Abstract
At some point during pregnancy women are typically encouraged to undergo a screening test in order to estimate the likelihood of fetal chromosomal aberrations. While timelines vary, the majority of pregnant women are screened within their first trimester (De Graaf et al., 2002). In the event of a positive test result, an invasive diagnostic assessment is usually recommended, namely amniocentesis or chorionic villus sampling (CVS). The combined test, widely considered to be the most feasible and effective screening procedure, involves an integrated assessment of: maternal age, fetal Nuchal Translucency (NT), maternal serum pregnancy-associated plasma protein A (PAPP-A), and free β human chorionic gonadotropin (β-hCG). This assay is most reliable when performed nearest to the 11th week of gestation (Malone et al., 2005), at which its detection rate and false positive rate for trisomy 21, in optimal conditions, are approximately 95 and 5%, respectively (Nicolaides, 2004). A variety of competing screening techniques are available in the first trimester, and though we focus on the combined test in our example below, the point raised in this article applies to each of them.
Highlights
A first-trimester screening assay carrying a relatively low false-positive rate might seem a reasonable option for women already considered to be at low risk—the vast majority of the pregnant population
Following such prenatal screening for trisomy 21, most women who test positive for high risk proceed with invasive diagnostic testing
The follow-up diagnostic tests are associated with serious procedure-related health risks, including a ∼1% increased chance of miscarriage
Summary
A first-trimester screening assay carrying a relatively low false-positive rate might seem a reasonable option for women already considered to be at low risk—the vast majority of the pregnant population. Following such prenatal screening for trisomy 21, most women who test positive for high risk proceed with invasive diagnostic testing.
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