Pitfalls of Prenatal Diagnosis Associated With Mosaicism TOG

DOI: 10.1111/tog.
12850 2023;25:28–37
The Obstetrician & Gynaecologist
Review
http://onlinetog.org
Pitfalls of prenatal diagnosis associated with mosaicism

Kelly Reilly MBBChBAO MRCOG DipCE,a Samantha Doyle MD MRCPI MSc,b,c Susan J Hamilton BSC Genetics DpiRCPath,d
Mark D Kilby DSc MB BS MD FRCP I FRCOG,e,f Fionnuala Mone PhD MSc (Genomics) MRCOG FRCPI PGCEg*
a
ST6 Obstetrics and Gynaecology, and Clinical Research Fellow, Centre for Public Health, Queen’s University Belfast, Institute of Clinical Science
Block A, Royal Victoria Hospital, Grosvenor Road, Belfast BT12 6BA, UK
b
Consultant in Clinical Genetics and Genomics, National Maternity Hospital, Holles Street, Dublin 2, Ireland
c
Consultant in Clinical Genetics, National Maternity Hospital, Holles Street, Dublin 2, Ireland
d
Principal Genomic Scientist, West Midlands Regional Genetics Laboratory, Birmingham Women’s and Children’s NHS Foundation Trust,
Birmingham BT15 2TG, UK
e
Dame Hilda Lloyd Professor of Fetal Medicine, Institute of Metabolism and Systems Research, College of Medical and Dental Sciences, University
of Birmingham, Edgbaston, Birmingham, UK
f
Fetal Medicine Centre, Birmingham Women’s and Children’s Foundation Trust, Birmingham, B15 2TG, UK
g
Clinical Lecturer in Maternal Fetal Medicine, Centre for Public Health, Queen’s University Belfast, Institute of Clinical Science, BT12 6BA, Belfast,
Block A, Royal Victoria Hospital, UK
*Correspondence: Fionnuala Mone. Email: f.mone@qub.ac.uk
Accepted on 28 June 2022. Published online 20 December 2022
Key content To appreciate underlying principles in NIPT and genomic testing

Fetal placental mosaicism, of which confined placental mosaicism strategies in relation to mosaicism.
is a subtype, occurs in 2–3% of pregnancies. To follow suggested clinical management principles in relation to
Confined placental mosaicism may lead to a false positive result on prenatal test counselling.
non-invasive prenatal testing (NIPT) for common aneuploidies.
Ethical issues
The risk of mosaicism in a chorionic villus sample (CVS) following
Clinicians face a dilemma following a high-risk NIPT result in the
a positive NIPT result is 2, 4, 22 and 59% for trisomy 21, 18, 13
setting of normal ultrasound. Awaiting long-term culture, as
and 45, X respectively.
opposed to short-term culture on CVS, or amniocentesis delays
Following a positive NIPT result in the absence of a significant fetal
potential termination of pregnancy.
structural anomaly (FSA), care is required in selecting the optimal
Sex chromosome abnormalities on NIPT without an identifiable
diagnostic invasive test. Discussion of the limitations and
FSA cannot be interpreted reliably. Hence, NIPT should not be
implications is essential and referral to clinical genetics may
offered for sex chromosome aneuploidy.
be warranted.
Keywords: aneuploidy / chorionic villus sampling / confined
Learning objectives

placental mosaicism / fetal placental mosaicism / non-invasive
To understand the embryological causes for and types of fetal
prenatal testing
placental mosaicism.
Please cite this paper as: Reilly K, Doyle S, Hamilton SJ, Kilby MD, Mone F. Pitfalls of prenatal diagnosis associated with mosaicism. The Obstetrician &
Gynaecologist 2023;25:28–37. https://doi.org/10.1111/tog.12850
trisomy 21, 18 or 13 on first trimester combined screening.3

Introduction
cffDNA analyses the fetal fraction of DNA in maternal blood,
Non-invasive prenatal testing (NIPT), also known as cell-free derived from apoptosis of the external layer of the placenta,
fetal DNA (cffDNA) testing, has transformed prenatal or trophoblast.4,5 When used as part of universal screening,
screening internationally for common chromosomal interpretation of NIPT in the setting of a normal first
anomalies and has considerably reduced the need for trimester anatomy ultrasound may be challenging.
invasive testing.1 However, some have described it as a Different methods exist for reporting the accuracy of NIPT,
‘disruptive technology’.2 NIPT is currently approved by the depending on the statistical assessment used and the evidence
National Health Service (NHS) Fetal Anomaly Screening evaluated: sensitivity and specificity, positive predictive value
Programme (FASP) in England, Scotland and Wales as a (PPV) and negative predictive value (NPV), and false positive
contingent screening test in those at high risk (defined as a and false negative rates. These may be confusing for clinicians
risk from 1 in 2 to 1 in 150 on combined screening) of providing pre-test and post-test counselling. The PPV is
28 ª 2022 The Authors. The Obstetrician & Gynaecologist published by John Wiley & Sons Ltd on behalf of Royal College of Obstetricians and Gynaecologists.
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium,
provided the original work is properly cited.
17444667, 2023, 1, Downloaded from https://obgyn.onlinelibrary.wiley.com/doi/10.1111/tog.12850 by Cochrane Malta, Wiley Online Library on [13/07/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
Reilly et al.
useful because it accounts for the sensitivity of the test, the and extrafetal compartments that occurs shortly after
pre-test risk and the prevalence of the trisomy in question. fertilisation.9 If it occurs beforehand, it can lead to true
These aspects must be considered when counselling patients, fetal mosaicism (TFM), where the abnormal cell line is
because the accuracy is less optimal in those groups deemed present in placenta and fetus or fetus alone; or CPM when it
to be low risk by combined screening.6 Other reasons for is isolated to the placenta.4 CPM is the commonest type of
reduced accuracy include maternal chromosome mosaicism, mosaicism; extrafetal (i.e., placental) tissues undergo more
inadequate (low) fetal fraction, maternal malignancy and mitotic divisions and therefore are at greater risk of
vanishing twin (caused by silent twin miscarriage). NDJ errors.4
Persistently inadequate fetal fraction (<4%) at second blood Mosaicism (encompassing CPM and TFM) occurs in
draw is also associated with an increased risk of trisomy 13 1–2% of chorionic villus samples. The incidence of CPM
and 18, as well as assisted reproductive techniques and from chorionic villus sampling (CVS) is 0.02% based on
elevated maternal body mass index.4 The commonest reason current laboratory processing procedures.14 The incidence
for a retrospectively false positive NIPT result is secondary to of CPM increases with advancing gestation, most notable
confined placental mosaicism (CPM), which is a subtype of beyond 20 weeks, potentially explained by somatic
chromosomal mosaicism.7 mosaicism. For this reason, beyond 15 weeks, amniocentesis
is the recommended invasive test.15 When analysing a CVS
sample, both layers of the placenta are normally reviewed.
The biological basis of chromosomal
This involves the cytotrophoblast, originating from the
mosaicism
trophoblast (external placental layer), and the mesenchymal
Chromosomal mosaicism occurs when two or more cell lines core, which is more representative of the fetal karyotype
exhibiting different karyotypes (chromosome complements) because it has a similar embryological origin.1,4,16 CPM can
are detected in a single embryo.8,9 This is associated with be further classified (Table 1). CPM Type 1 is typically only
either a non-disjunction (NDJ) error (that is, failure of sister evident on short-term culture (STC), CPM Type 2 on long-
chromatids to separate correctly) during mitotic cell division, term culture (LTC) and CPM Type 3 in both LTC and STC.4
or during meiosis with an NDJ error followed by a CPM 1 and 2 are the commonest types.17 By performing a
postzygotic mitotic trisomic rescue.7–9 The former is the CVS following a high-risk NIPT result, CPM types 1 and 3
primary mechanism for chromosomal mosaicism. This may be detected, which can lead to a false positive result
occurs when an initially normal diploid embryo with 46 (Table 1). This phenomenon is commoner for trisomy 13, 18
chromosomes undergoes an error during mitosis, creating a and the sex chromosomes.4 TFM is mainly identified via
trisomic cell line and a monosomic cell line, as well as the second trimester amniocentesis to identify fetal cells and can
unaffected cells having a normal chromosome complement. also be further classified.4 For all TFM types, the fetus is
In the case of the 44 autosomes, the monosomic cell line is abnormal, versus CPM where the fetus is normal (Table 1).
naturally disadvantaged and discontinues, leaving the CPM alone is associated with an increased risk of FGR and
trisomic and normal cell lines. In NDJ involving the X fetal loss, warranting regular fetal growth assessment
chromosome, however, all cell lines tend to in pregnancy.18,19
continue (Figure 1).8-10
The second mechanism is when a meiotic NDJ error
Prenatal aneuploidy testing methods
occurs followed by a postzygotic mitotic trisomy rescue via
anaphase lag where a diploid complement is restored.10,11 Initially, genetic analysis of prenatal samples is typically
Depending on the chromosome lost, this could lead to performed via quantitative fluorescence polymerase chain
biparental (one maternal and one paternal chromosome) or reaction (QF-PCR).20 This requires a small amount of DNA,
uniparental disomy (both chromosomes from the one which is amplified and displayed as peaks via an automated
parent).8,12 Uncommonly, uniparental disomy (UPD) may method. A normal diploid sample will typically reveal two
have a phenotypic effect on the fetus, leading to imprinting peaks in a 1:1 ratio for each chromosome analysed, whereas
disorders, a greater incidence of autosomal recessive trisomy may present with three peaks (1:1:1 ratio), a 2:1 ratio
disorders and fetal growth restriction (FGR).8,12 peak or rarely a homozygous pattern with a single peak.21
The rapid turnaround time (24–48 hours) and automation of
QF-PCR means it has now taken over the STC aspect of
Subtypes of fetal placental mosaicism
analysis of CVS samples.22 Owing to the reporting of
An abnormal cell line can be present in both the fetus and discrepancies between LTC karyotypes and QF-PCR results,
placenta, only in the fetus, or only in the placenta (Figure 2). UK-wide sample preparation has been streamlined so that the
This depends on the timing of the NDJ error and whether it sample used is representative of the biopsy and that the
occurs after or before the embryological separation of fetal mesenchymal core, in particular, is present in adequate
ª 2022 The Authors. The Obstetrician & Gynaecologist published by John Wiley & Sons Ltd on behalf of Royal College of Obstetricians and Gynaecologists. 29
Confined placental mosaicism
(a) (b) (c)
Mitotic non Mitotic non Meiotic non

disjunction disjunction (sex disjunction
(autosomes) chromosomes)
NON DISJUNCTION
MEIOSIS
(GAMETES)
46, N 46, XX 47,+chr
ZYGOTE
TRISOMY RESCUE
NON DISJUNCTION NON DISJUNCTION
/ANAPHASE LAG
47,+chr 45,-chr 47,XXX 45,X
47,+chr 46,N
MITOSIS
46,N 46,XX
Present Present Present

MOSAIC 46,N/47,+chr
46,N/47,+chr 46,XX/47,XXX/45,X
Figure 1. Mechanisms of chromosomal mosaicism. Reproduced from Grati (2014)8 with permission. (a) mitotic non disjunction error involving an
autosome (mosaic 46,N/47,+chr); (b) mitotic non disjunction error involving a sex chromosome (mosaic 45,X/47,XXX/46,XX); (c) meiotic non-
disjunction error followed by trisomy rescue (mosaic 46,N/47,+chr).
amounts in the specimen.14,17,23–26 QF-PCR may reveal structural anomaly (FSA) and the absence of aneuploidy on
suspicion of CPM through the presence of abnormal biallelic QF-PCR, a chromosome microarray should be the next
trisomy, in which case the laboratory report may be returned investigation performed.23
recommending a full culture.8,27 The gold standard testing
strategies for genetic CVS analysis are to analyse both the
False positive and negative NIPT results
cytotrophoblast and the mesenchyme via QF-PCR and LTC/
karyotype (mesenchymal core). However, notably, QF-PCR Autosomal aneuploidy
is considered a standalone diagnostic test for most Rates of false positive and negative (retrospectively
aneuploidy cases; specifically, in the presence of a 1:1:1 identified) results in respective aneuploidies are
marker pattern indicating trisomy of meiotic origin (that is, demonstrated in Table 2. These show wide variation,
the chromosomal condition was present at conception and dependent on (i) the specific chromosome affected, (ii) the
reflects the fetal genotype) (Figure 3).27 In the case of a fetal woman’s pre-test risk based upon first trimester screening
Reilly et al.
Complete Confined placental

fetal-placental concordance mosaicism
Fetal-placental Non-mosaic fetus, Fetal mosaicism,

mosaicism mosaic placenta non-mosaic placenta
Fetal mosaicism, Complete

normal placenta fetal-placental discordance
Figure 2. Types of fetal placental mosaicism. Reproduced with permission from Gardner, Sutherland and Shaffer (2011).13 Green indicates an
aneuploid cell line while white represents euploid.
represent the cytotrophoblast, risking a persistently false

Table 1. Different types of confined placental mosaicism/true fetal
mosaicism and the associated chorionic villus sampling and non- positive result if CPM is present. The implications of this will
invasive prenatal testing results be discussed (Figure 4).17 Where there is no clear evidence of
a meiotic origin to a positive result, mesenchymal core
Type Cytotrophoblast Mesenchyme Fetus NIPT result
culture and testing results should be awaited to
confirm karyotype.
CPM 1 Abnormal Normal Normal False positive
Sex chromosome aneuploidy
CPM 2 Normal Abnormal Normal Normal Monosomy X (45,X), Klinefelter syndrome (47,XXY or 48,
CPM 3 Abnormal Abnormal Normal False positive XXYY), Triple X syndrome (47,XXX) and 47,XYY are
classified as sex chromosome aneuploidies (SCA).30 With
TFM 4 Abnormal Normal Abnormal Abnormal NIPT, the detection rates for SCA aneuploidies are less
accurate than for autosomal aneuploidies.31,32 This is
TFM 5 Normal Abnormal Abnormal False negative
consistent with the low PPV of cffDNA for Monosomy X,
TFM 6 Abnormal Abnormal Abnormal Abnormal which has false positive NIPT rates as high as 91%.4,33 Factors
contributing to false SCA NIPT results include previously
CPM = combined placental mosaicism; NIPT = non-invasive prenatal undiagnosed maternal SCA associated with a normal
testing; TFM = true fetal mosaicism. Adapted and reproduced from phenotype, and mosaicism.34
Grati (2014)8 with permission.
Proposed prenatal management

(statistically a low risk of aneuploidy on first trimester Existing guidance and choice of confirmatory
screening confers a greater risk of false result on NIPT) and; invasive test
(iii) the presence or absence of a FSA.6,17,27 Reasons for false Current FASP recommendations for women with a high-risk
positive and negative NIPT results are demonstrated in combined or quadruple test include (i) no testing; (ii)
Box 1.28,29 NIPT is based upon ‘free fetal DNA’ released into invasive testing or (iii) additional screening with NIPT.27
the maternal circulation and derived from the placental Henceforth, women with a high-risk NIPT result may be
cytotrophoblast. UK best practice involves QF-PCR testing of offered (i) no testing or (ii) invasive testing. The challenge of
both cell lineages in a CVS, however it may still only which pathway for selection of the invasive test depends on
D21S11 D13S628 D13S634 D18S535

240 320 400 480
D13S742 D18S1002 D13S305

240 320 400 480
Figure 3. A QF-PCR electropherogram demonstrating trisomy 18. Three spikes are detected by probe sets D18S535 and D18S1002 (triallelic)
indicating trisomy 18.
Reilly et al.
Table 2. Performance of non-invasive prenatal testing for aneuploidy and low and high-risk pre-test screening groups
Risk of
mosaicism Detected
in a CVS mosaicism
False False after a high on CVS + fetal
negative positive Sensitivity Specificity PPV risk NIPT confirmation
Aneuploidy Risk rate rate (%) (%) (95% CI) result (%) by amniocentesis (%)
T21 High risk 1/135 1/7372 >99.9 99.8 94.1 (92.3–95.9) 2 44

Low risk 81.3 (77.3–88.4)
T18 High risk 1/64 1/7472 97.4 99.6 89 4 14

Low risk 68
T13 High risk 1/136 1/1754 87.5 >99 85 22 4

Low risk 61
MX (45,X) High risk 1/14 1/1421 95 99 39 59 26

Low risk 26
CPM = confined placental mosaicism; CVS = chorionic villus sampling; MX = monosomy X; NIPT = non-invasive prenatal testing; PPV = positive
predictive value.
DNA with a triallelic marker pattern on the

Box 1. Reasons for false positive and negative non-invasive
prenatal testing (NIPT) results electropherogram is considered diagnostic, although this
depends on which cell lines are assessed, and LTC with
Reasons for false positive tests karyotype is confirmatory. Trisomy 13 and 18 are different
Confined placental mosaicism because of the higher risk of CPM. Hence, in instances where
Maternal chromosome mosaicism no FSA is identified, careful counselling and documentation
Inadequate fetal fraction must be performed by the clinician. Advice from clinical
Maternal malignancy
Vanishing twin
genetics may be to offer (i) CVS with QF-PCR and LTC/
karyotype analysis performed in line with current guidance38
Reasons for false negative tests
or (ii) amniocentesis. This counselling should include the
Mosaicism risks and benefits of both the procedure and the risk of CPM,
Early gestational age of sample how to interpret the results, and the importance of the
Inadequate fetal fraction- increased maternal body mass index
Prolonged storage of samples
correct interpretation of these results.
Amniocentesis assesses DNA of truly fetal origin
(amniocytes), as opposed to CVS which assesses placental
cells (although fetal-placental mosaicism is possible but rare).
(a) the type of aneuploidy (i.e., trisomy 21, 13 or 18) and (b) NIPT testing is available from as early as 9 weeks of gestation
the presence of identifiable FSAs. In trisomy 21, first- and as part of the FASP evaluative roll out, up to 21 weeks
trimester screening for assessment of nuchal translucency and 6 days.39 Amniocentesis is not normally performed until
with secondary ultrasound markers has a sensitivity as high as after 15 weeks of gestation, with CVS performed from
91.7% for a false positive rate of 3%35, which is similar to 11 weeks of gestation. Sometimes, insufficient DNA can be
trisomy 13 and 18. Hence, clinicians must exert extreme obtained from amniocentesis at 15 weeks, so it is suggested it
caution in instances of a high-risk NIPT result and no be performed at 16 weeks of gestation in preference.40 This
identifiable FSA.36 Currently, first trimester detailed anatomy aforementioned scenario is highly complex and poses a
scanning is not part of the FASP screening programme, so scientific and clinical dilemma because the implications of
detection of anomalies in the first trimester is limited. making a false diagnosis, although unlikely, are considerable.
Continuing work by the National Congenital Anomaly and In such instances, liaison with the scientific staff in the
Rare Disease Registration Service collects data and quality genetics laboratory and clinical genetics service is advisable
assurance for detection of congenital anomalies and their and management may differ on a case-by-case basis. A
prevalence.37 For trisomy 21, QF-PCR analysis from fetal further challenge occurs in the instance of twin pregnancy
testing, and both informed counselling and full written

consent should be obtained. In the absence of an FSA,
patients must also be advised of the standard prenatal
screening pathway and options available to them, as well as
the pros and cons of each test. In relation to NIPT, as well as
false positive and negative results, the possibility of need for
re-draw or inability to obtain a result should also be outlined
and documented. With a positive NIPT result, post-test
counselling should be conducted by a fetal medicine specialist
with the chromosome, procedure-specific risks outlined to
the patient pertaining to risk with invasive testing (Table 2).
Laboratory reporting
Recent guidance recommends that genetic laboratory reports
should include (i) a full and clear interpretation of the test
results, taking into account any appropriate information
provided; (ii) and any further tests that may be indicated to
improve the accuracy of the interpretation. Furthermore, (iii)
Normal Abnormal Falsely abnormal
support should be available via/from clinical genetics where
fetus cytotrophoblast cfDNA
required.42 Clinical guidance is provided on laboratory
fragments in reports to alert the clinician to the risk of the potential for
maternal false positive and negative results associated with mosaicism.
Figure 4. Illustration portraying how false positive results occur in In CVS analysis, if the clinical scientist has identified a
non-invasive prenatal testing in confined placental mosaicism Types 1 potential for a false positive abnormal result (caused by a
and 3. Adapted and reproduced from Grati (2016)4 with permission. marker ratio pattern suggestive of a postzygotic mitotic
error), they should specifically inform the clinician of the
increased risk of CPM.23 In this instance, it is advised to
where there is an FSA and a high-risk NIPT result. Here, if await further karyotypic analysis by culture or, indeed, await
the patient requests selective reduction, most notably in an amniocentesis.23 Ideally, in suspected CPM or following a
monochorionic twin pregnancy where risk of demise or QF-PCR result that is not straightforward, it may be
impairment to the non-anomalous fetus (which although advisable for the obstetrician to liaise with a fetal medicine
rare, can be heterokaryotypic)41 is high, then an invasive specialist, laboratory scientist and, if required, clinical
diagnostic test of both fetuses should be considered in the geneticist in a multidisciplinary team setting such as a fetal
first instance, unless the FSA is so severe that grounds of medicine genetic clinic. The implications of acting upon a
Clause E of the Abortion Act are clearly met without such false positive result are considerable. Clear and directive
additional information. reporting by the laboratory of the test findings is thus
Updated best practice guidance for prenatal testing, mandatory so that clinical misinterpretation can be
interpretation and counselling is due to be published by the minimalised. Ideally, this is agreed nationally to avoid
Royal College of Obstetricians and Gynaecologists (RCOG) regional variability.
in 2022. Guidance regarding management of monochorionic
twin pregnancy and prenatal molecular and cytogenetic test
evaluation is also forthcoming from the Association for
Ethical implications
Clinical Genomic Science. The National Institute for Health and Care Excellence
(NICE) stipulates that abortion services should be available
Prenatal counselling to women who require them without delay – ideally
Patients must receive robust pre-test and post-test providing a termination of pregnancy (TOP) within 1 week
counselling regarding the risk of CPM and false positive of assessment.43 Where there is a potential false positive
findings by an appropriately trained clinician. Potential NIPT result, clinicians face a dilemma for women requesting
longer turnaround time to await LTC – or even TOP, which may have to be delayed by some weeks to permit
amniocentesis – must be discussed from the outset to appropriate testing to determine if the grounds for Clause E
manage expectations. Clinicians offering NIPT should of the Abortion Act are met, most notably in the absence of
remain up to date regarding the challenges surrounding an identifiable FSA. On the contrary, if TOP is performed
Reilly et al.
when there is a truly false positive result, the implications of midtrimester scan.27,39 Evidence regarding the optimal
this are more significant for the woman; hence, delaying screening strategy is conflicting. The American College of
pending further investigations is ethically the optimal Obstetricians and Gynecologists and Society for Maternal–
pathway. The woman can still decline the clinician’s advice Fetal Medicine both recommend that all women are offered
if further testing is required and proceed with TOP if under screening and testing for chromosomal abnormalities in the
24 weeks of gestation; however, this would fall under form of cffDNA, regardless of the level of risk.49 Based upon
alternative grounds, not Clause E, and would still require the principles of screening, for a test to be deemed suitable it
the support of two clinicians to proceed. Where there is no must fulfil the criteria as set out by Wilson and Jungner.50 A
FSA and trisomy 21 is suspected, clinicians may struggle to screening test is judged suitable and appropriate by many
ethically support TOP under Clause E because of the wide markers including sensitivity, specificity, PPV and NPV.
phenotypic spectrum of Down syndrome – particularly if NIPT has been shown to be more sensitive and specific for
there is a greater risk of fetal mosaicism where the phenotype trisomies 21, 18 and 13 in high-risk populations.51 Owing to
expressed may be very mild, if evident whatsoever.44 In this the generalised low prevalence of aneuploidy and risk of
instance, TOP may be supported under Clause C. biological phenomena such as CPM associated with the
A further ethical challenge posed by abnormal NIPT results chance of false positive or negative NIPT results, modelling
that do not fit with the fetal phenotype, is the potential for suggests that use of NIPT as a first line test rather than
underlying maternal chromosomal mosaicism and maternal contingent screening following standard first trimester
malignancy.7 Therein lies a further dilemma for the clinician: screening could lead to more unnecessary invasive testing,
do they practise ‘nonmaleficence’ and not break bad news to a with associated loss of a euploid fetus.52 In addition, a
woman (at a notably vulnerable time in life) regarding universal NIPT approach would probably lead to
potential malignancy based upon a test that is not designed considerably more challenging cases for clinicians in
to screen for it, or ‘beneficence’, and open the doors to further relation to counselling as to the optimal invasive testing
investigation and potential treatments.45 If previously strategy. A further issue regarding a universal NIPT screening
unknown maternal chromosomal mosaicism is detected, this approach is how pre-eclampsia screening might fit in, as this
can have profound effects on not only the woman’s mental would negate the requirement for serum tests for pregnancy
health, but her future health and – potentially – her medical associated plasma protein-A (PAPP-A). Although uterine
insurance.46 Such moral complexities highlight the importance artery Doppler may be a more optimal pre-eclampsia and
of a thorough consenting process by an appropriately FGR screening strategy, PAPP-A may the only available
trained professional.46 screening method within some centres. Hence, by ceasing the
Immunomodulatory drugs (excluding immunoglobulin) combined test, pre-eclampsia and FGR screening with
can lead to false positive NIPT results. If the benefit of initiation of prophylaxis and subsequent monitoring may
holding these drugs prior to testing outweighs the potential be lost.53 In 2019, the UK National Screening Committee
risks to the patient owing to their medical condition recommended a 3-year evaluation of the introduction of
worsening, then there is a consideration to be made for NIPT inclusive of scientific, ethical and user input to assess
halting the mother’s treatment temporarily. Advice from the its impact on the NHS FASP. Pending publication of the
relevant obstetric physician should be prospectively sought.39 evaluation’s findings, it may well become the case that
NIPT testing for the purposes of revealing fetal sex for contingent screening using NIPT moves to universal NIPT,
social reasons is unethical and associated with a high risk of a although this is awaited.48
false positive result for sex aneuploidy, hence this is
not recommended.47
Conclusion
While NIPT is a novel and exciting technology
Future considerations
revolutionising the field of prenatal diagnosis, the potential
Owing to the high sensitivity and specificity of NIPT for for false positive findings secondary to confined placental
common aneuploidies, it may be argued that an appetite is mosaicism carries uncommon but severe implications.
growing for universal first-line NIPT screening. Theoretically, Clinicians must be appropriately trained in offering
the adoption of such a screening programme may reduce the detailed pre-test and post-test counselling to women, with
number of women undergoing invasive testing and hence appropriate onward to fetal medicine specialists, and – in
reduce the miscarriage rate, as well as potentially facilitating some circumstances – clinical genetics, for the most
an earlier diagnosis.48 Currently NIPT is not used as an initial appropriate confirmatory invasive test and genetic analysis.
universal screening programme for the NHS. Women are This must be done before a woman has the opportunity to
offered the option of either the combined (first trimester) or act upon a result, most notably in the absence of an
the quadruple test (second trimester) in addition to a detailed identifiable fetal structural anomaly on ultrasound.
Disclosure of interests 9 Grati FR, Malvestiti F, Branca L, Agrati C, Maggi F, Simoni G. Chromosomal
mosaicism in the fetoplacental unit. Best Pract Res Clin Obstet Gynaecol
MDK is a member of Illumina’s International Perinatal 2017;42:39–52.
Advisory Group (but receives no payment for this) and is the 10 Kalousek DK, Langlois S, Robinson WP, Telenius A, Bernard L, Barrett IJ, et al.
fetal medicine representative for the Central and South Trisomy 7 CVS mosaicism: pregnancy outcome, placental and DNA analysis
in 14 cases. Am J Med Genet 1996;65:348–52.
genomics laboratory hub. He is also the Royal College of 11 Schinzel A, Kotzot D, Brecevic L, Robinson WP, Dutly F, Dauwerse H, et al.
Obstetricians and Gynaecologists’ (RCOG) representative on Trisomy first, translocation second, uniparental disomy and partial trisomy
the Joint Colleges Committee of Genomic and Genetic third: a new mechanism for complex chromosomal aneuploidy. Eur J Hum
Genet 1997;5:308–14.
Medicine and the RCOG Genomic Taskforce. FM and MDK 12 Engel E. Uniparental disomy revisited: The first twelve years. Am J Med
co-authored the recent RCOG Scientific Impact Paper no. 64. Genet 1993;46:670–4.
SH has co-authored the Association for Clinical Genomic 13 Gardner RJM, Sutherland GR, Shaffer LG, eds. Chromosome abnormalities
and genetic counselling. 4th ed. Oxford: Oxford University; 2011.
Science (ACGS) professional guidelines for quantitative 14 Hamilton SJ, Waters JJ. Completely discrepant results between prenatal QF-
fluorescence polymerase chain reaction (QF-PCR) for the PCR rapid aneuploidy testing and cultured cell karyotyping obtained from
diagnosis of aneuploidy best practice guidelines (2018). FM is CVS: Lessons from UK audit and re-audit of 22,221 cases. Prenat Diagn
2012;32:909–11.
editor of genetics for Ultrasound in Obstetrics & Gynecology, 15 Carroll SG, Davies T, Kyle PM, Abdel-Fattah S, Soothill PW. Fetal karyotyping
co-chair for the International Society of Prenatal Diagnosis by chorionic villus sampling after the first trimester. Br J Obstet Gynaecol
SIG for Fetal Imaging and Phenotyping as well as the BMFMS 1999;106:1035–40.
16 Coorens THH, Oliver TRW, Sanghvi R, Sovio U, Cook E, Vento-Tormo R, et al.
representative for the Fetal Genomics Group of the British Inherent mosaicism and extensive mutation of human placentas.
Society for Genomic Medicine. 2021;592:80–5.
17 Grati FR, Balaj K, Malvestiti F, Agrati C, Grimi B, Malvestiti B, et al. The type
of feto-placental aneuploidy detected by cfDNA testing may influence the
Contribution to authorship choice of confirmatory diagnostic procedure. Prenat Diagn
FM and MDK conceived and designed the manuscript. All 2015;35:994–8.
other authors contributed to drafting the article, providing 18 Del Gobbo GF, Yin Y, Choufani S, Butcher EA, Wei J, Rajcan-Separovic E,
et al. Genomic imbalances in the placenta are associated with poor fetal
final approval and agreement regarding accountability. All growth. Mol Med 2021;27:3.
authors approved the final version. 19 Kalousek DK, Howard-Peebles PN, Olson SB, Barrett IJ, Dorfmann A, Black
SH, et al. Confirmation of CVS mosaicism in term placentae and high
frequency of intrauterine growth retardation association with confined
Acknowledgements placental mosaicism. Prenat Diagn 1991;11:743–50.
The authors would like to thank Dr Stephanie Allen, Head of 20 Nicolini U, Lalatta F, Natacci F, Curcio C, Bui TH. The introduction of QF-PCR
Prenatal Diagnosis and Reproductive Medicine, West in prenatal diagnosis of fetal aneuploidies: time for reconsideration. Hum
Reprod Update. 2004;10:541–8.
Midlands Regional Genetics Laboratory, Birmingham 21 Mann K, Ogilvie CM. QF-PCR: application, overview and review of the
Women’s and Children’s NHS Foundation Trust. literature. Prenat Diagn 2012;32:309–14.
22 Atef SH, Hafez SS, Mahmoud NH, Helmy SM. Prenatal diagnosis of fetal
aneuploidies using QF-PCR: the Egyptian study. J Prenat Med 2011;5:83–9.
References 23 Mann K, Hamilton S, Evans J, Sibbring J, Dore J. ACGS best practice
guidelines for use of quantitative fluorescence-PCR for the detection of
1 Van Opstal D, Srebniak MI. Cytogenetic confirmation of a positive NIPT aneuploidy v4. London: Association for Clinical Genomic Science; 2018.
result: evidence-based choice between chorionic villus sampling and [https://www.acgs.uk.com/quality/best-practice-guidelines/].
amniocentesis depending on chromosome aberration. Expert Rev Mol Diagn 24 Waters JJ, Walsh S, Levett LJ, Liddle S, Akinfenwa Y. Complete discrepancy
2016;16:513–20. between abnormal fetal karyotypes predicted by QF-PCR rapid testing and
2 Warsof SL, Larion S, Abuhamad AZ. Overview of the impact of noninvasive karyotyped cultured cells in a first-trimester CVS. Prenat Diagn
prenatal testing on diagnostic procedures. Prenatal Diagn 2015;35: 2006;26:892–7.
972–9. 25 Grati FR, Malvestiti F, Grimi B, Gaetani E, Di Meco AM, Trotta A, et al. QF-
3 Togneri FS, Kilby MD, Young E, Court S, Williams D, Griffiths MJ, et al. PCR as a substitute for karyotyping of cytotrophoblast for the analysis of
Implementation of cell-free DNA-based non-invasive prenatal testing in a chorionic villi: advantages and limitations from a cytogenetic retrospective
National Health Service Regional Genetics Laboratory. Genet Res (Camb) audit of 44,727 first-trimester prenatal diagnoses. Prenat Diagn
2019;101:1–9. 2013;33:502–8.
4 Grati FR. Implications of fetoplacental mosaicism on cell-free DNA testing: a 26 Holgado E, Liddle S, Ballard T, Levett L. Incidence of placental mosaicism
review of a common biological phenomenon. Ultrasound Obstet Gynecol leading to discrepant results between QF-PCR and karyotyping in 22,825
2016;48:415–23. chorionic villus samples. Prenat Diagn 2011;31:1029–38.
5 Flori E, Doray B, Gautier E, Kohler M, Ernault P, Flori J, et al. Circulating cell- 27 NHS England. Prenatal diagnosis. In: Fetal anomaly screening programme
free fetal DNA in maternal serum appears to originate from cyto- and handbook. London: NHS England; 2021 [https://www.gov.uk/government/
syncytio-trophoblastic cells. Hum Reprod 2004;19:723–4. publications/fetal-anomaly-screening-programme-handbook/prenatal-
6 Wax J, Chard R, Cartin A, Litton C, Pinette MG, Lee Lucas F. Noninvasive diagnosis].
prenatal testing: the importance of pretest trisomy risk and posttest 28 Hu P, Liang D, Chen Y, Lin Y, Qiao F, Li H, et al. An enrichment method to
predictive values. Am J Obstet Gynecol 2015;212:548–9. increase cell-free fetal DNA fraction and significantly reduce false negatives
7 Grati FR, Malvestiti F, Ferreira JC, Bajaj K, Gaetani E, Agrati C, et al. and test failures for non-invasive prenatal screening: a feasibility study. J
Fetoplacental mosaicism: potential implications for false positive and false- Transl Med 2019;17:124.
negative non-invasive prenatal screening results. Genet Med 29 Van Opstal D, Srebniak MI, de Vries F, Govaerts LCP, Joosten M, et al. False
2014;16:620–4. negative NIPT results: risk figures for chromosomes 13, 18 and 21 based on
8 Grati FR. Chromosomal mosaicism in human feto-placental development: chorionic villi results in 5967 cases and literature review. PLoS One 2016;11:
implications for prenatal diagnosis. J Clin Med 2014;3:809–37. e0146794.
Reilly et al.
30 Wang Y, Li S, Wang W, Dong Y, Zhang M, Wang X, et al. Cell-free DNA 42 Cresswell L, Wallis Y, Fews G, Deans Z, Fratter C, Monkman L, et al. General
screening for sex chromosome aneuploidies by non-invasive prenatal testing genetic laboratory reporting recommendations. London: Association for
in maternal plasma. Mol Cytogenet 2020;13:10. Clinical Genomic Science; 2020 [https://www.acgs.uk.com/quality/best-
31 Grati FR, Bajaj K, Zanatta V, Malvestiti F, Malvestiti B, Marcato L, et al. practice-guidelines/].
Implications of fetoplacental mosaicism on cell-free DNA testing for sex 43 National Institute for Health and Care Excellence (NICE). 1.1 Service
chromosome aneuploidies. Prenat Diagn 2017;37:1017–27. organisation. In: Abortion care. NICE guideline [NG140]. London: NICE;
32 Nicolaides KH, Musci TJ, Struble CA, Syngelaki A, Gil MM. Assessment of 2019 [https://www.nice.org.uk/guidance/ng140/chapter/
fetal sex chromosome aneuploidy using directed cell-free DNA analysis. Fetal Recommendations#making-it-easier-to-access-services].
Diagn Ther 2014;35:1–6. 44 Royal College of Obstetricians and Gynaecologists (RCOG). The care of
33 Reiss RE, Discenza M, Foster J, Dobson L, Wilkins-Haug L. Sex chromosome women requesting induced abortion. Evidence-based Clinical Guideline No.
aneuploidy detection by noninvasive prenatal testing: helpful or hazardous? 7. London: RCOG; 2011 [https://www.rcog.org.uk/media/nwcjrf0o/
Prenat Diagn 2017;37:515–20. abortion-guideline_web_1.pdf].
34 Zhang B, Zhou Q, Chen Y, Shi Y, Zheng F, Liu J, et al. High false-positive non- 45 Benn P, Plon SE, Bianchi DW. Current controversies in prenatal diagnosis 2:
invasive prenatal screening results for sex chromosome abnormalities: Are NIPT results suggesting maternal cancer should always be disclosed. Prenat
maternal factors the culprit? Prenat Diagn 2020;40:463–9. Diagn 2019;39:339–43.
35 Wiechec M, Knafel A, Nocun A, Matyszkiewicz A, Juszczak M, Wiercinska E, 46 Bianchi D. Pregnancy: prepare for unexpected prenatal test results. Nature
et al. How effective is first-trimester screening for trisomy 21 based on 2015;522:29–30.
ultrasound only? Fetal Diagn Ther 2016;39:105–12. 47 Bowman-Smart H, Savulescu J, Gyngell C, Mand C, Delatycki MB. Sex
36 Wagner P, Sonek J, Hoopmann M, Abele H, Kagan KO. First-trimester selection and non-invasive prenatal testing: a review of current practices,
screening for trisomies 18 and 13, triploidy and Turner syndrome by evidence, and ethical issues. Ultrasound Obstet Gynecol 2020;40:398–407.
detailed early anomaly scan. Ultrasound Obstet Gynecol 2016;48: 48 UK National Screening Committee. UK NSC non-invasive prenatal testing
446–51. (NIPT) recommendation. London: UK National Screening Committee; 2016
37 Public Health England. National congenital anomaly and rare disease [https://legacyscreening.phe.org.uk/policydb_download.php?doc=570].
registration service: congenital anomaly statistics 2019. London: Public 49 American College of Obstetricians and Gynecologists’ Committee on
Health England; 2019 [https://assets.publishing.service.gov.uk/government/ Practice Bulletins—Obstetrics; Committee on Genetics; Society for Maternal-
uploads/system/uploads/attachment_data/file/1021335/NCARDRS_ Fetal Medicine. Screening for fetal chromosomal abnormalities: ACOG
congenital_anomaly_statistics_report_2019.pdf]. Practice Bulletin, Number 226. Obstet Gynecol 2020;136:e48–69.
38 Silva M, de Leeuw N, Mann K, Schuring-Blom H, Morgan S, Giardino D, 50 Wilson J, Junger G. Principles and practice of screening for disease. Geneva:
et al. European guidelines for constitutional cytogenomic analysis. Eur J World Health Organization; 1968 [https://apps.who.int/iris/bitstream/
Hum Genet 2019;27:1–16. handle/10665/37650/ WHO_PHP_34.pdf?sequence=17].
39 NHS England. Screening for Down’s syndrome, Edwards’ syndrome and 51 Taylor-Phillips S, Freeman K, Geppert J, Agbebiyi A, Uthman OA, Madan J,
Patau’s syndrome: NIPT. London: NHS England; 2021 [https://www.gov.uk/ et al. Accuracy of non-invasive prenatal testing using cell-free DNA for
government/publications/screening-for-downs-syndrome-edwards- detection of Down, Edwards and Patau syndromes: a systematic review and
syndrome-and-pataus-syndrome-non-invasive-prenatal-testing-nipt/ meta-analysis. BMJ Open 2016;6:e010002.
screening-for-downs-syndrome-edwards-syndrome-and-pataus-syndrome- 52 Mackie FL, Hemming K, Allen S, Morris RK, Kilby MD. The accuracy of cell-
nipt]. free fetal DNA-based non-invasive prenatal testing in singleton pregnancies:
40 Papp C, Papp Z. Chorionic villus sampling and amniocentesis: what are the a systematic review and bivariate meta-analysis. BJOG 2017;124:32–46.
risks in current practice? Curr Opin Obstet Gynecol 2003;15:159–65. 53 Royal College of Obstetricians and Gynaecologists (RCOG). The Investigation
41 Lewi L, Blickstein I, Van Schoubroeck D, Gloning K-P, Casteels M, and management of the small-for-gestational-age fetus. Green-top
Brandenburg H, et al. Diagnosis and management of heterokaryotypic guideline no. 31. London: RCOG; 2014 [https://www.rcog.org.uk/media/
monochorionic twins. Am J Med Genet A 2006;140:272–5. t3lmjhnl/gtg_31.pdf].

Pitfalls of Prenatal Diagnosis Associated With Mosaicism TOG

Uploaded by

Copyright:

Available Formats

Pitfalls of Prenatal Diagnosis Associated With Mosaicism TOG

Uploaded by

Document Information

Original Description:

Copyright

Available Formats

Share this document

Share or Embed Document

Sharing Options

Did you find this document useful?

Is this content inappropriate?

Copyright:

Available Formats

Pitfalls of Prenatal Diagnosis Associated With Mosaicism TOG

Uploaded by

Copyright:

Available Formats

DOI: 10.1111/tog.

Pitfalls of prenatal diagnosis associated with mosaicism

Accepted on 28 June 2022. Published online 20 December 2022

Key content  To appreciate underlying principles in NIPT and genomic testing

trisomy 21, 18 or 13 on first trimester combined screening.3

(a) (b) (c)

Mitotic non Mitotic non Meiotic non

46, N 46, XX 47,+chr

Present Present Present

Complete Confined placental

Fetal-placental Non-mosaic fetus, Fetal mosaicism,

Fetal mosaicism, Complete

represent the cytotrophoblast, risking a persistently false

Proposed prenatal management

D21S11 D13S628 D13S634 D18S535

D13S742 D18S1002 D13S305

T21 High risk 1/135 1/7372 >99.9 99.8 94.1 (92.3–95.9) 2 44

T18 High risk 1/64 1/7472 97.4 99.6 89 4 14

T13 High risk 1/136 1/1754 87.5 >99 85 22 4

MX (45,X) High risk 1/14 1/1421 95 99 39 59 26

DNA with a triallelic marker pattern on the

testing, and both informed counselling and full written

You might also like

Key content To appreciate underlying principles in NIPT and genomic testing