
Disclaimer
The practice guidelines of the National Society of Genetic
Counselors (NSGC) are developed by members of the NSGC
to assist genetic counselors and other health care providers in
making decisions about appropriate management of genetic
concerns; including access to and/or delivery of services. Each
practice guideline focuses on a clinical or practice-based issue,
and is the result of a review and analysis of current profes-
sional literature believed to be reliable. As such, information
and recommendations within the NSGC practice guidelines
reflect the current scientific and clinical knowledge at the time
of publication, are only current as of their publication date,
and subject to change without notice as advances emerge.
In addition, variations in practice, which take into account
the needs of the individual patient and the resources and
limitations unique to the institution or type of practice, may
warrant approaches, treatments, and/or procedures that differ
from the recommendations outlined in this guideline. There-
fore, these recommendations should not be construed as dic-
tating an exclusive course of management, nor does the use of
such recommendations guarantee a particular outcome. Ge-
netic counseling practice guidelines are never intended to
displace a health care provider’s best medical judgment based
on the clinical circumstances of a particular patient or patient
population. Practice guidelines are published by NSGC for
education and informational purposes only, and NSGC does
not “approve” or “endorse” any specific methods, practices, or
sources of information.
Background
The landscape of prenatal screening changed in 2007 with
the release of two ACOG Practice Bull etins stating all
women should be offered maternal serum screening (MSS)
and diagnostic testing regardless of maternal age, and that
healthcare provide rs should determine which screening
options would best serve their patients (ACOG Practice
Bulletin No. 77 & 88, 2007). ACOG’s assertion was subse-
quently echoed by the American College of Medical Genet-
ics (ACMG), leaving providers to reassess their screening
and diagnostic testing practices (Driscoll and Gro ss 2008).
Prenatal screening strategies have evolved greatly over the
years. Various combinations of firs t and second trimester
maternal serum analytes and fetal ultrasound findings have
been proposed as part of an ongoing quest to create a screen-
ing test with the highest detection and lowest false positive
rates. Recently, NIPT has been made commercially available
as an alternative option for chromosome aneuploidy screen-
ing. Screening options for chromosome aneuploidy are non-
invasive, which may make them attractive options for patients
who desired more individualized risk assessment information
prior to making a decision about whether or not to undergo
invasive diagnostic testing. The primary limitation of screen-
ing is that it does not provide a definitive diagnosis, leading to
the potential of increased anxiety in women with an unaffect-
ed pregnancy and the potential of false reassurance in women
who have a pregnancy with a chromosome aneuploidy. An-
other limitation of screening is the variability in the detection
rates, false positive rates, screening cut-offs, and anatomical
ultrasound markers included in the screen based on the par-
ticular laboratory and/or provider involved. In addition, detec-
tion rates for screening in multiple gestations are generally
decreased from those of singletons (Wald and Rish 2005). For
a more comprehensive review of the advantages and limita-
tions of the types of screening, refer to the individual sections
below and to Tables 1, 2,and3. A decision tree to assist
providers in selecting a screening method that is most suitable
for their practice is presented in Fig. 1.
In addition to the various prenatal screening options, diag-
nostic testing for chromosomal abnormalities (Table 4)is
available. Chorionic villus sampling, or CVS, is typically
performed between 10-13w6d of gestation, and involves a
transcervical or transabdominal aspiration of chorionic villi
from the developing placenta (Wapner 2005). Another option
for diagnostic testing is amniocentesis. Amniocentesis is tra-
ditionally performed after 15 weeks of gestation by trans-
abdominal removal of amniotic fluid (CDC 1995). Samples
obtained through CVS and amniocentesis are typically used
for chromosomal analysis by karyotyping, but may also be
used for rapid interphase fluorescence in situ hybridization
(FISH) to screen for chromosome aneuploidy, metaphase
FISH to evaluate for specific microdeletions or microduplica-
tions, chromosomal microarray, or other molecular testing.
A referral for genetic counseling has traditionally been
made for patients who have an increased risk for chromosome
aneuploidy, including those with a positive maternal serum
screen result, positive family history, a fetal anomaly identi-
fied on ultrasound, or those who are 35 years of age or older at
delivery. However, as screening options have expanded, it has
become more routine to refer patients for genetic counseling
when they are having difficulty deciding on a course of action
for screening or diagnostic testing. Appropriate screening and
diagnostic testing options are typically presented to patients in
the context of a prenatal genetic counseling session (CDC
1995). Genetic counselors are uniquely trained to explain
complex information in an understandable format to patients
and to facilitate the informed decision-making process.
First Trimester Screening Options
The first trimester analyt es pregnancy associa ted plasma
protein-A (PAPP-A) and free beta-human chorionic gonad-
otropin (β-hCG) may be measured between 9w0d-13w6d of
gestation, while the nuchal translucency (NT) measurement
NSGC Practice Guideline 5