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Cell-free DNA vs sequential screening for the detection of fetal chromosomal abnormalities

American Journal of Obstetrics and Gynecology, Volume 214, Issue 6, June 2016, Pages 727.e1 - 727.e6


Sequential and cell-free DNA (cfDNA) screening are both tests for the common aneuploidies. Although cfDNA has a greater detection rate (DR) for trisomy 21, sequential screening also can identify risk for other aneuploidies. The comparative DR for all chromosomal abnormalities is unknown.


To compare sequential and cfDNA screening for detection of fetal chromosomal abnormalities in a general prenatal cohort.

Study Design

The performance of sequential screening for the detection of chromosome abnormalities in a cohort of patients screened through the California Prenatal Screening Program with estimated due dates between August 2009 and December 2012 was compared with the estimated DRs and false-positive rates (FPRs) of cfDNA screening if used as primary screening in this same cohort. DR and FPR for cfDNA screening were abstracted from the published literature, as were the rates of “no results” in euploid and aneuploid cases. Chromosome abnormalities in the entire cohort were categorized as detectable (trisomies 13, 18, and 21, and sex chromosome aneuploidy), or not detectable (other chromosome abnormalities) by cfDNA screening. DR and FPR were compared for individual and all chromosome abnormalities. DR and FPR for the cohort were compared if “no results” cases were considered “screen negative” or “screen positive” for aneuploidy. DR and FPR rates were compared by use of the Fisher exact test.


Of 452,901 women who underwent sequential screening during the time period of the study, 2575 (0.57%) had a fetal chromosomal abnormality; 2101 were detected for a DR of 81.6%, and 19,929 euploid fetuses had positive sequential screening for an FPR rate of 4.5%. If no results cases were presumed normal, cfDNA screening would have detected 1820 chromosome abnormalities (70.7%) with an FPR of 0.7%. If no results cases were considered screen positive, 1985 (77.1%) cases would be detected at a total screen positive rate of 3.7%. In either case, the detection rate of sequential screening for all aneuploidies in the cohort was greater than cfDNA (P<.0001).


For primary population screening, cfDNA provides lower DR than sequential screening if considering detection of all chromosomal abnormalities. Assuming that no results cfDNA cases are high-risk improves cfDNA detection but with a greater FPR. cfDNA should not be adopted as primary screening without further evaluation of the implications for detection of all chromosomal abnormalities and how to best evaluate no results cases.

Key words: aneuploidy screening, cell-free DNA screening, noninvasive prenatal screening, noninvasive prenatal testing, sequential screening.

Related editorial, page 673

The introduction of cell-free DNA (cfDNA) screening has impacted prenatal testing for aneuploidy significantly as a result of the reported high sensitivity for trisomy 21 of >99% at a false-positive rate of ≤0.15%.1, 2, 3, 4, 5, and 6 Because initial data were all obtained from high-risk populations, professional societies recommended that cfDNA screening be reserved for women who are at high risk for trisomy 21.7, 8, and 9

Recent studies of cfDNA screening in low- and average-risk populations also have reported high sensitivities and specificities for trisomy 21, as well as other common aneuploidies,10, 11, and 12 which has led to consideration of the potential use of cfDNA as a primary screening test in all pregnant women. cfDNA screening, however, has limitations that require consideration, including the limited range of targeted aneuploidies, as well as the failure of some tests to provide a result.

Consideration of optimal primary screening policy requires a comparison of available screening options. Aneuploidy screening with serum analytes and nuchal translucency (NT) measurement has long been the mainstay of fetal aneuploidy screening. The most accurate approach is sequential or integrated screening, which is reported to detect 90−95% of Down syndrome cases at a 5% false-positive rate (FPR).13, 14, 15, and 16 Current screening algorithms generally target trisomies 18 and 21. Because serum and NT screening are nonspecific, many pregnancies with “false-positive” results for trisomy 18 and 21 are found to be affected with other chromosomal abnormalities17; however, detection of these requires that 1 woman in 20 undergo diagnostic testing. Screening with cfDNA also targets trisomies 18 and 21, as well as trisomy 13 and the sex chromosomal aneuploidies, but is very precise and has a far lower screen positive rate. Because of this chromosome specificity, however, cfDNA screening does not detect nontargeted aneuploidies. Analyses of cfDNA test performance have excluded cases with aneuploidies other than those that are targeted. Despite being individually rare, these other aneuploidies also can be associated with significant disability and in total account for as many as one-third of chromosome abnormalities detected prenatally and are therefore important to consider.18 and 19

Studies of cfDNA screening also have largely excluded cases in which a result is not obtained. Test failure rates vary by laboratory but occur in approximately 3% of screened pregnancies.1 Test failure has been found to be associated with an increased risk of aneuploidy10, 20, and 21 and is therefore an important component of test performance that becomes more significant as the test is more broadly applied for primary screening. Follow-up of all “no results” cases would increase significantly the effective screen positive rate if cfDNA screening were implemented on a broad scale.

Our objective was to compare the detection rate of sequential screening for all chromosomal abnormalities to the expected performance of primary cfDNA screening in a large, population-based cohort. We compared detection and FPRs for trisomy 21 and other individual common targeted aneuploidies in the cohort, as well as for all chromosome abnormalities if cases with “no result” were assumed to be euploid, and also if these cases were considered “high risk” and in need of follow-up.


Our cohort included participants in the California Prenatal Screening Program within the California Department of Public Health who underwent first- or first- and second-trimester (sequential) screening. California state regulations require that healthcare providers offer prenatal screening to all women seen before the 20th gestational week. MediCal (California’s low-income health coverage) and almost all insurers cover the cost of this screening. Women found to be screen positive are offered follow-up services, with all costs covered by the Program. Covered services include genetic counseling, ultrasound, diagnostic procedures including chorionic villus sampling or amniocentesis, and karyotyping, through contracted Prenatal Diagnostic Centers. All participants are tracked, and results of diagnostic testing are recorded centrally. The Genetic Disease Screening Program California Chromosomal Defect Registry collects information about chromosome abnormalities and pregnancy outcome on all California births, regardless of whether prenatal testing was performed.22

Details regarding the Prenatal Screening Program, including screening algorithms and detection rates for first-trimester and/or sequential screening, were published recently.16 To summarize in brief, since April 2009, the Program has provided first- and second-trimester serum screening with integration of NT ultrasound measurements into the risk algorithm. First-trimester serum screening uses maternal pregnancy-associated plasma protein-A and total human chorionic gonadotropin; patients who have an NT measurement performed are provided a first-trimester risk assessment for Down syndrome and trisomy 18. Second-trimester serum testing uses alpha-fetoprotein, total human chorionic gonadotropin, unconjugated estriol, and dimeric inhibin-A; these are integrated with first-trimester results for a final risk calculation Patients in whom an NT is not performed have serum sequential screening results calculated by the use of the results of the first- and second-trimester serum analytes.

We included data from all women with singleton pregnancies who underwent first-trimester only or first- and second-trimester sequential aneuploidy screening from April 2009 through December 2012. Karyotypes of fetuses or infants were categorized as normal or abnormal; abnormal results were further analyzed as to type of abnormality and whether the abnormality would be detectable by routine cfDNA screening. All abnormal karyotypes were included; although some karyotypes may be associated with a normal outcome, there is a range of potential outcomes with any chromosomal abnormality. The number of cases likely to have a completely normal phenotype (balanced translocations and confined placental mosaicism) was very small (<5% of total abnormalities). Infants with no karyotyping performed in their first year were presumed to be euploid. Although cfDNA laboratories provide somewhat different analyses, for the purpose of this study, nonmosaic trisomy 13, 18, or 21, or sex-chromosomal aneuploidy were considered detectable. Robertsonian translocations causing trisomy 13 or 21 also were considered detectable by cfDNA screening.3 and 6 Other rare trisomies, triploidy, structural rearrangements, including unbalanced translocations other than Robertsonian translocation trisomy 13 or 21, duplications and deletions, and all forms of mosaicism were considered not detectable by cfDNA screening.

We compared the frequency of chromosomal defects detected with sequential screening to the frequency with which they would have been detected by primary screening with cfDNA. Detection by either method was defined as screen positive for any condition in a fetus or infant found to be affected by any chromosomal abnormality. Detection and FPRs of cfDNA were based on the recent meta-analysis by Gil et al.1 The number of chromosomal defects of each type that would be detected by cfDNA was determined by taking into account the reported detection rate, as well as the percentage of each aneuploidy that is undetected as the result of failed cfDNA screening.1, 2, 3, 6, 10, and 20 Because the failure rate varies by laboratory and method, a weighted average was calculated on the basis of the primary validation study reported by each of the major laboratories. For each aneuploidy, we calculated the number of affected infants in the cohort in which a result would be successfully obtained; the published detection rates were then applied only to the number of cases for which test results would be available. In a separate analysis, the detection and FPRs of sequential screening also were compared with the rates that would result from primary cfDNA screening if those cases with no result were considered to be screen positive and referred for follow-up testing.

In sum, the overall performance of sequential screening was compared with cfDNA under 2 different assumptions. In the first model, cases of aneuploidy with “no result” were presumed to be normal and therefore false negatives, and the detection rate was lowered accordingly, whereas the screen-positive rate reflected only those cases identified as aneuploid. Under the second assumption, all patients with “no result” were considered to be at increased risk; therefore, aneuploid cases with no result would be detected with appropriate follow-up. In this model, the screen-positive rate reflects those cases identified as aneuploid, as well as the “no results” cases. All rates were compared with the χ2 test. The study was approved by the Committee for the Protection of Human Subjects within the Health and Human Services Agency of the State of California.


The study population included 452,901 pregnant women who underwent first- or first-and-second trimester screening. The mean maternal age was 31 years at term; 73.6% of the sample was younger than 35 years of age. The cohort was racially and ethnically diverse and representative of the California population: participants were Hispanic (40.6%), non-Hispanic white (30.8%), Asian (14.2%), black (4.2%), and multiracial or other (10.1%) (Table 1). Most (91.1%) participants underwent screening in both the first- and second-trimesters; the remaining 9% had only first-trimester screening.

Table 1 Maternal characteristics of the 452,901 women screened

Characteristic Number of women screened (%)
Maternal age at term, y
 <35 333,189 (73.6)
 ≥35 119,712 (26.4)
Maternal race or ethnic group
 Hispanic 184,078 (40.6)
 White 139,537 (30.8)
 Black 19,142 (4.2)
 Asian 64,418 (14.2)
 Other 29,002 (6.4)
 Multiple races 16,724 (3.7)

Norton et al. Cell-free DNA and sequential screening. Am J Obstet Gynecol 2016.

Of the 452,901 patients in the cohort, 22,504 (5.0%) had a positive screening result in the first and/or second trimester. In the entire cohort, there were 2575 pregnancies (1 in 176) with chromosomal abnormalities present; 1275 (49.5%) of these were Down syndrome. Of Down syndrome cases, 1184 had positive sequential screening, for a 92.9% detection rate. In addition, 313 of 336 (93.2%) cases of trisomy 18 were detected, as were 115 of 143 (80.4%) cases of trisomy 13. Of other, nonsex chromosomal aneuploidies, 59 of 80 (73.8%) were detected, as were 61 of 67 (91.0%) cases of triploidy. Of 256 sex chromosome abnormalities, 185 (72.3%) were identified; 129 of 161 (80.1%) cases of Turner syndrome (45,X), 19 of 36 (52.8%) cases of Klinefelter syndrome (47,XXY), and 37 of 59 (62.7%) cases of other sex chromosome aneuploidies. Details of detection rates for other chromosome abnormalities are described in Baer et al16 and in Table 2.

Table 2 Detection rates of sequential screening compared with cfDNA, assuming that “no results” cases are assumed to be normal

Aneuploidy N % “no result” by cfDNA No. with result by cfDNA DR cfDNA n (%) detected by cfDNA n (%) detected by SS DR of SS vs cfDNA
T21 1275 3.3% 1233 99.2% 1223 (95.9) 1184 (92.9) P = .001
T18 336 10.3% 301 96.3% 290 (86.3) 313 (93.2) P = .005
T13 143 12.5% 125 91.0% 114 (79.7) 115 (80.4) P = 1.0
45,X 161 17.2% 133 90.3% 120 (74.5) 129 (80.1) P = .29
Other SCAa 95 17.2% 79 93.0% 73 (76.8) 56 (58.9) P = .013
Otherb 601 4.3% 575 0% 0 323 (53.7) P < .0001
All 2575 5.0% 2446 70.7% 1820 (70.7) 2101 P < .0001

aXXY, n=36; XXX, n=20; XYY, n=18; other, n=9

bIncludes other trisomies (n=80, of which 59 [73.8%] were screen positive), including trisomy 2 (n=9); 4 (n=1); 6 (n=1); 7 (n=3); 8 (n=2); 9 (n=12); 10 (n=2); 12 (n=2); 14 (n=3); 15 (n=3); 16 (n=19); 20 (n=6); 22 (n=12); trisomies for multiple chromosomes (n=4); triploidy (n=67, of which 61 [91%] were screen positive) and other polyploidy (n=4); unbalanced Robertsonian translocations (n=33, of which 8 [24%] were screen positive); large duplications and deletions (n=117, of which 48 [41%] were screen positive); extrastructurally abnormal chromosomes (n=34, of which 13 [38%] were screen positive); and other (n=230, of which 113 (49%) were screen positive), including other cases of reported CVS mosaicism, other translocations, additions, duplications, inversions, and ring chromosomes.

cfDNA, cell-free DNA; DR, detection rate; SCA, sex chromosome abnormalities; SS, sequential screening.

Norton et al. Cell-free DNA and sequential screening. Am J Obstet Gynecol 2016.

With regard to cfDNA screening, the calculated test failure rate for each aneuploidy varies from 3.3% for trisomy 21, to 17.2% for 45,X (Table 2). Rates for other uncommon aneuploidies are not calculable from the published literature, and we therefore used a weighted average for all aneuploidies calculated from each laboratory’s primary validation study,1, 2, 3, 6, 10, and 20 yielding a weighted average test failure rate in chromosomally abnormal pregnancies of 4.3%.

Of the 1275 trisomy 21 cases in the cohort, if 3.3% had a failed cfDNA screening result, 1233 cases would have been successfully provided with a result. At a detection rate of 99.2% in such cases, 1223 cases overall, or 95.9%, would be detected. This result is greater than the 1184 (92.9%) trisomy 21 cases detected by sequential screening (P = .001). Using the same methodology, we calculated that the detection rate for sex chromosomal aneuploidies other than 45,X would likewise be greater with cfDNA than with sequential screening. In contrast, detection of trisomy 13 and 45,X would not differ between the 2 techniques, and detection of trisomy 18 would be lower with cfDNA than with sequential screening (86.3% vs 93.2%) (P = .005). Sequential screening detected 53.7% (323/601) of the other chromosomal abnormalities (rare aneuploidies, mosaics, and large deletions and duplications); none of these would have been detected by cfDNA screening (Table 2). Of all chromosomal abnormalities in the cohort, 1820 (70.7%) would be detected with cfDNA screening vs 2101 (81.6%) with sequential screening (P < .0001). Comparing only detection of those aneuploidies currently included on all cfDNA panels (ie, excluding the 601 rare aneuploidies) would result in overall detection by sequential screening of 1778 chromosomal abnormalities (69.0%), which is not different than the 1820 predicted to be detected by cfDNA (P = .21).

If patients with a failed cfDNA screen were considered high risk and referred for follow-up, the detection rates would increase accordingly (Table 3). Under this assumption, the detection rate of cfDNA screening is greater than sequential screening for trisomies 13, 18, and 21, as well as the sex chromosomal abnormalities. If the rate of test failure in rare aneuploidies is the same as the 4.3% seen across other aneuploid fetuses as described previously, 26 of the 601 rare aneuploidies also would be detected on the basis of this “screen-positive” category; however, the detection rate for all chromosomal abnormalities in the cohort under this model would be 77.1% (n=1985), which is still lower than the 81.6% (n = 2101) detected with sequential screening (P < .0001).

Table 3 DRs of sequential screening compared with cfDNA, assuming that “no results” cases are considered “screen positive” and referred for follow-up

Aneuploidy N % with no result by cfDNA No. with no result by cfDNA DR cfDNA n (%) detected by cfDNA n (%) detected by SS DR of SS vs cfDNA
T21 1275 3.3% 42 99.2% 1265 (99.2) 1184 (92.9) P < .0001
T18 336 10.3% 35 96.7% 325 (96.7) 313 (93.2) P = .05
T13 143 12.5% 18 92.3% 132 (92.3) 115 (80.4) P = .005
45X 161 17.2% 28 91.9% 148 (91.9) 129 (80.1) P = .004
Other SCAa 95 17.2% 16 93.7% 89 (93.7) 56 (58.9) P < .0001
Otherb 601 4.3% 26 4.3% 26 (4.3) 323 (53.7) P < .0001
All 2575 5.0% 165 77.1% 1985 (77.1) 2101 (81.6) P < .0001

aXXY, n=36; XXX, n=20; XYY, n=18; other, n=9

bIncludes other trisomies (n=80, of which 59 [73.8%] were screen positive), including trisomy 2 (n=9); 4 (n=1); 6 (n=1); 7 (n=3); 8 (n=2); 9 (n=12); 10 (n=2); 12 (n=2); 14 (n=3); 15 (n=3); 16 (n=19); 20 (n=6); 22 (n=12); trisomies for multiple chromosomes (n=4); triploidy (n=67, of which 61 [91%] were screen positive) and other polyploidy (n=4); unbalanced Robertsonian translocations (n=33, of which 8 [24%] were screen positive); large duplications and deletions (n=117, of which 48 [41%] were screen positive); extra structurally abnormal chromosomes (n=34, of which 13 [38%] were screen positive); and other (n=230, of which 113 [49%] were screen positive), including other cases of reported CVS mosaicism, other translocations, additions, duplications, inversions, and ring chromosomes.

cfDNA, cell-free DNA; DR, detection rate; SCA, sex chromosome abnormalities; SS, sequential screening.

Norton et al. Cell-free DNA and sequential screening. Am J Obstet Gynecol 2016.

With regard to FPRs, sequential screening returned a positive result in 22,504, of which 19,929 were euploid. This finding corresponds to a screen-positive rate of 5.0% and a FPR of 4.5%. On the basis of Gil et al,1 the corresponding FPR for cfDNA screening is 0.72% (P < .0001). If the 3.0% of cases of failed cfDNA screening in euploid fetuses also are considered screen positive, the overall percentage of false-positive, euploid cases that would be referred for follow up is 3.72%. This is lower than the FPR of sequential screening (P < .001) (Table 4).

Table 4 Detection rate and false-positive rate of sequential screening and cfDNA screening for all aneuploidies

Detection rate for all chromosomal abnormalities False-positive rate
Sequential screening 81.6% 4.5%
cfDNA, “no results” assumed normal 70.7% 0.7%
cfDNA, “no results” assumed high risk 77.1% 3.7%

cfDNA, cell-free DNA.

Norton et al. Cell-free DNA and sequential screening. Am J Obstet Gynecol 2016.


We determined that, when considering all chromosomal abnormalities in this large cohort, the detection rate of sequential screening was greater than would have been achieved with cfDNA screening as a primary screening test. This is largely attributable to identification of rare chromosomal abnormalities that occur because sequential screening is a less-specific screening test that uncovers many nontargeted abnormalities and also because some chromosomal aneuploidies (particularly trisomy 13 and 18) are not detected by cfDNA screening because of test failure. Even if all patients with failed cfDNA screens are considered screen positive, however, the detection rate of cfDNA remains lower than sequential screening. Under either assumption, the FPR of cfDNA screening is lower, although referral of all failed cases for follow-up would increase substantially the effective FPR.

Studies that have evaluated cfDNA screening have included only selected subjects, and true cohort studies or randomized trials comparing this technique to traditional screening have not been reported, because essentially all studies have excluded pregnancies in which the test failed to provide a result and/or the fetus was affected with a chromosome abnormality other than those targeted by the screen.1, 2, 3, 6, 10, and 20 Thus, it has been difficult to determine the true detection rate in the population if cfDNA screening were used as a primary screening tool.

Failed cfDNA screens can occur as the result of low quantities of fetal DNA, failed sequencing, or high sequencing variance. Failed results have been associated with a number of factors, including fetal aneuploidy.10, 20, and 21 The rate of failed tests varies substantially between laboratories and in published reports, from less than 1% to 12% or greater.1, 2, 3, 6, 10, 20, and 23 Much of the difference is explained by variation in each laboratory’s measurement and use of the fetal fraction (FF), that is, the percentage of cfDNA that is of fetal origin. Greater FF results in better discrimination of normal from abnormal,2 and 21 although the benefits of requiring a minimum percentage of fetal DNA have not been specifically studied. On the basis of the concern that a very low FF is less accurate, some laboratories routinely measure this and report a “no call” or failed test if inadequate fetal DNA is present (usually less than 4%). This results in greater rates of failed tests but also means that aneuploidies associated with biologically lower fetal DNA fractions are less likely to result in a false-negative result.

Our study demonstrates how test performance is impacted by the management of failed test results and might vary in clinical practice between laboratories as a result. Theoretically, laboratories that measure FF will have some trisomy fetuses in the “no results” group, whereas testing of those same pregnancies without measuring FF would result in a false-negative test. In the absence of head-to-head testing, only modeling such as that reported in this paper can begin to assess the implications of these different strategies. Because low fetal DNA resulting from smaller placentas is most likely in cases of trisomy 18, failure to measure FF results in detection rates that are lowered 10%; in contrast, sequential screening, which measures placental proteins, is more likely to detect trisomy 18 because of the small placental size and low serum analyte levels. Detection of trisomy 13 is virtually identical between the techniques despite not being a specific target of sequential screening by the California program. If, in contrast, failed cases are considered screen positive, the detection rate of cfDNA screening is greater than that of sequential for all targeted aneuploidies. With trisomy 21, the FF is as high, or higher than in euploid cases, resulting in fewer failed tests and a greater rate of detection than with sequential screening. Overall, the detection rate of all target aneuploidies would be identical between cfDNA and sequential screening when considering “no result” cases as “not detected.”

The results of this analysis with regard to other rare aneuploidies are consistent with previous studies of traditional screening,17 and 18 cytogenetic registries,19 and cfDNA screening.12 In a previous study, we assessed the predicted performance of cfDNA screening in a cohort of women found to be at increased risk, primarily for trisomy 21, after traditional screening. In that report, we found that 83% of chromosome abnormalities detected by traditional screening were potentially detectable by cfDNA screening, whereas a chromosome abnormality undetectable by cfDNA was present in 2% of screen-positive patients.17 In this current study of an average risk population, a greater percentage of chromosome abnormalities (23% or 29%, depending how failed cases are handled) would remain undetected by cfDNA screening. This difference is attributable to a lower proportion of targeted abnormalities in a lower risk cohort, as well as consideration of failed cfDNA tests, which was not modeled into our prior report.

It is likely that many women who choose to undergo prenatal testing for aneuploidy are concerned not solely with trisomy 21 but with all causes of intellectual disability and serious birth defects. The limited coverage of cfDNA screening for all chromosome abnormalities has been the subject of relatively little discussion. Although traditional screening has been criticized as having a greater FPR, many of these “false”-positive cases are affected with other chromosomal or structural abnormalities or risk for adverse perinatal outcomes.24 Therefore, the overall benefits of traditional screening extend beyond simple screening for Down syndrome.

Although our study includes a large, diverse cohort, it is not without limitations. The reporting of diagnosed chromosome abnormalities, although mandatory, may not be complete, and some cases likely remain undiagnosed. In the absence of a reported chromosomal abnormality, we assumed that the screen-positive infants were normal. Therefore, the impact of this limitation in ascertainment on the study findings is likely an overestimate of the FPR and an underestimate of the detection rate for rare abnormalities. Data regarding ultrasound abnormalities also were not available. Our analysis was based on potential detection by cfDNA, not on actual testing. Although we considered mosaicism to be not detectable by cfDNA because of exclusion from initial validation studies, detection is likely possible in some cases.2, 3, and 6 Not all cfDNA laboratories report risks for the same abnormalities; some methods can detect some cases of triploidy.25 and 26 Some laboratories test for trisomies 16 and 22, which were not specifically assessed in our study, but contributed 19 and 12 cases, respectively, of which 18 of 19 and 11 of 12 were detected by sequential screening. Finally, we studied sequential screening, which is not used by all programs and providers, and did not specifically compare with other methods such as first-trimester combined or second-trimester biochemical screening.

In summary, when comparing cfDNA with sequential screening, sequential screening has better detection when considering all chromosomal abnormalities. Whether “no results” cases are presumed to be normal or are referred for high-risk follow-up changes the difference in performance overall, and for trisomies 13 and 18 in particular. Clearly, further study is needed in prospectively collected cohorts regarding the clinical utility of cfDNA vs traditional screening to accurately assess this trade-off, particularly in low or average risk pregnant women.


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a Department of Obstetrics, Gynecology and Reproductive Sciences, University of California, San Francisco, San Francisco, CA

b Genetic Disease Screening Program, California Department of Public Health, Richmond, CA

c University of California San Diego, Department of Pediatrics, La Jolla, CA

d Department of Obstetrics and Gynecology, Columbia University Medical Center, New York, NY

e Department of Epidemiology and Biostatistics, University of California, San Francisco, San Francisco, CA

Corresponding author: Mary E. Norton, MD.

Disclosures: M.E.N. has received research funding from Ariosa Diagnostics and Natera; R.J.W. has received research funding from Ariosa Diagnostics, Natera, Sequenom, and Illumina; M.K. has received unrestricted research funding from Illumina; the other authors declare no conflicts of interest.

Cite this article as: Norton ME, Baer RJ, Wapner RJ, et al. Cell-free DNA vs sequential screening for the detection of fetal chromosomal abnormalities. Am J Obstet Gynecol 2016;214:727.e1-6.