Trisomy 13 presents as constellation of congenital anomalies that result from the presence of an extra chromosome 13, either whole or translocated onto another chromosome. It is not known how the presence of the extra chromosome disrupts so many different systems during organogenesis.

Although the clinical findings of the condition were previously known, the association of the extra chromosome with the clinical syndrome was not described until 1960 (Patau et al., 1960). Synonyms for trisomy 13 include trisomy D or Patau syndrome.

Trisomy 13 is the third most common liveborn autosomal aneuploidy. The incidence of trisomy 13 has been variously reported as 1 in 2206 to 1 in 7602 livebirths (Taylor, 1968). Most authors give the approximate incidence figure of 1 in 5000 livebirths (Wladimiroff et al., 1989). The incidence of trisomy 13 is equal among all races. It is thought that there are equal numbers of conceptuses of both genders. However, a slight excess of females exists at birth presumably due to a survival advantage (Jacobs et al., 1987).

Fetuses with trisomy 13 are more active in utero than comparable fetuses with trisomy 18. In addition, fetuses with trisomy 13 have a higher frequency of major congenital anomalies than fetuses with trisomy 21. The most common major abnormality seen in trisomy 13 is holoprosencephaly, which can be seen as early as 12 weeks of gestation (Figure 129-1). Other important findings associated with trisomy 13 include an abnormal midface with hypotelorism, cleft lip or palate, and even cyclopia (see Chapter 14) (Figure 129-2). Many studies have shown that there is a greatly increased incidence of congenital heart disease in fetuses affected with trisomy 13 (Wladimiroff et al., 1995). The most common cardiac defects include ventricular septal defect (VSD), hypoplastic left ventricle, or double outlet right ventricle. Other common sonographic findings include ulnar/fibular polydactyly and echogenic or polycystic kidneys. In one study, all fetuses with trisomy 13 had one or more sonographic abnormalities or abnormal measurements present (Seoud et al., 1994).

Benacerraf et al. (1988, 1992, 1994) developed a sonographic scoring index to identify fetuses with aneuploidy. She and her co-authors gave 2 points for nuchal thickening of greater than 6 mm, 2 points for demonstration of a major structural defect, and 1 point each for a short femur, short humerus, and the presence of renal pyelectasis. This scoring system identified 2 of 2 fetuses with trisomy 13. In another report, Benacerraf et al. (1986) demonstrated that 6 of 6 affected fetuses with trisomy 13 had holoprosencephaly and a severely malformed face.

In the largest study of sonographic abnormalities in 33 consecutive fetuses with trisomy 13, Lehman et al. (1995) demonstrated that 91% of affected fetuses had one or more sonographically detectable abnormalities. In this study, 18 of the 33 fetuses described were at less than 20 weeks of gestation, and 15 of the affected fetuses were at greater than 20 weeks of gestation. The mean gestational age for the fetuses in the study was 20.7 weeks. Thirty of the 33 fetuses had structural abnormalities, as documented in Table 129-1. Only 3 of the fetuses had no sonographically detectable abnormalities. The most common abnormalities observed were holoprosencephaly, other central nervous system abnormalities, facial abnormalities, and cardiac defects (Table 129-1).

An increased nuchal translucency measurement for gestational age is also associated with trisomy 13 (Pandya et al., 1994). In the Pandya study, 80% of fetuses with trisomy 13,18, or 21 had a nuchal translucency of at least 3 mm. Omphalocele is also somewhat frequent in fetuses with trisomy 13. In the presence of an omphalocele, the risk of trisomy of 13 or 18 is increased by 340-fold (Snijders et al., 1995).

First trimester sonographic findings in fetuses with trisomy 13 include increased nuchal translucency measurement, early onset fetal growth restriction, fetal tachycardia (2/3 of cases), holoprosencephaly, megacystis and omphalocele (Snijders et al., 1999; Nicolaides, 2004). Nonspecific sonographic markers seen in trisomy 13 include mild dilatation of the lateral cerebral ventricles, echogenic bowel, and echogenic intracardiac foci (EIF). The combination of EIF and a hypoplastic left heart is characteristic of trisomy 13 (Nyberg and Souter, 2001).

In summary, fetuses with holoprosencephaly, cardiac abnormalities, evidence of facial clefting or midface anomalies, and possibly polydactyly should strongly suggest a diagnosis of trisomy 13 (Twining and Zuccollo, 1993).