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Article #226: Who Has Down Syndrome Babies?

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Most people associate having a child with "false-positives" (test states the
Down syndrome with older women. While it patient has the condition when the
is true that women over 35 do have an patient really doesn't) and
increased risk of having a child with "false-negatives" (patient has the
Down syndrome, 80% of these children are condition but the test states he/she
born to those women under age doesn't).
thirty-five. Maternal Serum Screening
In the United States, approximately 5,000 The mother's blood is checked for three
babies with Down syndrome are born every items: alpha-fetoprotein (AFP),
year. A woman's chance of having another unconjugated estriol (uE3) and human
baby with Down syndrome is approximately chorionic gonadotropin (hCG). These three
1 in 100. are independent measurements, and when
Prenatal Screening for Down Syndrome taken along with the maternal age
Over the last 10 years, new technology (discussed below), can calculate the risk
has improved the methods of detection of of having a baby with Down syndrome.
Down syndrome. While there are ways to A very important consideration in the
diagnose Down syndrome by obtaining fetal screening test is the age of the fetus
tissue samples by amniocentesis or (gestational age). The correct analysis
chorionic villus sampling, it would not of the different components depends on
be appropriate to examine every pregnancy knowing the gestational age precisely.
this way. Besides greatly increasing the The best way to determine that is by
cost of medical care, these methods do ultrasound.
carry a slight amount of risk to the Test results are sometimes reported to
fetus. doctors as "Multiples of the Median
So screening tests have been developed to (MoM)." The "average" value is therefore
try to identify those pregnancies at called 1.0 MoM. Down syndrome pregnancies
"high risk." These pregnancies are then have lower levels of AFP and estriol, so
candidates for further diagnostic their levels would be less than 1.0
testing. MOM.hCG in a Down syndrome pregnancy
Screening Vs Diagnostic Test would be greater than 1.0 MoM.
What is the difference between a Finally, the calculated risk is used to
screening test and a diagnostic test? In modify the risk already statistically
diagnostic tests, a positive result very calculated based on the mother's age. We
likely means the patient has the disease already know that as the mother's age
or condition of concern. In screening advances, the risk of having a baby with
tests, the goal is to estimate the risk Down syndrome increases.
of the patient having the disease or For example: Let's say the test results
condition. come back in the typical range for a
Diagnostic tests tend to be more pregnancy not associated with Down
expensive and require an elaborate syndrome (that would be 1.0 MoM for all
procedure; screening tests are quick and components). This result reduces the
easy to do. However, screening tests have woman's risk of having a child with Down
more chances of being wrong: there are syndrome four-fold.






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