44 1/25 1/38 1/15 1/24 5. Does the etus have a major anomay? If so, diagnostic testing 1/19 1/30 1/12 1/19 is recommended rather than screening.
Data from Hook EB, Cross PK, Schreinemachers DM: live-born infants, JAMA. 1983 Apr 15;249(15):2034-2038.
ofered aneuploidy screening or diagnostic testing early in pregnancy (American College of Obstetricians and Gynecologists, 2016c). Considerations prior to screening are as follows: 1.
Has the patient elected screening? At least 20 percent of women elect not to receive aneuploidy screening, even when financial barriers are removed. Fewer than 40 percent of women with a positive screening result elect prenatal diagnosis (Dar, 2014; Kuppermann, 2014).
2.
ould the patient preer prenatal diagnosis? Diagnostic testing is safe and efective, and chromosomal microarray
TABLE 14-2. Maternal Age-Related Risk for Down Syndrome and Any Aneuploidy at Midtrimester and at Term in Dizygotic Twin Pregnanciesa
The American College of Obstetricians and Gynecologists (2016c) has airmed that screening for aneuploidy should be an informed patient choice, with an underlying foundation of shared decision making that its her clinical circumstances, values, interests, and goals. Elements of counseling prior to aneuploidy screening are listed in Table 14-3.
Aneuploidy screening can be challenging because the test characteristics of each option may vary with maternal age and with whether the test is analyte-based or cell-free DNA-based. The test sensitivity is the detection rate-that is, the proportion of aneuploid fetuses identiied by the screening test. Its converse, the false-negative rate, is the percentage of cases that the test is expected to miss. A first-trimester screening test with a sensitivity of 80 percent is expected to miss 1 in 5 cases. The sensitivity of screening tests for Down syndrome has increased steadily during the past 30 years, from just 25 percent with serum AFP alone to more than 90 percent with integrated or sequential screening.
Another key characteristic is the false-positive rate, the percentage of unafected pregnancies that will "falsely" screen positive. This approximates 5 percent for first-trimester screening, quadruple-marker screening, or integrated screening options (Baer, 2015; Kazerouni, 2011; Malone, 2005b; Nor 1/160 1/257 1/91 1/126 ton, 2015). The converse of false-posi tive rate is speciici ty percent of unafected pregnancies. lthough test sensitivity 1177 1/118 1/44 1/67 has improved, the false-positive rate has been held constant for many diferent aneuploidy screening tests (Table 14-4).
1/47 1172 1/27 1/44 Both statistics are relevant for counseling. An additional con aRisk applies to one or both fetuses.
Data from Meyers C, Adam R, Dungan J, et al: Aneuploidy in twin gestations: when is maternal age advanced? Obstet
Gynecol. 1997 Feb;89(2):248-251l.
sideration is that with all analyte-based screening tests, women 35 and older have higher rates of positive results (Kazerouni, 2011; Malone, 2005b).