For more than a decade after its introduction, serum aneuploidy screening was intended for women younger than 35, because it simply did not have suicient sensitivity to be ofered to women who had higher a priori risk. This is also no longer the case. And, because the prevalence of fetal aneuploidy rises sharply with maternal age, the positive-predictive value of all aneuploidy screening tests-whether analyte-based or cellfree DNA tests-is higher in women aged 35 years or older. Women 35 and older now make up more than 15 percent of deliveries in the United States (Fig. 14-1). At Parkland Hospital, this age group accounts for half of births with Down syndrome (Hussamy, 2017).
Aneuploidy is the presence of one or more extra chromosomes, usually resulting in trisomy, or loss of a chromosomemonosomy. Data from population-based registries that include births, fetal deaths, and pregnancy terminations indicate an overall prevalence of 4 such abnormalities per 1000 births (Wellesley, 2012). Aneuploidy accounts for more than 50 percent of first-trimester abortions, about 20 percent of second-trimester losses, and 6 to 8 percent of stillbirths and early-childhood deaths (Reddy, 2012; Stevenson, 2004; Wou, 2016). Of recognized pregnancies with chromosomal abnormalities, trisomy 21 composes approximately half of cases; trisomy 18 accounts for 15 percent; trisomy 13, for 5 percent; and the sex chromosomal abnormalities-45,X, 47,XX, 47,Y, and 47,XY-for approximately 12 percent (Wellesley, 2012).
The risk for fetal trisomy increases with maternal age, particularly after age 35 (Fig. 13-2, p. 255). When counseling, a provider includes specific maternal-age-related aneuploidy risks (Tables 14-1 and 14-2). Other important fetal aneuploidy risk factors include a numerical chromosomal abnormality or structural chromosomal rearrangement in the woman or her partner-such as balanced translocation-or a prior pregnancy with autosomal trisomy or triploidy.
Broadly speaking, there are two types of aneuploidy screening tests, those that are traditional or analyte-based and those that are cell-free DNA-based. All pregnant women should be
TABLE 14-1. Maternal Age-Related Risk for Down Syndrome and Any Aneuploidy at Midtrimester and at Term in Singleton Pregnancies analysis provides information about genetic conditions that screening tests and karyotyping alone currently do not (American College of Obstetricians and Gynecologists, 2016b). This is discussed further on page 291 and in Chapter 13 (p. 271).
3. Is this a multetal gestation? All traditional (analyte-based) aneuploidy screening tests are significantly less efective in multifetal gestations, and cell-free DNA screening is not cur rently recommended with multifetal gestations (p. 285).
37 1/149 1/227 1/83 1/1l30 4. What method will be used or neural-tube deect screening?
1/89 1/137 1/53 1/81 that does not include second-trimester serum analytes, 41 1/53 1/81 1131 1/50 arately, either with MSAFP assessment or with sonography 42 1/41 1/64 1/25 1/39 (American College of Obstetricians and Gynecologists, 43 1/31 1/50 1/19 1/30 2016c).