Trisomy l8-Edwards Syndrome. he association between this constellation of abnormalities and an autosomal trisomy was irst described by Edwards (1960). In population-based series, prevalence of trisomy 18 approximates 1 in 2000 recognized pregnancies-including abortuses, stillbirths, and live births, and approximately 1 in 6600 liveborn neonates (Loane, 2013; Parker, 2010). he diference in prevalence is explained by the high in-utero lethality of the condition and the termination of many afected pregnancies. Perhaps not surprisingly, survival of liveborn neonates is likewise bleak. .10re than half die within the irst week, and the I-year survival rate approximates only 2 percent (Tennant, 2010; Vendola, 2010). The syndrome is three-to fourfold more common in females (Lin, 2006; Rosa, 2011) . Unlike Down and Patau syndromes, which involve acrocentric chromosomes and thus may stem from a robertsonian translocation, Edwards syndrome uncommonly results from a chromosomal rearrangement.
Virtually every organ system can be afected by trisomy 18. Common major anomalies include heart defects in more than 90 percent-particularly ventricular septal defects, as well as cerebellar vermian agenesis, myelomeningocele, diaphragmatic hernia, omphalocele, imperforate anus, and renal anomalies such as horseshoe kidney (Lin, 2006; Rosa, 2011; Yeo, 2003).
Sonographic images of several of these are shown in Chapter 10.
Cranial and extremity abnormalities are also frequent and include a prominent occiput, posteriorly rotated and malformed ears, micrognathia, clenched hands with overlapping digits, radial aplasia with hyperflexion of the wrists, and rockerbottom or clubbed feet (Fig. 13-5). A "strawberry-shaped" cranium is noted in approximately 40 percent of cases, abnormally wide cavum septum pellucidum in more than 90 percent, and choroid plexus cysts in up to 50 percent (Abele, 2013; Yeo, 2003). Importantly, isolated choroid plexus cysts are not associated with trisomy 18. These cysts only raise the risk for trisomy 18 if fetal structural abnormalities or an abnormal aneuploidy screening test result is also present (Reddy, 2014).
Pregnancies with trisomy 18 that reach the third trimester often develop fetal-growth restriction, and the mean birthweight is <2500 g (Lin, 2006; Rosa, 201l). Because abnormal fetal heart rate tracings are common during labor, mode of delivery and management of heart rate abnormalities should be discussed in advance. In older reports, more than half of undiagnosed fetuses underwent cesarean delivery for "fetal distress" (Schneider, 1981).
Trisomy l3-Patau Syndrome. This constellation of fetal abnormalities and their association with an autosomal trisomy was irst described by Patau and colleagues (1960). The prevalence of trisomy 13 approximates 1 in 12,000 live births and 1 in 5000 recognized pregnancies, which includes abortuses and stillbirths (Loane, 2013; Parker, 2010). As with trisomy 18, trisomy 13 is highly lethal, and most afected fetuses are lost or terminated.