Dr. Warsof is a Professor and Director of Genetics &
Prenatal Diagnosis Department of OB/GYN Division of Maternal Fetal
Medicine, Eastern Virginia Medical School; and Ms. Valenti is a
Genetic Counseling Supervisor, Department of OB/GYN Division of Maternal
Fetal Medicine, Eastern Virginia Medical School, Norfolk, VA.
childbirths in the United States exceeded 4.2 million in 2007. The
average age of mothers at birth has been steadily increasing since 1975.
In 2006, 14% of all neonates were born to women aged 35 years or older.1 These trends have continued and are also noted in many European countries.2
syndrome (DS) is the most common chromosomal abnormality, affecting
1:700 live births. Its frequency is at least a third greater if
stillbirths and spontaneous losses, which inherently have a greater
burden of aneuploidies, are included. With the aging of the obstetrical
population the incidence has grown as high as 1:500 live births.
Identifying pregnancies at risk for DS has been a major goal of prenatal
care since safe prenatal diagnostic testing became available in the
Experts have long recognized that DS occurs more frequently in older women (Table 1)3,4
and in women with a family history of DS. These two criteria (maternal
age >35, and family history) were the traditional risk factors for
offering diagnostic testing when amniocentesis and karyotyping became
clinically available. Unfortunately, these criteria have proven to be
poor screening tools for DS, exposing 15% to 20% of pregnant women to
invasive testing, with its inherent pregnancy loss rate, while detecting
only 30% to 35% of affected pregnancies.
Recently, due to the
results of large population-based studies in the United States and
Europe, myriad new options have been introduced for Down syndrome risk
assessment that can be done either in the first trimester or by
combining first- and second-trimester measurements. This article
presents options for fetal aneuploidy risk assessment during pregnancy.
Second trimester risk assessment
trimester risk assessment for aneuploidy primarily involves serum
biochemical markers. The association between DS and low maternal serum
alpha-fetoprotein (MSAFP) levels in the second trimester was recognized
in 1984.5 This association was expanded in the late 1980s,
when elevated levels of human chorionic gonadotropin (hCG) and decreased
levels of unconjugated estriol were also reported in DS fetuses.
Combining these three biochemical markers into what is currently known
as the “Triple Screen” was shown to detect approximately 60% of DS
fetuses for a 5% screen positive rate when offered to women between 15
and 22 weeks’ gestation.6 In the 1990s, Dimeric Inhibin A was shown to be increased in DS fetuses independently of the other biochemical analytes.7
By adding this fourth serum analyte to the Triple Screen, the “Quad
Screen” enhanced detection of DS fetuses to 81% with a 5% screening
false positive rate.8,9 These biochemical markers were also found to be effective in assessing risk for Trisomy 18.
Ultrasound risk assessment for DS in the second trimester
with ultrasound-detected congenital anomalies show an increased risk
for DS. Congenital heart disease, cystic hygroma, omphalocele, and
duodenal atresia have the highest burden of aneuploidy, even though at
least 50% of infants with DS have no recognizable major congenital
anomalies. Therefore, many sonographic studies have been attempted to
identify DS by unusual ultrasound features often referred to as markers.
Frequently referred to as a “genetic sonogram,” identification of these
markers will significantly alter the risk of DS, providing individual
risk assessments for DS rather than age-specific risks. (Table 2).10-12
None of these markers should be considered diagnostic for DS.
Similarly, the absence of these markers will reduce the risk of DS by
approximately 50% to 80%, but it will not exclude the possibility of
First trimester risk assessment
late onset, limited, or complete lack of prenatal care continue to be
problematic in the United States, more than 80% of pregnant women
initiate care prior to 12 weeks’ gestation. The desirability of first
trimester detection of DS is obvious. First trimester risk assessment
for aneuploidy was introduced in Europe in the mid-1990s. It involves a
combination of serum biochemistry and ultrasound measurement of the
nuchal translucency (NT) (Figure 1). The biochemical markers are
Pregnancy Associated Plasma Protein A (PAPP-A), which is lower in DS
fetuses, and HCg, which is increased in DS fetuses. Although NT or
serum analytes alone can be used for risk assessment, when taken
together with maternal age, the Combined First Trimester Screen is the
most sensitive technique in the first trimester.13
describes an anechoic area in the posterior nuchal region of the fetus
and is typically observed in the first trimester (Figures 1–4). NT
increases with gestational age between 10 and 14 weeks. A combination of
NT with serum biochemistry between 10 and 14 weeks’ gestation in
several population-based studies demonstrates 79% to 90% sensitivity for
DS fetuses, with a false positive rate of 5%.8,9 Table 3 lists prospective study outcomes on first trimester DS risk assessment.9,13,14
be useful NT measurements must be performed with accuracy and
reproducibility. Errors of tenths of a millimeter can greatly alter
calculated DS risk (Table 4). As a result, the Nuchal Translucency
Quality Review (NTQR) in the U.S. and the Foundation for Maternal Fetal
Medicine (FMFM) have developed strict criteria, specific credentialing,
and quality review for NT measurements (Table 5). In clinical practice,
the largest NT measurement that meets the criteria should be reported to
the laboratories. The crown-rump length window for NT assessment is
between 38 and 84 mm, which correlates between 10 and 13 weeks’
gestation. NT is also a marker of fetal congenital abnormalities,
especially cardiac malformations. Due to the association of congenital
heart disease with an NT greater than 3.4 mm, it is generally accepted
practice to perform a detailed evaluation of the fetal heart in the
Aside from its effectiveness in screening for
DS and other chromosomal abnormalities, first trimester risk assessment
may provide many additional benefits. These include either establishment
or confirmation of precise gestational age, early recognition and
determination of chorionicity in multiple gestations, identification of
many major congenital abnormalities—including cystic hygromas that are
associated with an up-to-50% incidence of chromosomal anomalies.
Additionally, studies show that, similar to abnormal values of second
trimester serum markers, first trimester markers correlate with preterm
delivery, preeclampsia, and fetal growth restriction.
Combining first and second trimester DS risk assessment
most efficient screening test is one with the highest sensitivity
(detection rate) with the lowest screening false positive rate. The most
efficient screening for Down syndrome is achieved when both first
trimester and second trimester markers are combined. The Fully
Integrated Screening model involves measuring NT and PAPP-A between
approximately 10 weeks and 6 days and 13 weeks and 6 days, followed by
serum measurement of AFP, UE3, HCG and Inhibin at approximately 16 to 18
weeks’ gestation. A single risk assessment for DS is given at this
time. Both the SURRUS and FASTER trials reported a 94% to 95% detection
rate in DS with a 5% false-positive rate.8,9 While the
detection rates are quite high, the disadvantage of fully integrated
screening is that it leads only to a second trimester result. Women are
not given results after the first part of their screening. Integrated
screening would deny a patient the opportunity to pursue a first
trimester diagnosis by chorionic villus sampling (CVS) with earlier and
safer terminations if needed.
To provide information to patients
after the first part of their screening and allow them the option to
pursue CVS, the Stepwise Sequential Screening and the Sequential
Contingency Screening methods have been introduced (Figures 5 and 6). In
the Stepwise Sequential Screening method, the patient is given a result
following the first part of her screening (NT plus PAPP-A and hCG). At
that point she has the option of pursuing a diagnostic test if she is at
high risk for Down syndrome. If her initial risk is low, she will
proceed to the second part (AFP, UE3, HCG and Inhibin) at approximately
16 to 18 weeks of pregnancy. At that point, a final overall risk
assessment can be provided. By manipulating cut-offs, Stepwise
Sequential Screening is believed to offer a DS detection rate that is
very similar to the Fully Integrated Screening approach while
maintaining a 5% false positive rate.9
Sequential Contingency Screening method, a patient would be given a risk
after the first part of her screening. The risk would be classified as
high, intermediate, or low. Women at high risk would be given the option
to pursue a diagnostic test. Women at low risk would not need to pursue
any further screening or testing. Only those with an intermediate risk
would have the second trimester blood screening. An overall risk
assessment employing both the first and second trimester screening would
then be provided. The detection rate for DS with the Sequential
Contingency Screening method is considered similar to that of the Fully
Integrated Screening model. The SURUSS trial suggested that only 25% of
patients who pursue Contingency Screening would need the second
trimester part.8 In a meta-analysis,15 detection
rates of > 90% were achieved with contingency screening, with only
15% requiring the second trimester portion. Although this strategy is
already used in many patients, formal Contingency Screening is not
The Genetic Sonogram can also be used to
assess DS risk, and it can be used either sequentially or contingently
alone or in addition to second trimester serum analytes.12
translucency measurements may not be obtainable in 10% to 15% of
patients. In these cases Serum Integrated Screening should be
considered. This option is similar to Fully Integrated Screening but
without the NT measurement. A screening risk assessment is provided only
after the second trimester biochemical markers have been evaluated. The
FASTER trial reported an 87% DS detection rate, which was better than
MSAFP4, with the same 5% false-positive rate.19
to 14 weeks’ gestation, the nasal bone (Figure 7) is reportedly absent
in about 60% to 70% of fetuses with DS, while absent in fewer than 1% of
chromosomally normal fetuses. Significant population variation has
been noted. A nasal bone contingency test has been proposed which
appears to have similar effectiveness for DS detection with the further
advantage of completing screening and diagnostic testing by the end of
the first trimester (Figure 8).16-18
Investigations continue exploring additional ultrasound parameters for DS risk assessment,21 Recent studies have found that blood flow through the fetal Ductus Venosus 19,20, fetal tricuspid regurgitation, and the frontomaxillary facial angles22
are all promising. All of these techniques are unavailable for
large-scale population screening and remain investigational at this
Which is the best test?
combined testing (NT plus serum analytes) gives the earliest results but
leaves approximately 15% of DS cases undetected. The second trimester
quad test and the genetic sonogram are appropriate for late registrants
but leave 20% to 25% of DS cases undetected. The integrated test has the
highest detection rates but leads only to a second trimester result,
which may be unacceptable to many patients. Stepwise Sequential testing
detection rates are very close to the integrated test and allow women
with very abnormal first trimester results to undergo early invasive
testing, but it requires two tests. The contingency test may have
slightly lower detection rates than stepwise sequential testing, but
higher than first trimester combined testing alone, and it allows most
women to complete screening after a single study early in pregnancy.
Again, this testing is not currently clinically available. All studies
that require first and second trimester results require significant
The nasal bone contingency test,
which can be completed in 1 first-trimester visit, appears to have high
detection rates and significantly reduced screen positive rates. This
testing requires performance by an individual with expertise not only in
NT measurement but also in nasal bone assessment. Additionally, the
presence or absence of the fetal nasal bone varies with ethnicity. This
form of screening requires further study.
Some patient factors can
direct the testing performed. Does the patient want the earliest test
or the one that is least likely to falsely label her as being screen
positive? Is her fear of losing a normal pregnancy greater than that of
missing an aneuploidy? A single testing approach does not fit all
It must be emphasized that definite answers rely on a
diagnostic test. All risk assessment strategies are designed only to
determine the risk of a problem, and they will always have less than
100% sensitivity. As long as the only definitive way of diagnosing the
karyotype of a fetus involves an invasive procedure that can cause the
loss of a normal pregnancy, there simply is no substitute for explaining
the options and their downsides to all patients. Patients will in turn
be empowered to make decisions that are best for them and their
pregnancy (Table 6).
- Shin M, Besser LM, Kucik JE, et al. Prevalence of Down syndrome
among children and adolescents in 10 regions of the United States. Pediatrics. 2009;124:1565-1571.
- New Cronos Database: Demographic statistics. Eurostat.
Accessed February 7, 2012.
- Hecht CA, Hook EB. Rates of Down syndrome at live birth by one-year
maternal age intervals in studies with apparent close to complete
ascertainment in populations of European origin: A proposed revised rate
schedule for use in genetic and prenatal screening. Am J Med Genet. 1996;62:376-385.
- Hook, EB. Chromosome abnormalities in older women by maternal age. Am J Med Genet.1990;35:184-187.
- Merkatz IR, Nitowsky HM, Macri JN, et al. An association between low MSAFP and fetal chromosomal abnormalities. Am J Obstet Gynecol. 1984;148:886-894.
- Wald NJ, Cuckle HS, Densem JW, et al. Maternal serum screening for Down’s syndrome in early pregnancy. Br Med J. 1988;297:883-897.
- Wald NJ, Densem JW, George L, et al. Prenatal screening for Down’s syndrome using Inhibin-A as a serum marker. Prenat Diag. 1996;16:143-153.
- Wald NJ, Rodeck C, Hackshaw AK, et al. First and second trimester
antenatal serum screening for Down’s syndrome: The results of the SURUSS
Study. J Med Screen. 2003;10:56-57.
- Malone FD, Canick JA, Ball RH, et al. First- and second-trimester
evaluation of risk (FaSTER) research consortium. First-trimester or
second-trimester screening, or both, for Down’s syndrome. N Eng J Med. 2005;353:2001-2011.
- Nyberg DA, Luthy DA, Resta RG, et al. Age-adjusted risk assessment
for fetal Down’s syndrome during the second trimester: Description of
the method and analysis of 142 cases. Ultrasound Obstet Gynecol. 1998;12:8-14.
- Smith-Bindman R, Hosmer W, Feldstein VA, et al. Second-trimester
ultrasound to detect fetuses with Down syndrome: A meta-analysis. JAMA. 2001;285:1044-1055.
- Bromley B, Lieberman E, Shipp TD, et al. The genetic sonogram: A
method of risk assessment for Down syndrome in the second trimester. J Ultrasound Med. 2002;21:1087-1096.
- Nicolaides KH, Spener K, Avgidou K, et al. Multicenter study of the
first trimester screening for trisomy 21 in 75,821 pregnancies. Ultrasound Obstet Gynecol. 2005;25:221-226.
- Wapner R, Thom E, Simpson JL, et al. First trimester maternal serum
biochemistry and fetal nuchal translucency screening (BUN) study group.
N Engl J Med. 2003;349:1405-1413.
- Cuckle HS, Malone FD, Wright D, et al. Contingent screening for Down syndrome-results from the FaSTER trial. Prenat Diagn. 2008;28:89-94.
- Cicero S, Sonek JD, McKenna DS, et al. Nasal bone hypoplasia in trisomy 21 at 15-22 weeks’ gestation. Ultrasound Obstet Gynecol. 2003;21:15-18.
- Cicero S, Rembouskos G, Vandecruys H, et al. Likelihood ratio for
trisomy 21 in fetuses with absent nasal bone at the 11-14 week scan. Ultrasound Obstet Gynecol. 2004;23:218-223.
- Sonek JD, Cicero S, Neiger R, et al. Nasal bone assessment in prenatal screening for trisomy 21. Am J Obstet Gynecol.. 2006; 195:1219-1230.
- Matias A, Gomes C, Flack N, et al. Screening for chromosomal
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Ultrasound Obstet Gynecol.1998;12:380-384.
- Antolin E, Comas C, Torrents M, et al. The role of ductus venosus
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- Falcon O, Faiola S, Huggon I, et al. Fetal tricuspid regurgitation
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- Sonek J, Borenstein M, Downing C, et. al. Frontomaxillary facial angles in screening for trisomy 21 at 14-23 weeks’ gestation. Am J Obstet Gynecol. 2007;197:160 e1-e5.