Prenatal Screening

Prenatal screening estimates the likelihood of a fetal condition. It does not establish that the condition is present. That distinction determines the next clinical action. A screening result may be positive, negative, or nonreportable, but none of those labels alone replaces diagnostic testing. Chorionic villus sampling (CVS) and amniocentesis provide diagnostic chromosome testing, although both are invasive procedures with procedural risks that require informed consent. Screen performance is described using several related terms: - Sensitivity is the proportion of affected pregnancies detected. - Specificity is the proportion of unaffected pregnancies correctly classified as negative. - False-positive rate is the proportion of unaffected pregnancies that screen positive. - Positive predictive value (PPV) is the probability that a positive result represents an affected pregnancy in that particular population. PPV is shaped by prevalence. A positive result for a relatively uncommon condition may have a much lower PPV than a positive result for trisomy 21, even when the same laboratory test is used. Maternal age changes the baseline prevalence of some aneuploidies; it does not make cfDNA technically more accurate in an older...