FIRST
What is First Trimester Screening?
First trimester screening combines information from a
nuchal translucency measurement (an ultrasound measurement),
biochemical analysis of free beta hCG and PAPP-A,
and maternal age to calculate a risk for Down syndrome
and Trisomy 18.
What is "screening"?
Most tests are done to identify the presence
or absence of a condition; these are called diagnostic
tests. Other tests, called screening tests, are not intended
to diagnose a condition, but are used instead to identify
individuals at an increased risk for a specific condition.
Screening tests are often used during pregnancy to estimate
the risk of certain birth defects. If the risk is high,
a woman can then choose to have diagnostic testing. Screening
tests typically do not pose a risk to the pregnancy,
while diagnostic testing, through amniocentesis or CVS,
carries a risk for complications, including miscarriage.
How do you screen for Down syndrome?
In the past, the risk of having a child with
Down syndrome was based only on the age of the mother
-- the older the mother, the higher the chance of having
a child with Down syndrome. For example, a woman at the
age of 25 has a risk of about 1 in 1250, but a woman
at the age of 40 has a higher risk - about 1 in 94.
It has been found that the levels of certain
chemicals in the mother's blood can be used along with
the mother's age to better predict the risk that a child
will have Down syndrome. This has led to a blood test
at 15-22 weeks of pregnancy called maternal serum screening
(also known as the triple or quad screen). This screening
has been used in clinical practice for more than 20 years.
More recently, other chemicals have been used at 11-13
weeks of pregnancy to evaluate the risk of Down syndrome
earlier in pregnancy. A specific ultrasound measurement
can also be used to evaluate the risk of Down syndrome.
In early pregnancy, babies with Down syndrome will often
have an increased space at the back of the neck. The
ultrasound measurement of this area is called the nuchal
translucency.
Who performs the ultrasound?
Accurate measurement of the space behind the
baby's neck is not a standard ultrasound measurement
currently done in doctor's offices. This risk prediction
is very dependent on precise measurement of this space,
and the most accurate results are obtained when the ultrasound
in performed by individuals trained in this specific
measurement.
Who can have first trimester screening?
Most pregnant women between 11 and 13 weeks
gestation.
How is first trimester screening
performed?
1. A small amount of blood is obtained by either
by a blood draw or a finger prick to measure the levels
of PAPP-A and free beta-hCG.
2. An ultrasound evaluation to date the pregnancy and measure the nuchal translucency
is performed.
3. The mother's blood sample and ultrasound data is sent to the laboratory
for analysis. The results calculate the risk for Down syndrome and Trisomy
18.
What do the results of the first
trimester screen mean?
It is important to remember that this is a screening
test and does not provide a diagnosis; instead it calculates
the chance the baby could have a problem. A result of “screen
negative” or “low risk” indicates that
the measurements of nuchal translucency, PAPP-A and free
beta-hCG were within normal range and that the chance
of the baby having Down syndrome or trisomy 18 is low.
A result of “screen positive” or “high
risk” indicates that the measurements were not
within normal range and that the chance of the baby having
Down syndrome or trisomy 18 is increased.
If the results show an increased
risk, what additional testing will be needed?
If the first trimester screen indicates an increased
risk for either Down syndrome or trisomy 18, you will
be offered diagnostic testing to determine if the baby
has one of these conditions. The following options are
available based on your gestational age:
10-12 weeks: Diagnostic
testing by chorionic villus sampling (CVS)
15 weeks or later: Diagnostic testing by amniocentesis
How accurate is first trimester screening?
Current available data indicates that combining the nuchal
translucency measurement with analysis of free beta-hCG
and PAPP-A along with maternal age can identify 90%
of pregnancies with Down syndrome.
Is second trimester multiple marker
screening necessary after first trimester screening?
First trimester screening cannot predict the
presence of spina bifida and other neural tube defects.
Second trimester alpha-fetoprotein (AFP) screening and/or
an ultrasound are necessary to evaluate the risk of the
baby having this condition.
