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Why is Down syndrome not always correctly detected in pregnancy tests?
Wiki User
∙ 14y ago
Pregnancy screenings for Down syndrome have a 5-8% false positive rate (test comes back positive, but the baby does not have Down syndrome) and a 35-40% false negative rate (test is negative, but baby has Down syndrome). This is due to a number of different factors that the screening depends on, such as the mother's age and weight, the age of the fetus, etc.
Since the tests results come back as risk factors- the chance that the baby will have Down syndrome, for example 1 out of 270- and there is usually an arbitrary cutoff, such as 1 in 250, where the test counts as a "positive", this is why sometimes it is not correct. The "positive" or "negative" depends on chance, and obviously a high chance the fetus has Down syndrome does not necessarily mean it does have it.
Wiki User
∙ 14y ago
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