Approximately 80 percent of pregnancies with Patau syndrome result from trisomy 13. The remainder is caused by a robertsonian translocation involving chromosome 13. The most frequent structural chromosomal rearrangement is a translocation between chromosomes 13 and 14, der(l3; 14) (q 1 O;q 10). This translocation is carried by approximately 1 in 1300 individuals, although the risk of an afected liveborn neonate is less than 2 percent (Nussbaum, 2007).
FIGURE 13-5 Trisomy 18-Edwards Syndrome. A. This transventricular sonographic view shows fetal choroid plexus cysts and an angulated "strawberry-shaped" skull. B. Radial clubhand is manifested as a single forearm bone (radius), with the hands in a fixed, hyperflexed position at right angles to the forearms. C. This three-dimensional (3-�) sonographic image shows the characteristic hand position of clenched fists with overlapping digits. D. 3-D sonographic image displays a rockerbottom foot.
Trisomy 13 is associated with abnormalities of virtually every organ system. One characteristic finding is holoprosencephaly (Fig. 10-15, p. 195). his is present in approximately two thirds of cases and may be accompanied by microcephaly, hypotelorism, and nasal abnormalities that range from a single nostril to a proboscis. Cardiac defects are found in up to 90 percent of fetuses with trisomy 13 (Shipp, 2002). Other abnormalities that suggest trisomy 13 include neural-tube defectsparticularly cephalocele, microphthalmia, cleft lip-palate, omphalocele, cystic renal dysplasia, polydactyly, rockerbottom feet, and areas of skin aplasia (Lin, 2007). For the fetus or newborn with a cephalocele, cystic kidneys, and polydactyly, the diferential diagnosis includes trisomy 13 and the autosomalrecessive Meckel-Gruber syndrome. Sonographic images of several of these are shown in Chapter 10 (p. 192).
Few fetuses with trisomy 13 survive until birth. Of those that do, the I-week survival rate approximates 40 percent, and I-year survival rate is only about 3 percent (Tennant, 2010; Vendola, 2010). Counseling regarding prenatal diagnosis and management options is similar to that described for trisomy 18.
Unlike other aneuploidies, fetal trisomy 13 confers risk to the pregnant woman. Hyperplacentosis and preeclampsia develop in up to half of pregnancies with trisomy 13 carried beyond the second trimester (Tuohy, 1992). Chromosome 13 contains the gene for soluble fms-like tyrosine kinase-1 (sFlt-l), which is an anti angiogenic protein associated with preeclampsia (Chap. 40, p. 716). Investigators have documented overexpression of the slt-1 protein by trisomic 13 placentas and in serum of women with preeclampsia (Bdolah, 2006; Silasi, 2011).
Other Trisomies. In the absence of mosaicism, discussed later (p. 263), other autosomal trisomies rarely yield a liveborn neonate. Case of live births with trisomy 9 and with trisomy 22 have been noted (Kannan, 2009; Tinkle, 2003). Trisomy 16 is the most common trisomy found with first-trimester losses and accounts for 16 percent of these losses. However, it is not identiied later in gestation. Trisomy 1 has never been reported.