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Routine Antenatal Care > Early Pregnancy Screening > Flashcards

Early Pregnancy Screening Flashcards

1
Q

What is the most common aneuploidy seen in live born infants?

A

Trisomy 21

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2
Q

What is the background risk of T21 if the maternal age is

35?

40?

A

35 - 1:300

45: 1:100

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3
Q

At what gestation should combined first trimester screening (CFTS) be undertaken?

A

11+0 - 13+6/40

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4
Q

What four factors are used to calculate risks for CFTS

A

Maternal age
USS measurement of NT
PaPP-A (Pregnancy associated plasma protein-A)
B-HCG

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5
Q

What is associated with low-PaPP-A, increasing false positives?

A

Assisted Reproductive Technology

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6
Q

At what gestation can NIPT / cell-free DNA testing be performed?

A

Any gestation from 10/40

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7
Q

What is the risk of test failure of NIPT for IVF compared to spontaneously conceived pregnancies?

A

IVF: 5.2%

Spontaneously conceived: 2.2%

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8
Q

At what gestation can MSS2 be performed?

A

15-20/40

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9
Q

At what gestation can amniocentesis be performed from?

A

15/40

If performed before, increases risk of adverse outcomes like talipes

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10
Q

What is the sensitivity and specificity of MSS1?

A

Sensitivity = 85%

Specificity = 95%

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11
Q

What is the sensitivity and specificity of MSS2?

A

Sensitivity = 70-75%

Specificity = 93%

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12
Q

What is the sensitivity and specificity of NIPT?

A

Sensitivity = 99%

Specificity = 99%

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13
Q

At what gestation can CVS be performed from?

A

11/40

Before this gestation, CVS is associated with an increased risk of transverse limb reduction defects

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14
Q

What PAPP-A level is defined as low?

A

<0.4 MoM (5th centile) on first trimester screen

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15
Q

What adverse outcomes is a low PAPP-A associated with

With normal chromosomes

A 
Stillbirth
Infant death
IUGR
PTB
PET
Pregnancy loss <20/40
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16
Q

What is an abnormal NT result?

A

=3.5mm (95th centile)

17
Q

What is an abnormal NT result associated with

With normal chromosomes

A

Miscarriage
IUFD
Specific structural abnormalities, especially cardiac
Genetic syndromes

Should be referred for tertiary anatomy scan (regardless of MSS result)

18
Q

What is PAPP-A

A

Pregnancy associated plasma protein A
Large glycoprotein produced by the placenta and decidua

Thought to have several functions including

  • prevention of recognition of the fetus by the maternal immune system
  • matrix mineralisation
  • angiogenesis
19
Q

What is genetic carrier screening?

A

The detection of carrier status of autosomal and X-linked recessive diseases in couples or people who do NOT have an a priori increased chance of being a carrier based on their or their partners’ personal or family history

20
Q

Who should genetic carrier testing be offered to?

As per RANZCOG

A

Everyone

Not publicly funded, costs $400
Inequitable

21
Q

What conditions are tested for in Genetic Carrier Screening?

A

Thalassaemia
Cystic fibrosis
Spinal muscular dystrophy
Fragile X syndrome

22
Q

What are the two options for Genetic Carrier Screening?

A
  1. Sequential
    - cheaper. If normal (majority), no need for partner to get tested
    - gives information on individual, which can inform cascade testing
  2. Couple
    - quicker timeframe
    - probably for couple
    - but do not necessarily get access to individual results
23
Q

Outline your counselling points for antenatal aneuploidy screening:

A
  • MSS screens for Down syndrome, Edward and Patau syndrome.
  • MSS-1 and MSS-2 funded but not compulsory.
  • Detects 75% of of total range of prenatal testable conditions. Negative testing does not guarantee a completely normal baby.
  • Screening gives parents options to terminate pregnancy if significant abnormality or make preparations for baby.
  • High risk screening requires further invasive testing to confirm diagnosis of an abnormality. Invasive testing has some inherent risks.
  • Process may reveal other anomalies not expected.
24
Q

List the antenatal screening tests available including parameters tested and testing gestation etc.

A

Combined first trimester screening:

  • Parameters: NT, maternal age, BhCG, PAPP-A
  • Gestation 11+0 to 13+6

MSS-2:

  • Parameters: maternal age, alpha-fetoprotein, oestriol, BhCG, inhibin
  • Gestation: 15+0 to 20+0 weeks

NIPT/cell-free fetal DNA:
- Gestation: from 10 weeks.

25
Q

List the confounding factors for CFTS:

A
  • IVF conception
  • Maternal weight
  • Smoking
26
Q

What is the fetal loss rates for CVS and amniocentesis?

A

0.5-1% but does not vary between procedures in meta-analysis with experienced operators.

27
Q

Outline the issues with invasive diagnostic testing:

A
  • Can have maternal cell contamination
  • Failure of cell culture to grow
  • Rapid FISH analysis can only test limited number of chromosomes, not full karyotype.
  • Some karyotype abnormalities do not predict the phenotype.
28
Q

What complications are associated with CVS and amniocentesis?

What risks are associated specifically with CVS?

A
  • Bleeding
  • Miscarriage
  • Amniotic fluid loss
  • Infection
  • Rh sensitisation.

CVS-specific risks:

  • Technical failure
  • Risk of mosaicism.
29
Q

What are the relative contraindications for amniocentesis?

A
  • Active PV bleeding
  • Unfused amniotic chorionic membranes
  • Intercurrent maternal febrile illness (defer for 1 week).
30
Q

List the three major ways chromosomes can be assessed:

A
  • Conventional (G-banded) karyotype
  • Rapid aneuploidy test e.g. FISH analysis
  • Chromosomal microarray analysis
31
Q

Can MSS and cfDNA be used in triplets and higher order pregnancies?

A

No

32
Q

What is the altered performance of screening tests in twin pregnancies?

A

CFTS sensitivity 72-80%; improved to 89% when assessing nasal bones.

cfDNA: increased failure rate and less performance data. Higher unreportable rate >5%

33
Q

What are benefits and limitations of CFTS (compared with NIPT)?

A

Benefits:
- Fully funded

Limitations:

  • Positive predictive value 7-10%
  • Limited gestation of use 11+0 to 13+6
  • Lower sensitivity (85%) and specificity (95%)
34
Q

What are benefits and limitations of NIPT (compared with CFTS)?

A

Benefits:

  • High sensitivity 99% and specificity 99%
  • Positive predictive value 45%
  • A low risk test reduces need for invasive diagnostic testing and fetal loss.
  • Assesses sex chromosome abnormalities, fetal sex.

Limitations:

  • Not funded and expensive $500-$1000.
  • Does not provide information on structural abnormalities.
35
Q

Explain the underlying principles of the currently available non-invasive prenatal tests (NIPT) used to detect Trisomy 21:
(5 marks)

A

• Cell-free DNA testing involves taking a maternal blood sample and collecting maternal and fetal DNA fragments (fetal DNA fragments are placental in origin) and comparing them against a reference genome to determine its chromosome of origin and then counting the number of DNA fragments from each chromosome.
• Done from 10 weeks as needs sufficient fetal DNA fragments in maternal plasma to analyse.
• 99% sensitivity and specificity for Trisomy-21. An appropriate screening tool particularly for trisomy 21
• Is not a diagnostic test; if she has an abnormal test result she should be offered diagnostic testing.
Can assess risk of autosomal aneuploidy, sex chromosome aneuploidy and some other conditions and fetal sex

36
Q

Discuss the epidemiology of having a baby with cystic fibrosis (CF) in Australia and New Zealand (5 marks):

A
  • Most common life-threatening inherited disorder in NZ.
  • CF affects 1 in 3,500 newborns.
  • CF most common amongst Northern Europeans and Ashkenazi Jews. Carrier status incidence 1:25.
  • Less common amongst Africans, Maori and Pacific Islanders.
37
Q

Discuss the genetic basis of cystic fibrosis:

A
  • Cystic fibrosis results from an abnormality in the cystic fibrosis transmembrane conductance regulator protein (CFTR). The CFTR gene is a single large gene encoded on chromosome 7.
  • Inheritance is autosomal recessive i.e. need two CFTR gene defects. Can be homozygous (same CFTR gene defect) or heterozygous (different CFTR gene defects) with variable disease phenotype.
  • Most common mutation type is F508del-CTR (75%).
  • A couple who are both carriers of CFTR gene mutation have a 1 in 4 chance of having a child with cystic fibrosis.
38
Q

A Caucasian couple with a 2 year old child with CF are referred to you by their general practitioner for pre-pregnancy planning. BOTH parents are carriers with an identified CF mutation.

List 4 screening or treatment options available for this couple to avoid having another child affected with CF and provide 1 limitation for each:

A
  1. Preimplantation genetic diagnosis:
    - Limitation: requires IVF.
  2. Non-carrier donor gamete:
    - Limitation: donor may have unidentified gene mutation.
  3. Antenatal amniocentesis:
    - Limitation: risk of miscarriage; termination if CF affected.
  4. cell-free fetal DNA (cfDNA):
    - Limitation: expensive ($500-$1000) and not funded; still requires invasive diagnostic testing if abnormal.

Routine Antenatal Care (5 decks)

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