First Trimester
Diagnostic Testing
Any woman at any age can have a baby with a chromosome
abnormality, but the risk for these conditions increases with
age. Traditionally diagnostic testing was only available for
pregnant women 35 years of age or older. The American
College of Obstetricians and Gynecologists now recommends
that diagnostic testing should be available to all pregnant women
regardless of age.
Chorionic villus sampling (CVS)
CVS testing is performed between 10-14 weeks of pregnancy.
It involves the removal of a small amount of placental tissue for
chromosome analysis. 99.9% of chromosome abnormalities
can be detected with this test. CVS carries an increased risk
for pregnancy complications including miscarriage. The risk of
complication from this procedure is 0.5-1% (1/200-1/100).
1st Trim. Screen Fetal Cell Free DNA
Timing
Today, women of all ages have many options for screening or testing
for fetal chromosome abnormalities. Some women want earlier
screening, some prefer to go directly to diagnostic testing, and some
choose to neither screen nor test. These decisions can be complex
and overwhelming. Genetic counselors are available to help families
understand their risks and options as they choose the course of action
most appropriate for them. If you have questions about screening or
diagnostic tests in pregnancy, please call the MUSC Health Connection
at 792-1414 or 1-800-424-MUSC.
10 wks & greater
Detect Down
Syndrome85%
False positive
3 - 5%
Additional test
information
Trisomy 13 & 18
10 - 14 wks
99.1% 99.99%
Risk
Blood draw
Blood draw
< 1%
0.5 - 1%
(1/200 - 1/100)
risk pregnancy loss
MUSC
Women’s Care
Very rare
Trisomy 13, 18 All chromosomes
& presence of Y
are evaluated
Second Trimester
Amniocentesis
Amniocentesis is performed at 15 weeks or later in pregnancy.
It involves the removal of a small amount of amniotic fluid.
Chromosome analysis is performed on the baby’s skin cells
which can be found in the amniotic fluid. 99.9% of chromosome
abnormalities can be detected with this test. The amniotic fluid is
also tested for AFP levels which can be an indicator for open neural
tube defects as well as some genetic conditions. Amniocentesis
carries an increased risk for pregnancy complications including
miscarriage. The risk of complication from this procedure is 0.250.5% (1/400-1/200).
10 -14 wks
CVS
2nd Trim.
Quad Screen
Timing
15 - 22 wks
Fetal Cell
Free DNA
Amniocentesis
15 wks & greater 15 wks & greater
Detect Down
Syndrome80% 99.1% 99.99%
Risk
Blood draw
Blood draw
False positive
3 - 5%
Additional test
information
Spina bifida
Trisomy 18
< 1%
0.25 - 0.5%
(1/400 - 1/200 )
risk pregnancy loss
Very rare
Trisomy 13, 18 All chromosomes
& presence of Y
evaluated
& Spina bifida
If you are scheduled to visit with a genetic
counselor please complete a pre-appointment
questionnaire at MUSChealth.com/genetichistory.
Prenatal Screening and
Diagnosis
First Trimester Screening
Combined Screening
Ultrasound
Although most babies are born
healthy, the risk of having a child with
a birth defect concerns many parents.
Risks for certain conditions, such
as chromosome abnormalities (an
example is Down Syndrome) and open
neural tube defects, can be assessed
by a variety of prenatal screening and
testing options.
The first trimester screen is performed between 11-14 weeks of
pregnancy and screens for Down Syndrome (trisomy 21), trisomy
18 and trisomy 13 (a chromosome disorder similar to trisomy
18). Testing involves an ultrasound and blood test (finger stick).
Ultrasound alone can never diagnose or exclude fetal
chromosome abnormalities such as Down Syndrome (trisomy 21)
and trisomies 18 and 13. However, a detailed ultrasound at 20
weeks can be used to detect some birth defects and soft markers
associated with fetal chromosome abnormalities. Markers
are fetal features that increase the risk of a fetal chromosome
abnormality, but can also be variations of normal. Therefore,
the detection of a marker does not necessarily mean that a baby
has a chromosome abnormality. It means that the baby has an
increased risk of having a chromosome abnormality and more
definitive testing will be offered.
In 1985, maternal serum alpha-fetoprotein (AFP) screening for
open neural tube defects became a standard of second trimester
obstetric care. Since that time, additional chemical markers
have been added to this testing in the second trimester, enabling
screening for chromosome abnormalities like Down Syndrome
and trisomy 18, a condition resulting in severe mental retardation,
physical birth defects, and usually death shortly after delivery.
The levels of the proteins combined with the NT measurement
and the mother’s age are used to provide a risk assessment for
Down Syndrome and trisomies 18 and 13. The first trimester
combined screen detects approximately 85% of pregnancies with
Down Syndrome and 95% of pregnancies with trisomies 18 and
13.
In 2004, first trimester screening for Down Syndrome and trisomy
18 emerged, providing an earlier screening option. Most recently
(2011), fetal cell-free DNA testing became available for patients
with a known increased risk, although in the future this test may
be available for all pregnant women.
The information provided by screening/diagnostic tests is
beneficial for some families; other families do not feel this
information is helpful to have prior to delivery. This brochure is
intended to help you explore these options but participation in
screening or diagnostic tests is your choice. Your obstetric health
care provider or a genetic counselor can also provide you with
more information.
•
Ultrasound: measures an area of fluid at the back of the baby’s neck (the nuchal translucency or NT). This pocket of fluid is a normal finding. Increased NT measurements may indicate increased risk for chromosome abnormalities, heart defects, and other genetic syndromes.
• Blood test: measures the levels of two proteins, free Beta-
hCG and PAPP-A, which are found in the blood of pregnant women.
Second Trimester Screening
The second trimester screen is performed between 15-22 weeks
of pregnancy and screens for open neural tube defects, Down
Syndrome (trisomy 21), and trisomy 18. This testing involves the
measurement of four substances found in the blood of pregnant
women, AFP, hCG, estriol, and inhibin A.
The results of the blood work are combined with the mother’s age
to provide a risk assessment for Down Syndrome, trisomy 18 and
open neural tube defects. The second trimester screen detects
approximately 80% of open neural tube defects, 80% of
pregnancies with Down Syndrome, and 60% of pregnancies with
trisomy 18.
A normal screening test does not guarantee
a normal baby, nor do abnormal test results
definitely mean a baby with a birth defect.
Women with an abnormal result from either of these tests will be
offered more definitive testing.
Fetal Cell-Free DNA Testing
Fetal cell-free DNA from the placenta can be found in the mother’s
blood during her pregnancy. Recently, testing has become
available to measure the amount of specific chromosomal DNA
from a sample of the mother’s blood. Because pregnancies with
Down Syndrome (trisomy 21), trisomy 18 and trisomy 13 have too
much chromosome material, these conditions can be detected by
this method. The testing can be performed at 10 weeks or later
in pregnancy. Cell free DNA testing can detect at least 99% of
pregnancies with Down Syndrome (trisomy 21) and trisomy 18
and at least 91% of pregnancies with trisomy 13.
This blood test is currently only available for women who meet
one or more of the following criteria:
• 35 years or older at the time of delivery
• A previous pregnancy with a chromosome abnormality
• An abnormal screening test result
• Ultrasound findings suggestive of a chromosome abnormality