First-Trimester Screening for Down Syndrome and Trisomy 18

Researchers learned that when another marker, PAPP-A (found in lower than normal amounts in a Down syndrome pregnancy), was performed on the mother's blood at the same time, sensitivity increased dramatically. One study, with close to 100,000 participants, demonstrated that Down syndrome as well as trisomy 13 and 18 could be detected accurately in approximately 81 percent with about an 8 percent rate of false-positive results when nuchal translucency was used alone. When combined with the blood test for PAPP-A, detection rates as high as 90 percent were seen. False positives fell to 5 percent.

Nuchal translucency screening is currently offered predominantly in metropolitan areas under the care of a specialist in high-risk pregnancies. It requires a specialized ultrasound scan and is performed during a small window of time, generally in the 11th to 13th week of pregnancy.

Advantages of Screening
Although amniocentesis continues to be the gold standard in fetal genetic screening, as chromosomes can be retrieved and directly analyzed, integrated serum screening has the advantage of safety while maintaining an acceptable degree of accuracy for most women and couples. About 1 in 200 pregnancies are lost as a result of amnio, but there are no direct fetal risks associated with nuchal translucency combined with PAPP-A at 11 to 13 weeks. Amniocentesis can then be reserved for those with positive screens.

Why Test At All?
Clearly, genetic risk screening is not for everyone and should always be presented as an option and never a requirement for care. For some women, it's important to know all that is possible to know about their baby. Nuchal translucency and integrated screening should be reserved for those with risk factors for chromosomal defects (for example, a previous affected baby, a woman who is age 35 or greater, familial or ethnic risks). A consultation with a genetic counselor and/or perinatologist can help delineate such risks and provide screening recommendations.

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