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