Prenatal Screening Flashcards

1
Q

What are some differences between prenatal screening & diagnostic testing

A

Screening determines chance, negative is low risk but not zero, positive is increased chance, identifies patients to offer additional testing to, widely available & affordable
Dx testing provides or rules out diagnosis, negative usually rules out, positive provides diagnosis, risk associated with procedure, usually more costly

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2
Q

What are prenatal screening options

A

MSS, carrier screening, cfDNA, routine blood work, US, MRI, fetal echo

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3
Q

Where is AFP made, how can it be detected, & what is it associated with

A

Fetal liver & yolk sac, detected in MSS or amniocentesis, high levels are associated with neural tube defects & abdominal wall defects

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4
Q

What trimester is AFP measured in

A

Second trimester

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5
Q

What is the cutoff for at risk AFP levels

A

Greater or equal than 2.5 is high risk for NTDS

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6
Q

What factors can affect AFP

A

Incorrect dating, twins, increased maternal weight, ethnicity, diabetes, other birth defects

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7
Q

When is a first trimester screen offered, what does it screen for, and what does it measure

A

10-13.6 weeks, trisomy 21,18,13, nuchal translucency, crown rump length, nasal bone, PAPP-A, bHCG,

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8
Q

What is the cutoff value for nuchal translucency, & what are the associated risks

A

Greater than 3 mm, trisomy 21, trisomy 18, Turner syndrome, noonan syndrome, congenital heart defects

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9
Q

When is second trimester screening offered, what does it measure, & what does it screen for

A

15-21.6 weeks, bHCG, AFP, uE3, IA, trisomy 21, 18, neural tube defects & abdominal wall defects

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10
Q

What factors can affect second trimester screening

A

Gestational age, maternal diabetes, multiples, family history of Down syndrome or NTDS, correct age & weight

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11
Q

What is uE3 associated with

A

Smith Lemli opitz, x linked icthyosis

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12
Q

What screening results would indicate trisomy 21 in the first & second trimesters

A

First: high NT, low PAPPA high BhCG
Second: low AFP, high BhCG, low uE3, high iA

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13
Q

What is the screening results would indicate trisomy 18 in the first & second trimester

A

First: high NT, low PAPPA, low bHCG
Second: low AFP, low BhCG,, low uE3, low iA

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14
Q

What is the serum marker patter for trisomy 13

A

Low bHCG, low PAPP-A

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15
Q

What should be done next when a prenatal screen is abnormal

A

Targeted US, confirm gestational age, offer diagnostic testing

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16
Q

When can can NIPT be offered

A

As early at nine weeks for some labs, screening after 12 weeks most helpful

17
Q

What can NIPT screen for

A

Trisomy 18, 22, 13, sex chromosome aneuploidies, some micro deletions, some companies do single gene

18
Q

Where does fetal DNA come from

A

Placental cells undergoing apoptosis

19
Q

What kind of maternal results can NIPT give

A

Sex chromosome aneuploidy, 22q, somatic cell variation, maternal malignancy

20
Q

What are the two methods for NIPT

A

Counting method - massive parallel shotgun sequencing or targeted sequencing, or SNP (Natera only)

21
Q

How does the counting method work

A

MPSS sequences entire genome, targeted amplifies regions of interest, chromosome origins determined for each fragment, then compared to reference genome, quantitative amount compared to reference to give ratio

22
Q

What are benefits of the counting method for NIPT

A

Targeted is less expensive, faster results than SNP, less reliant on fetal fraction

23
Q

What are downsides of the counting method

A

Can’t differentiate between fetal & maternal DNA

24
Q

How does the SNP based method for NIPT work

A

Targeted sequencing of SNPS, selected based on reference genome, sequence data analyzed for allelic measurement, put through bioinformatics

25
What are benefits of the SNP based method
Can differentiate between maternal & fetal dna, can screen for triploidy & vanishing twins, can provide separate fetal sexes in twin pregnancies
26
What are downsides of the SNP based method
More reliant on fetal fraction, can take longer for results
27
What are possible result types for NIPT
Low risk negative screen, High risk positive screen, no results due to lab errors/failure in quality metrics/ low fetal fraction, partial results - pending microdeletions, no call on Y analysis or fetal sex result
28
What characteristics would indicate someone is not a candidate for NIPT
auto immune conditions - can cause fetal dna to degrade, cancer, very high BMI