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Year 1 POM > Prenatal testing > Flashcards

Prenatal testing Flashcards

1
Q

What are the stages in a normal pregnancy?

A
  • Positive pregnancy test – no longer confirmed at GP
  • Book into antenatal care – see midwife
    1. Nuchal scan – 10-14 weeks gestation. Different tests dependent upon NHS Trust e.g. nuchal translucency, combined test etc.
    2. Mid-trimester anomaly scan
    3. Ultrasound examination is the main method for prenatal diagnosis of fetal abnormalities. 4. All pregnant women should be offered routine ultrasound scans at 11-14 weeks and again at 20-22 weeks.
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2
Q

What are the aims of the 12 week scan?

A
  • To date the pregnancy accurately.
  • To diagnose multiple pregnancy.
  • To diagnose major fetal abnormalities.
  • To diagnose early miscarriage.
  • To assess the risks of Down Syndrome and other chromosomal abnormalities.
  • Taking into account the maternal age, blood hormone levels, nuchal translucency thickness, nasal bone, blood flow through the fetal heart and fetal abnormalities.
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3
Q

When is the Nuchal translucency test?

A
  • 10-14 weeks
  • Look at thickness of fluid at back of fetal neck
  • NT is screening test and is not diagnostic
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4
Q

What can it indicate if the thickness at back of fetal neck is greater than 3mm?

A 
1. Chromosome abnormalities 
 (e.g. Downs, Edwards, Patau, Turners)
NT + maternal age detects up to 75% of Down syndrome with 5% false positive rate
2. Birth defects: - 
Cardiac anomalies
Pulmonary defects (diaphragmatic hernia)
Renal defects
Abdominal wall defects
3. Skeletal dysplasias
(Check)
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5
Q

When is prenatal testing arranged?

A
  1. Following abnormal findings at nuchal scan or mid-trimester scan
  2. Following results of combined test which give an increased risk of Down Syndrome
  3. If previous pregnancy affected with a condition e.g. DS, CF
  4. If parent(s) carrier of chromosome rearrangement or genetic condition, e.g. t(13;14), DMD, HD.
  5. FH of genetic condition
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6
Q

What are the aims of prenatal testing?

A

1 .To inform and prepare parents for the birth of an affected baby
2. To allow in utero treatment
3. Manage the remainder of the pregnancy
4 .To be prepared for complications at or after birth
5. To allow termination of an affected fetus

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7
Q

What are the scanning prenatal tests?

A

Ultrasound/MRI

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8
Q

What are the non-invasive prenatal tests?

A

Maternal blood test

Cell-free fetal DNA

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9
Q

What are invasive pre-natal tests?

A

Chorionic villus sampling (CVS)

Amniocentesis

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10
Q

What are the different ultrasounds in pregnancy?

A

Early / dating scan
Nuchal translucency (NT) & nasal bone
High level / anomaly scan

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11
Q

When is there usually a metal MRI?

A

Usually around 20 weeks+

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12
Q

What does examination of metal profile increase sensitivity of screening for?

A

Downs syndrome

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13
Q

What is maternal serum screening? What does it find?

A
  1. Tests maternal serum markers in the blood to detect:
    - increased risk of fetal trisomy 21, trisomy 18 and/or neural tube defects
  2. 1st trimester maternal serum screening (with nuchal translucency measurement): 11-14 weeks [hCG, PAPP A]
  3. 2nd trimester maternal serum screening (triple screen): 16-20 weeks [AFP, uE3, hCG]
  4. Nuchal translucency measurement: 11-14 weeks
  5. Other variations combining 1st and 2nd trimester screening results available privately
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14
Q

What is cell free metal DNA>

A
  • Non-invasive prenatal diagnosis (NIPD) works by analysing the DNA fragments present in the maternal plasma during pregnancy (cell-free DNA).
  • Most of this DNA comes from the mother
  • 10%-20% of it comes from the placenta, which is representative of the unborn baby (cell-free fetal DNA).
  • Cell-free fetal DNA (cffDNA) is first detectable from about 4 -5 weeks’ gestation, but cannot accurately be detected on testing until around 9 weeks
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15
Q

What is NIPD? When is it free?

A

•Maternal blood test at around 9 weeks of pregnancy

  • Achondroplasia - testing is free
  • Thanatophoric dysplasia - testing is free
  • Apert syndrome- testing is free
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16
Q

When is sexing offered and when is non invasive test required?

A

SEXING
•Currently offered when there is a X-linked condition in the family e.g. DMD.
•Test detects SRY gene on Y chromosome, enabling us to determine if male or female fetus
–If male-go on to prenatal test
–If female -no invasive test required

17
Q

When is NIPD offered privately via NHS?

A

•Autosomal dominant single gene disorders inherited from the father or arisede novo
–NF1
•NIPD is also possible to alter management of pregnancies at risk of recessive conditions when the mother and father carry different altered genes.
–if the paternal alterationhas been inherited by the fetus invasive prenatal testing can be offered.
- Cystic fibrosis – haplotyping (RHDO) can test for both maternal and paternal mutation

cffDNA testing for Aneuploidy (NIPT)

•Offered privately (Harmony) or via research studies
•Harmony currently test for T13, T18, T21 and this identifies:
–99% of fetuses with trisomy 21
–97% of fetuses with trisomy 18
–92% of fetuses with trisomy 13.
•Is this accurate enough to make a decision?

18
Q

What are the basic symbols?

A
  • Translation of non-invasive prenatal diagnosis (NIPD) for Duchenne and Becker Muscular Dystrophy (DMD/BMD) into a clinical setting.
  • Evaluating early non-invasive prenatal diagnosis (NIPD) based on cell-free fetal (cff) DNA and RNA in maternal plasma.
19
Q

What are the limitations of NIPD and NIPT?

A

1 .Multiple pregnancies - It is not possible to tell which fetus the DNA is from when carrying twins/triplets etc.

  1. The relative proportion of cell-free fetal DNA is reduced in women with a high BMI as they have more of their own cell-free DNA.
  2. Although it is just a blood test, it has the same implications as an invasive test.
  3. Women may prepare themselves more for the implications of an invasive test result
  4. Women must consider the consequences of the results. Do they want this information?
  5. An invasive test may still be required to confirm an abnormal result.
20
Q

What are the benefits of NIPD and NIPT?

A
  1. The number of invasive tests carried out is likely to reduce as a result
  2. There is no increased risk of miscarriage.
  3. Less expertise is required to perform a blood test than an invasive test.
  4. In many cases we can offer NIPD /NIPT earlier than traditional invasive testing, thereby getting a result much earlier.
21
Q

When are invasive tests offered?

A 
•Offered if there is a ‘known risk’
–Chorionic villus sampling (CVS)
–Amniocentesis
•Molecular, cytogenetic and biochemical tests
•Ultrasound guidance
•Outpatient basis
22
Q

When is chorionic villus sampling (CVS) offered? What is the risk of miscarriage?

A

–11-14 weeks
–1-2% risk of miscarriage
–Transabdominal or transvaginal
–Takes sample of chorionic villi – part of developing placenta – same DNA as fetus
–Allows patient to have an earlier result than amnio - important for many patients re. TOP decision

23
Q

What are the different methods of CVS? What does it entail?

A

–Transabdominal or transvaginal
–Takes sample of chorionic villi – part of developing placenta – same DNA as fetus
–Allows patient to have an earlier result than amnio - important for many patients re. TOP decision

24
Q

When is amniocentesis carried out?

A

–From 16 weeks

–Takes sample of amniotic fluid which contains fetal cells

25
Q

What are the risks of amniocentesis?

A

–Up to 1% risk
of miscarriage
–Infection
–Rh sensitisation

26
Q

What tests are done with the DNA sample?

A
  1. Test for the genetic disorder in question
    Timing for results dependent upon condition
  2. Karyotype if chromosomal abnormality in family
    Results 2 weeks (dependent upon the cells growing)
  3. QF-PCR for all:
    We send samples to Guy’s Hospital laboratory
    Looks for t13, 18 & 21 (plus sex chromosomes if sex chromosome disorder suspected)
    Result within 24-48 hours
27
Q

What is a CGH array? When is it carried out?

A
  • If there are concerns on 20 week scan the gold standard is to offer CGH array
  • Looks for small/large imbalances in chromosomes (picks up microdeletions and duplications)
  • If something found on array we standardly test parents to see if either is a carrier. This can help with interpretation
  • Neurosusceptibility loci: 1q21.1 dup, 16.11.2 dup, 15q11.2 del etc etc. Uncertainty regarding penetrance. Be wary about discussing PND/PGD
28
Q

What is trio exome? When is it in use?

A
  • Consider where fetus in previous pregnancy had significant anomalies e.g. heart, brain, skeletal or where baby has been born with developmental delay, dysmorphic features (and array normal)
  • Exome is the coding region of the genome. Take DNA from fetus/baby and parents
  • Good pick up (40%+) where referrals are appropriate
  • Started with Deciphering Developmental Disorders (DDD) study
  • Whole genome sequencing is coming!
29
Q

What are some reproductive options offered?

A
  1. Where there is a known reproductive risk, the options for family planning include:
  2. Conceive naturally, no prenatal testing
  3. Conceive naturally, have prenatal testing
  4. Use of egg and/or sperm donors
  5. Adoption
  6. Choose not to have children
  7. Pre-implantation genetic diagnosis (PGD)
30
Q

Describer egg and sperm donation

A
  • No longer anonymous, children conceived have the right to contact donor when 18
  • Best to go through a UK HFEA licensed fertility centre – conform to strict medical, ethical and legal standards
  • Can privately find own donor
  • Some couples may consider going abroad
31
Q

What happens in process of adoption?

A

Two stages:
1.Registration and checks
Registering interest with adoption agency
Medical and criminal background checks; three written references
Usually takes ~2 months
2.Assessment and approval
Home visits by social worker
Compilation of ‘prospective adopters report’, taken to adoption panel
Panel review information and make a decision whether a couple is suitable to adopt
Takes ~4 months

32
Q

What is pre implantation genetic diagnosis?

A
  • Uses IVF with an additional step to genetically test the embryo before implantation
  • PGD is particularly used by people who do not want TOP
33
Q

What is the process of PGD?

A
  1. Stimulation of the ovaries
  2. Egg collection
  3. Insemination
  4. Fertilisation
  5. Embryo biopsy
  6. Embryo testing
  7. Embryo transfer
  8. Pregnancy test
    - Following hyperstimulation of the ovaries, many eggs are hopefully removed.
    - Each egg is surrounded by sperm to allow fertilisation.
34
Q

What is intracytoplasmic sperm injection (ICSI)?

A

-single sperm injected into the centre of each egg
- used for conditions caused by a single faulty gene to reduce the amount of non-embryo DNA (including sperm DNA) which could make the risk
of a wrong diagnosis higher.

35
Q

When do you use PGD?

A

-Our patients use PGD for many genetic disorders;
Translocation carriers
HD
DMD – only implant female embryos (where mutation in family in unknown)
CF
BRACA 1/2
-PGD is now nationally funded
-A licence is required from the HFEA for each genetic condition or indication

36
Q

What is the eligibility criteria for PGD?

A
  1. Female partner is under age 39
    2 . Female partner has a BMI of 19-30
  2. Both partners are non-smokers
    4 .Couple are living together in a stable relationship
  3. No living unaffected children from the relationship
  4. Known risk of having a child affected by a ‘serious’ genetic condition (at least 10%)
  5. Female partner has hormone levels that suggest she will respond to treatment
  6. An accurate genetic test is available
  7. No welfare concerns for the unborn child
  8. A licence is required from the HFEA for each genetic condition or indication
  9. Eligible couples are usually funded for three rounds of PGD.
37
Q

Why is PGD not easy?

A

-PGD not always an easy option:
Emotional and physical implications
Can be lengthy process
Success rates ~30% per cycle; ~40% per embryo transfer

38
Q

What is the role of GC in prenatal testing?

A
  1. Arrange & explain CVS, amniocentesis, PGD, cffDNA
  2. Facilitate decision-making
  3. Give results
  4. See patients in clinic following a diagnosis in utero
  5. Arrange termination if necessary
  6. Discuss recurrence risks and plans for future pregnancies
39
Q

What does facilitating decision making entail?

A

In the context of:

  • Previous experience
  • Family situation
  • Religion
  • Personal beliefs
  • Psychosocial situation
  • Balancing miscarriage risk with genetic risk
  • Dealing with indecision
  • Couples do not always agree

Year 1 POM (39 decks)

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