First-Trimester Screening for Down Syndrome and Trisomy 18

What is a Screening Test?
Let's start by defining what such a test is not. It is not a diagnostic assessment. It cannot tell a couple that they will or will not have a "normal" baby, and it cannot tell them that the baby they have conceived will or will not have a genetic birth defect. By definition, a screening test classifies a couple by their likelihood of having a baby with a specific condition.

The Best Screening Tests:
-- Are inexpensive
-- Are easy to administer
-- Impose minimal discomfort
-- Are reliable and sensitive
-- Are useful in making decisions and planning for outcomes

We want all screening tests to be "sensitive" and "specific." A test is sensitive when people with the condition actually have a positive test (there are few false negatives). A test is specific if the majority of those without disease come up negative.

The "cutoff," or level above or below which the test will be read as negative or positive, is important. If the cutoff is high, we might miss those who actually have the condition. If it's set too low, we might identify those who actually do not have the problem, wasting valuable medical care resources to prove that they don't have cause for alarm, or treating those who don't need treatment and frightening those who never had to worry in the first place.

What is Down Syndrome?
The condition we now call Down syndrome was first described by Langdon Down in 1866. In this condition, also known as trisomy 21, either parent contributes an extra 21st chromosome and the child may be born with characteristics such as mental retardation, heart abnormalities and some typical physical features such as eyes that may be slanted upward and outward, folds of tissue under the eyes and low-set ears.

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