Twin zygosity nipt

DOI: 10.1002/pd.4132
ORIGINAL ARTICLE
Noninvasive twin zygosity assessment and aneuploidy detection by
maternal plasma DNA sequencing
Tak Y. Leung1†, James Z. Z. Qu2,3†, Gary J. W. Liao2,3, Peiyong Jiang2,3, Yvonne K. Y. Cheng1, K. C. Allen Chan2,3, Rossa W. K. Chiu2,3
and Y. M. Dennis Lo2,3*
1Department of Obstetrics and Gynaecology, The Chinese University of Hong Kong, Hong Kong SAR, China
2Centre for Research into Circulating Fetal Nucleic Acids, Li Ka Shing Institute of Health Sciences, The Chinese University of Hong Kong, Hong Kong SAR, China
3Department of Chemical Pathology, The Chinese University of Hong Kong, Hong Kong SAR, China
*Correspondence to: Y. M. Dennis Lo. E-mail: loym@cuhk.edu.hk
†These authors contributed equally.
ABSTRACT
Objective This study aimed to provide an individualized assessment of fetal trisomy 21 and trisomy 18 status for twin
pregnancies by maternal plasma DNA sequencing.
Method Massively parallel sequencing was performed on the plasma/serum DNA libraries of eight twin pregnancies
and 11 singleton pregnancies. The apparent fractional fetal DNA concentrations between genomic regions were
assessed to determine the zygosities of the twin pregnancies and to calculate the fetal DNA concentrations of
each individual member of dizygotic twin pairs. Z-scores were determined for the detection of trisomy 18 and
trisomy 21.
Results Circulating DNA sequencing showed elevated chromosome 21 representation in one set of twins and elevated
chromosome 18 representation in another pair of twins. Apparent fractional fetal DNA concentration analysis
revealed both sets of twins to be dizygotic. The fractional fetal DNA concentrations for each individual fetus of the
dizygotic twin pregnancies were determined. Incorporating the information about the fetal DNA fraction, we
ascertained that each fetus contributed adequate amounts of DNA into the maternal circulation for the aneuploidy
test result to be interpreted with confidence.
Conclusion Noninvasive prenatal assessment of fetal chromosomal aneuploidy for twin pregnancies can be
achieved with the use of massively parallel sequencing of cell-free DNA in maternal blood. © 2013 John Wiley
& Sons, Ltd.
Funding sources: The study is supported by the University Grants Committee of the Government of the Hong Kong Special Administrative Region, China, under the
Areas of Excellence Scheme (AoE/M-04/06), a Sponsored Research Agreement from Sequenom, Inc. and by funding from the S. K. Yee Foundation. Y. M. D. Lo is
supported by an endowed chair from the Li Ka Shing Foundation.
Conflicts of interest: T. Y. L., P. J., K. C. A. C., R.W.K. C. and Y. M. D. Lo hold patents or have filed patent applications on noninvasive prenatal diagnosis using fetal
nucleic acids in maternal plasma. Part of this intellectual property portfolio has been licensed to Sequenom, Inc. R.W.K. C. and Y. M. D. Lo have received research
support from, are consultants to, and hold equities in Sequenom, Inc.
INTRODUCTION
Noninvasive prenatal testing of fetal chromosomal aneuploidies
has been achieved with the use of massively parallel maternal
plasma DNA sequencing.1 The test is based on detecting a
change in representation among DNA molecules (from
mother and fetus) present in maternal plasma as a result of
the abnormal chromosome dosages contributed by the
aneuploid fetus. For example, the genome of a trisomy 21
fetus has a third copy of chromosome 21 that results in
increased proportions of chromosome 21 DNA content in
maternal plasma. Massively parallel sequencing-based
noninvasive prenatal testing (MPS-NIPT) has been shown by
several studies to provide highly sensitive and specific
detection of trisomies 21, 18 and 13 among high risk singleton
pregnancies.2–6
Canick et al.7 applied MPS-NIPT for fetal aneuploidy
detection in multi-fetus pregnancies. It was shown that Down
syndrome was correctly classified in seven sets of twin
pregnancies (two sets of twins where both twin fetuses had
Down syndrome and five sets of twins where only one of the
two twin fetuses had Down syndrome). Trisomy 13 was also
correctly classified in one set of twin pregnancy. For the 17 sets
of euploid twins and two sets of euploid triplets, Down
syndrome was correctly excluded. However, the sensitivity of
MPS-NIPT for aneuploidy detection is governed by the
fractional fetal DNA concentration of the affected fetus.2,4
Prenatal Diagnosis 2013, 33, 675–681 © 2013 John Wiley & Sons, Ltd.

676 T. Y. Leung et al.
Maternal plasma samples with higher fetal DNA fractions are
associated with a greater degree of overrepresentation of
DNA amounts from the trisomic chromosome. For example,
with the sequencing depth used for a number of the currently
used MPS-NIPT laboratory protocols, the lower limit of fetal
fraction for aneuploidy detection is 4%.2,4 A fetal DNA fraction
lower than this limit may lead to false-negative detection.
For aneuploidy diagnosis by MPS-NIPT, monozygotic twins
can be interpreted like a singleton pregnancy. This method
can be applied as long as the combined fetal DNA fraction
contributed by the pair of monozygotic twins reached the
minimum requirement, that is, the aforementioned 4%. On
the contrary, for dizygotic twins, the fetal DNA fraction
contributed by each individual twin member would need to
reach the minimum requirement in order for the chromosomal
representation of each twin to be adequately assessed by MPS-NIPT.
However, currently adopted methods for quantifying the
fetal DNA fractions do not determine the contributions from
individual dizygotic twin fetuses. In the study performed by
Canick et al., each twin fetus was assumed to have contributed
adequate amounts of fetal DNA into maternal plasma.
Recently, our group developed a noninvasive approach for
determining the zygosity of twins by looking for evidence of
genetic differences between the twins as inferred by
differences in the apparent fractional fetal DNA concentrations
across multiple genomic regions.8 With this approach, we can
also estimate the fractional fetal DNA concentrations of
individual members of dizygotic twins. Here, we incorporate
this approach for achieving MPS-NIPT trisomy 21 and trisomy
18 detection for twin pregnancies.
MATERIALS AND METHODS
Sample collection and processing
This study was approved by the Joint Chinese University of
Hong Kong – Hospital Authority New Territories East Cluster
Clinical Research Ethics Committee. With informed consent,
we recruited eight twin pregnancies and 11 singleton
pregnancies (Table 1). The 11 singleton pregnancies involved
euploid fetuses as confirmed by full karyotyping. Six of the twin
pregnancies were part of the cohort previously studied,8 and
healthy fetuses were observed at birth. Two of the twin
pregnancies in the previous study did not have adequate
amounts of sample available to be included in this study.
Two other twin pregnancies in the present study, cases 01
and 02, each involving one aneuploid fetus and one euploid
fetus, were recruited after the completion of the earlier study
on twin zygosity assessment. Maternal peripheral blood
samples were taken on one occasion before any invasive
obstetrics procedures between 11 and 36 weeks of gestation
for all cases.
For case 01, chorionic villus sampling was performed, and
full karyotyping revealed one euploid fetus and one trisomy
21 fetus. For case 02, trisomy 18 was suspected after observing
multiple structural abnormalities during ultrasound scanning
in the second trimester. Prenatal diagnosis was declined, and
trisomy 18 in one child was confirmed postnatally. The other
child was healthy. No fetal tissues were available for this study,
but a maternal serum sample taken just before delivery was
retrieved for analysis. For all the other cases in this study,
maternal plasma was used. DNA from chorionic villus biopsy
Table 1 Clinical cases and the sequencing output
Case
Gestational age
(weeks + days) Twins/singleton Karyotype Utility Sequencing counts Chr21 Z-scores Chr18 Z-scores
01 14 + 6 Twins T21 + Euploid Test 16 919 203 14.25 0.05
02 36 + 3 Twins T18 + Euploid Test 21 797 758 0.05 38.65
03 21 + 5 Twins Euploid2 Test 23 937 979 2.83 2.16
09 16 + 4 Singleton Euploid Reference 21 248 214 NA NA
Prenatal Diagnosis 2013, 33, 675–681 © 2013 John Wiley Sons, Ltd.

Noninvasive twin zygosity and aneuploidy detection 677
or cord blood from each twin fetus, except case 02, was
genotyped. The degree of concordance in the genotypes
between each twin pair was used to confirm if the twins were
monozygotic or dizygotic. For case 02, the different postnatal
phenotypes of the two neonates served as the evidence
indicating that they were dizygotic.
Maternal blood samples were centrifuged twice9 to separate
the plasma/serum portion from the blood cell portion. Plasma
or serum DNA was extracted. For each twin pregnancy, two
analyses were performed. One analysis was based on target
capture sequencing for the purpose of determining the
zygosity of the twins.8 The other analysis was performed for
the purpose of noninvasive aneuploidy detection.1–5
Noninvasive zygosity assessment
The noninvasive zygosity assessment for the six euploid twin
pregnancies was performed in our recent study,8 and the same
protocol was applied to cases 01 and 02. Briefly, the plasma or
serum DNA libraries were constructed with the Paired-End
Sample Preparation Kit (Illumina), and target capturing of
selective genomic regions was performed using the SureSelect
Target Enrichment System (Agilent).8,10,11 The capture library
covered 5.5Mb of genomic regions distributed over 14
chromosomes, which were designed to target single nucleotide
polymorphisms (SNPs) on the Affymetrix Genome-Wide
Human SNP Array 6.0 system. As previously reported, to
achieve accurate genotype deduction for the mother and fetus
using the plasma DNA sequenced reads from samples
containing at least 5% fetal DNA, each informative SNP
would need to be sequenced to an average depth of 100
times.8 Therefore, the plasma DNA library of case 01 was
sequenced using four lanes in a flowcell on the Genome
Analyzer IIx sequencer (Illumina), and case 02 was
sequenced using two lanes in a flowcell on the HiSeq 2000
sequencer (Illumina). The choice of the model of sequencer
used was based on which instrument was available at the
time of the experiments.
After sequencing, the FetalQuant algorithm12 was used to
analyze the ratio of alleles of SNPs that were located within
the captured genomic regions to identify loci where the mother
was homozygous and at least one of the twin fetuses was
heterozygous, termed informative SNPs. The ratio of the
fetal-specific allele to the major allele was calculated for each
locus. Allelic ratios of adjacent SNP loci were combined to
obtain an apparent fractional fetal DNA concentration for a
chromosomal region. Apparent fractional fetal DNA
concentrations were calculated for multiple chromosomal
regions. For monozygotic twins, because the fetuses are
genetically identical, the apparent fractional fetal DNA
concentrations should be similar across different genomic
regions. On the contrary, for dizygotic twins, because they are
not genetically identical and each fetus is heterozygous at
different sets of loci, the apparent fractional fetal DNA
concentrations would vary across different genomic regions.
Noninvasive aneuploidy detection
For aneuploidy testing, plasma or serum DNA libraries
were constructed with the TruSeq DNA Sample Preparation
Kit (Illumina). Each DNA sample was ligated with a unique
adaptor and then pooled to form two multiplex libraries.
Each multiplexed pool was composed of nine or ten
samples. For MPS-NIPT aneuploidy testing by the random
sequencing protocol,1–5 only a low sequencing depth is
needed. Therefore, each multiplexed pool of samples was
sequenced using three lanes of a flowcell using the HiSeq
2000 sequencer.
For aneuploidy testing, sequence reads from chromosomes
X and Y were excluded for calculating the genomic
representation of each autosome. Z-scores were calculated
for chromosome 21 and chromosome 18 as previously
described.1,4 A Z-score3 is considered to indicate an
abnormally increased chromosomal representation and hence,
the presence of trisomy.
Plasma DNA sequencing and alignment
DNA libraries were sequenced by the standard paired-end
protocol (Illumina) with a read length of 75 bp for each
end on the HiSeq 2000 sequencer (Illumina) and a read
length of 50 bp for each end on the Genome Analyzer IIx
sequencer (Illumina). All sequenced reads were aligned to
the nonrepeat-masked human reference genome (Hg18)
using SOAP2.13
RESULTS
Determination of twin zygosity by maternal plasma DNA analysis
Microarray-based genotype analysis of the chorionic villus
biopsy or cord blood DNA provided the gold standard for
determining the zygosities of the twins in case 01 (Table 2).
The noninvasive determination of twin zygosity by maternal
plasma DNA sequencing for the six pairs of euploid twins was
carried out in our previous study.8 Four of these previously
studied twin pregnancies were monozygotic (Table 2). The
noninvasive twin zygosity test for cases 01 and 02 had not been
performed previously. For case 01, 187106 raw paired-end
reads were sequenced, and 157106 reads were mappable.
Each SNP locus in the target captured regions was covered an
average of 210 times. For case 02, 462106 raw paired-end
reads were sequenced, and 387106 reads were mappable.
Each SNP locus in target captured regions was covered an
average of 493 times. We calculated the average apparent
fractional fetal DNA concentration for every 1000 informative
SNPs on contiguous genomic blocks within the targeted
regions. We compared if these average values varied from
block to block. Large fluctuations of average apparent
fractional fetal DNA concentrations were observed for both
cases 01 and 02 (Figure 1). The proportions of data points
lying outside the extent of stochastic variations expected for
monozygotic twins were 73% for case 01 and 55% for case
02. As a reference, the proportions of data points lying
outside these boundaries ranged from 0% to 1.93% for
the monozygotic twin cases as previously reported. These
data suggested that both cases 01 and 02 were dizygotic
twin pregnancies.

Table 2 Zygosity status and the fractional fetal DNA concentrations of the twin fetuses
Fractional fetal DNA concentrations of individual members
of twins
The range of apparent fetal DNA concentrations as shown by
every 1000 SNPs was noted. The overall distribution of the
observed apparent fractional fetal DNA concentrations of both
fetuses and the combined total concentration was assumed to
be governed by a mixture of three binomial distributions.8 By
adopting the binomial mixture model, we resolved two of the
distributions and their associated peaks of fetal DNA
concentrations that had the highest likelihood of producing
the combined total amounts of circulating fetal DNA observed
in cases 01 and 02. For case 01, the data suggested that one
fetus contributed 10.7% of the DNA in maternal plasma, the
other fetus contributed 16.9% and the combined fractional
concentration of circulating fetal DNA from both fetuses was
27.6%. For case 02, one fetus contributed 8.80% of the DNA
in maternal plasma, the other fetus contributed 16.3% and
the combined fractional concentration of circulating fetal
DNA from both fetuses was 25.1%. For case 01, to validate the
estimated fetal DNA fraction, we retrieved the microarray-based
genotype information of each individual twin. The
massively parallel sequencing data covering confirmed
informative SNPs were retrieved to calculate the fractional fetal
DNA concentration of each individual twin member. The
concentrations estimated using the binomial mixture statistics
were consistent with those calculated using the known fetal
genotype information (Table 2). The data also suggested that
each fetus among the twin pregnancies had fractional fetal
DNA concentrations that reached the lowest requirement,
namely at least 4%, for aneuploidy detection by one of the
currently used MPS-NIPT protocols.4
Noninvasive prenatal diagnosis of fetal chromosomal aneuploidy
For the diagnosis of fetal chromosomal aneuploidy, the 11
euploid singleton cases served as the reference controls. The
eight twin pregnancies were the test cases. A median of
16106 (range: 3106 to 24106) aligned (i.e. mapped)
paired-end reads were sequenced from the plasma or serum
Case
Zygosity assessment
Predicted fetal
DNA %
Measured fetal DNA % using
microarray-based genotypes
% Concordance in genotypes by microarray Monozygotic or dizygotic Twin 1 Twin 2 Twin 1 Twin 2
01 78.8 Dizygotic 10.7 16.9 13.8 15.3
02 — Dizygotic 8.8 16.3 — —
03(6774)a 78.0 Dizygotic 7.0 11.4 7.9 10.9
04(6870)a 99.6 Monozygotic — — — —
05(6905)a 79.6 Dizygotic 13.9 7.0 11.2 7.8
06(7585)a 99.7 Monozygotic — — — —
07(8114)a 99.8 Monozygotic — — — —
08(8207)a 99.6 Monozygotic — — — —
aThese cases were part of a cohort investigated in a previous study.8 The case numbers adopted in the previous study8 are shown in parentheses. Two other dizygotic twin
cases included in the previous study did not have sufficient plasma DNA available for the present study.
Figure 1 Regional variation in apparent fractional fetal DNA concentrations. Solid line shows the average apparent fractional fetal DNA
concentrations across blocks of 1000 consecutive informative single nucleotide polymorphisms (SNPs) for the twin pregnancy. Grey shadow
shows the results of the computer simulation analysis of the background level of stochastic variation. Dashed lines show the maximum and
minimum values of the predicted stochastic variation. The proportion of data points outside the boundaries is given at the top of each plot. Case
numbers are given in the upper right corner. Large fluctuations are expected for dizygotic twin pregnancies

DNA libraries. Paired-end reads with identical start and end
coordinates in the genome were removed. The number of
reads originating from each autosome was counted and
then expressed as a percentage of the total reads sequenced
from all autosomes for the sample. This percentage value is
the genome representation of that chromosome in the
maternal plasma sample. Guanine–cytosine correction was
performed for the calculation of genome representation
of each chromosome. For the reference group, the mean
genomic representation of chromosome 21 (%chr21) was
1.311 (standard deviation (SD) was 0.007), and the mean
genomic representation of chromosome 18 (%chr18) was
2.808 (SD was 0.004). The %chr21 values for the six euploid
twin cases ranged from 1.297 to 1.331, and the z-scores were
all 3 (Table 1 and Figure 2). The %chr18 values for the
six euploid twin cases ranged from 2.812 to 2.818, and the
z-scores were all 3 (Table 1 and Figure 2). For case 01,
the %chr21 was 1.412, which resulted in a z-score of 14.25
(Figure 2). Because the z-score for chromosome 21 was 3,
the result suggested that case 01 involved at least one fetus
with trisomy 21. For case 02, the %chr18 value was 2.975,
which gave a z-score of 38.65 (Figure 2). Because the z-score for
chromosome 18 was 3, the result suggested that case 02
involved at least one fetus with trisomy 18. Case 01 had normal
representation for chromosome 18, whereas case 02 had normal
representation for chromosome 21.
Aneuploidy assessment of dizygotic twin fetuses
Theoretically, the genomic representation of the aneuploid or
trisomic chromosome (%chrT) should have a linear relationship
with the fractional fetal DNA concentration for all trisomy cases2
but not the euploid cases. The reason is the elevated genomic
representation of a chromosome is contributed by the aneuploid
fetus. One should be able to predict the %chrT of an aneuploid
fetus when the mean value of %chrT of the reference group
and the fetal DNA fraction of the trisomy fetus is known. We
established the relationship between the %chr21 or %chr18
values and the fetal DNA fraction using the following:
%chrTsample ¼ mean%chrTreference
þ 0:5f mean%chrTreference
where %chrTsample is the %chrT value of the test sample, %
chrTreference is the %chrT value of the reference group and f
is the fetal DNA fraction. The correlation plots are shown
in Figure 3.
For case 01, the correlation line for %chr21 intersected with
the observed genomic representation of the affected twin
pregnancy when the fractional fetal DNA concentration was
15.4%. For case 02, the correlation line for %chr18 intersected
with the observed genomic representation of the affected twin
pregnancy when the fractional fetal DNA concentration was
11.9%. For both cases, the correlation lines intercepted at fetal
DNA concentrations that were much lower than the combined
fetal DNA concentrations contributed by both twins. The
results therefore suggested that only one fetus had aneuploidy
in each twin pair. Our results also confirmed that each twin
fetus contributed adequate amounts of DNA into maternal
plasma to render the MPS-NIPT aneuploidy test valid for
interpretation, that is, not a false-negative result due to
inadequate fetal DNA percentage.
There were seven male fetuses among the 11 euploid
singleton cases. We determined the fetal DNA concentration
of these cases using a real-time polymerase chain reaction
assay targeting a gene on chromosome Y, the sex-determining
region Y (SRY) gene. All values deviated from the correlation
lines for %chr21 and %chr18. This was as expected because
there was no elevation in %chr21 and %chr18 values in the
euploid cases; hence, there should be no relationship with
the fractional fetal DNA concentration.
DISCUSSION
Previously, Canick et al. investigated the use of MPS-NIPT
for chromosomal aneuploidy detection among multi-fetus
pregnancies.7 However, on the basis of the MPS-NIPT
aneuploidy assessment alone, it is not known if one or both
fetuses were affected. More important, for dizygotic twins, it is
not known if each fetus contributed adequate amounts of DNA
to pass the minimum quality control requirement for valid
assessment. Here, we have demonstrated that our recently
developed noninvasive twin zygosity test provides useful
information for the MPS-NIPT assessment of chromosomal
aneuploidy among twin pregnancies.8 When the twins are
determined to be monozygotic, the MPS-NIPT aneuploidy test
can be interpreted in the same manner as for singleton
pregnancies. When the twins are determined to be dizygotic,
the fractional fetal DNA concentration of each fetus can be
calculated. This has allowed us to confirm if the fetus
contributing the lesser amount of fetal DNA also has a value that
reaches the minimum requirement. In this study, we have
further shown that by exploiting the relationship between the
genomic representation of the aneuploid chromosome and the
Figure 2 Genomic representations (GR) of chromosome 21 and
chromosome 18 for the reference and test cases. Broken lines
indicate the cut-off value of genomic representation corresponding
to a z-score that is equal to 3. The chromosome 21 genomic
representation of case 01 is elevated, and the data point lies
above the cut-off. The chromosome 18 genomic representation of
case 02 is elevated, and the data point lies above the cut-off. All
singleton control pregnancies and all other twin pregnancies show
genomic representations for chromosomes 21 and 18 that are
below the cut-off

fetal DNA fraction,2 we could further determine if one or both
fetuses were affected. Although the method cannot pinpoint
which one of the twins had aneuploidy, the aneuploidy
assessment of the twin fetuses could be made with greater
certainty to justify the need for invasive confirmatory testing.
Because of the proof-of-principle nature of this study,
further studies with larger sample size are needed. For case
02, trisomy 18 was diagnosed postnatally; hence, fetal tissues
were not available for analysis in this study. However, our
results were consistent with the postnatal diagnosis.
Although the noninvasive twin zygosity test provides useful
information for interpreting the MPS-NIPT for fetal
chromosomal aneuploidy testing, it is an added procedure
and additional costs are involved. With the rapid reduction in
sequencing costs, the zygosity test could be considered as an
optional quality control step for additional reassurance in
A
B
Figure 3 Determination of the number of twin fetus(es) with chromosomal aneuploidy. The red line shows the expected relationship between
the maternal plasma genomic representation of the aneuploid chromosome and the fetal DNA fraction. The dashed line marks the genomic
representation of the affected twin pregnancy. The black solid lines mark the fetal DNA percentage estimated for each fetus of the dizygotic
twin pair and the combined fetal DNA percentage from both fetuses. The genomic representation and fetal DNA percentage values for the
seven male singleton control pregnancies are depicted by squares. (A) Plot of %chr21 and fetal DNA percentage values for case 01. The
correlation line for %chr21 intercepts with the observed genomic representation of the affected twin pregnancy when the fractional fetal DNA
concentration is 15.4%. (B) Plot of %chr18 and fetal DNA percentage values for case 02. The correlation line for %chr18 intercepts with the
observed genomic representation of the affected twin pregnancy when the fractional fetal DNA concentration is 11.9%. Each plot suggests that
there is only one fetus affected by aneuploidy in each twin pair

interpretation of the aneuploidy test results. In our opinion, as
twin pregnancies represent only a minority of pregnancies, the
increased costs of adding this quality control step to the overall
prenatal healthcare costs in the population as a whole should
be affordable. On the other hand, we believe that the
noninvasive twin zygosity test may provide additional clinical
utility. For example, it is known that a number of pregnancy-related
complications are associated with abnormal levels of
circulating fetal DNA.14,15 With our approach, we could
monitor the dynamic changes of the fetal DNA concentration
of each dizygotic twin member for monitoring the well-being
of each twin.
CONCLUSION
With the use of massively parallel sequencing of maternal
plasma DNA, we have successfully determined the zygosity of
twins with chromosomal aneuploidies and estimated the fetal
DNA fraction of each dizygotic twin member. Our approach
has made it possible to confidently interpret the MPS-NIPT
results for trisomy 21 and trisomy 18 diagnoses among
twin pregnancies.
ACKNOWLEDGMENTS
We thank Yongjie Jin for performing the sequencing and Coral
Lee for bioinformatics support.
WHAT’S ALREADY KNOWN ABOUT THIS TOPIC?
• By using massively parallel maternal plasma DNA sequencing,
noninvasive prenatal detection of fetal chromosomal aneuploidies
has been achieved. The sensitivity of the maternal plasma DNA
sequencing test for aneuploidy detection is governed by the
fractional fetal DNA concentration.
WHAT DOES THIS STUDY ADD?
• We have recently developed a maternal plasma DNA sequencing-based
method for the noninvasive assessment of twin zygosity. The
method also allows the determination of the fetal DNA fraction
contributed by each member of a pair of dizygotic twin in maternal
plasma. In this study, we applied the new method to two dizygotic
twin pregnancies involving one aneuploid fetus each. We
confirmed that each fetus contributed adequate amounts of DNA
into maternal plasma so that the aneuploidy test result could be
interpreted with confidence.
REFERENCES
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fetal chromosomal aneuploidy by massively parallel genomic
sequencing of DNA in maternal plasma. Proc Natl Acad Sci USA
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2. Chiu RWK, Akolekar R, Zheng YWL, et al. Non-invasive prenatal
assessment of trisomy 21 by multiplexed maternal plasma DNA
sequencing: large scale validity study. BMJ 2011;342:c7401.
3. Palomaki GE, Kloza EM, Lambert-Messerlian GM, et al. DNA
sequencing of maternal plasma to detect Down syndrome: an
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