down’s syndrome

Answer 23

• 1 in 650 births
• Maternal age effect
• Risk> population risk by mid 30’s
• Recurrence risk is 1 in 100 or twice the risk for age if due to non-disjunction (95%)
• Mosaicism
• Robertsonian translocation of 21 onto usually 14 (maternal= 12%, paternal= 3%)

Question 24

How is CVS performed?

Answer 24

• CVS will provide a piece of placenta, providing material for DNA extraction and chromosome analysis. It is generally performed from about 11 weeks gestation and carries a miscarriage risk of around 1.5 - 2% as a result of the test.
• As DNA can be extracted directly without having to culture the villi a result is generally available within 3 days providing that the test is PCR based.

Question 25

How is amniocentesis performed?

Answer 25

Amniocentesis is a diagnostic test that involves the removal of 10-20mls of amniotic fluid under ultrasound control. The fluid contains cells from the baby and placental membranes which can be cultured for chromosome analysis or can provide a source of DNA for molecular analysis. Amniocentesis is generally performed from 15 weeks gestation onwards and carries a 0.5-1% miscarriage risk.

Question 26

What is Amnio-PCR?

Answer 26

Amnio-PCR is a rapid, PCR based test involving amplification of polymorphic markers on chromosomes 21,18 and 13 directly from amniotic fluid. The test is based on a limited number of PCR cycles which allows quantification of results.

Question 27

How does foetal sexing on maternal blood work?

Answer 27

Cell-free fetal DNA (cffDNA) originates from placental trophoblast and is shed into the maternal blood stream. cffDNA circulates freely in the maternal blood stream and can be used for non-invasive prenatal diagnosis. The amount of cffDNA increases as the pregnancy progresses, representing 3 – 6% of total DNA in maternal plasma and is cleared rapidly from the maternal circulation post delivery. From 8 weeks gestation a maternal blood sample can be analysed to detect a Y-specific sequence from a male foetus. This technique is currently used for the early non-invasive prenatal determination of sex for foetuses at risk of X-linked disorders with the aim to avoid invasive CVS for foetuses predicted to be female.

Question 28

What is haemoglobinopathy screening?

Answer 28

All pregnant women will be offered screening for thalassaemia based on a formal process of inspection of routine blood indices. Additionally, using a Family Origin (Ancestry) Questionnaire (FOQ) to assess risk status, women in high risk groups, or women whose partners are in high risk groups, will be offered screening for sickle cell disorders and other haemoglobin variants

Question 29

What is Meckel Gruber Syndrome?

Answer 29

- Meckel Gruber syndrome is a rare heterogeneous autosomal recessive disorder characterised by post axial polydactyly, cystic kidneys and encephalocele. - It is almost invariably lethal. - The gene MKS1 was found to be mutated in families in whom the condition was linked to chromosome 17 in 2006. - Since then another 7 genes have been identified and mutations in the eight genes known to be associated with Meckel Gruber syndrome account for about 75% of all cases. - Currently prenatal molecular diagnosis is not feasible unless the familial mutations have previously been identified.

Question 30

What is DMD?

Answer 30

- DMD is an X-linked neuromuscular disorder caused by mutations in the dystrophin gene. - Approximately 50-65% of boys with DMD will have a detectable deletion in the gene, 5-10% have a duplication and sequencing can detect pathogenic variants in approximately 20-35%. - Boys with DMD are often slow to walk and some may have evidence of more general developmental delay. - Muscle weakness develops through childhood as muscle fibres are replaced with fat and boys become usually wheelchair bound in their teens. - Cardiac and respiratory muscles are also affected and nocturnal non invasive ventilation may be required. - Early treatment with steroids may slow progression of the disease but death often occurs in young adulthood.

Question 31

What is Spina Bifida?

Answer 31

- Spina bifida is a developmental abnormality characterised by failure of fusion of the neural tube. - The condition is associated with neurological abnormalities of the lower limbs and often bowel and bladder. - About 90% of children with spina bifida will also develop hydrocephalus and require a shunt. - Intelligence is usually within the normal range. - Information and counselling is available through the patient group ASBAH (Association for Spina Bifida and Hydrocephalus).

Question 32

What are marker chromosomes?

Answer 32

- Marker chromosomes are unidentified pieces of chromosomal material found at karyotyping. - They may be inherited or arise de novo. - If inherited they are generally without phenotypic effect. - If the marker chromosome has arisen de novo it is important to assess whether it contains euchromatin (coding DNA) as this suggests that there will be an associated phenotypic abnormality.