The antenatal natural history of prenatally detected SCT is not as favorable as that of SCT presenting at birth. Well-defined prognostic factors for SCT diagnosed postnatally, as outlined in the AAPSS classification system, do not necessarily apply to fetal cases (Altman et al., 1974; Bond et al., 1990). While the mortality rate for SCT diagnosed in the newborn is at most 5%, the mortality rate for fetal SCT approaches 50% (Flake et al., 1986; Bond et al., 1990; Flake, 1993; Hedrick et al., 2004).
Most SCTs are histologically benign. The incidence of malignant elements present in fetal SCT has ranged from 7% to 30% (Hedrick et al., 2004; Heerema-McKenny et al., 2005). Malignancy appears to be more common in males, especially with solid versus complex or cystic tumors (Schey et al, 1977). The presence of histologically immature tissue does not necessarily signify malignancy (Carney et al., 1972; Gonzalez-Crussi, 1982). Calcifications occur more often in benign tumors but may also be seen in malignant tumors and are unreliable indicators of malignant potential (Hickey and Layton, 1954; Waldhausen et al., 1963; Grosfeld et al, 1976; Schey et al., 1977; Horger and McCarter, 1979). Although there is one reported case of malignant yolk sac differentiation in a fetal SCT, there has not been a case of metastatic teratoma in a neonate with a prenatally diagnosed SCT (Holzgreve et al., 1985; Flake, 1993).
The prenatal history of SCT is quite different from the postnatal natural history. Flake et al. (1986) reviewed 27 cases of prenatally diagnosed SCT. Five cases were electively terminated and 15 of the remaining 22 died, either in utero or shortly after delivery. The majority of these patients presented between 22 and 34 weeks of gestation with a uterus large for gestational age secondary to severe polyhydramnios. The International Fetal Medicine and Surgery Society reported a mortality rate of 52% among cases of prenatally diagnosed SCT (Bond et al., 1990). When SCT was seen in association with placentomegaly or hydrops, all affected fetuses died in utero. The indication for ultrasound examination was also found to be a predictive factor. If SCT was an incidental finding, the prognosis was favorable at any gestational age. However, if the ultrasound examination was performed for maternal indications, 22 of 32 (69%) fetuses died. In addition, diagnosis prior to 30 weeks was associated with a poor outcome.
Sheth et al. (1988) also reported significant perinatal mortality associated with SCT, with only 6 survivors among 15 cases diagnosed prenatally. Three of four cases associated with hydrops were rapidly fatal. The sole survivor was salvaged by emergency cesarean section at 35 weeks. This series was unusual because three cases had severe obstructive uropathy and secondary renal dysplasia. A more favorable outcome was reported by Gross et al. (1987) in which 8 of 10 fetuses with prenatally diagnosed SCT survived. However, no fetus had hydrops or placentomegaly, and the two nonsurvivors were electively terminated.
Hydrops in SCT is usually, but not always, fatal. Nakoyama et al. (1991) reported survival in two fetuses with SCT presenting with hydrops at 27 and 30 weeks of gestation. In addition, Robertson and Crombleholme (1995) were able to salvage a hydropic fetus at 26 weeks of gestation by staged resection of the SCT in the neonatal period. In this case, acute rapid growth of the SCT led to polyhydramnios and preterm delivery. After delivery, the newborn was noted to be in a high-output state from shunting through the tumor. In a staged resection, the tumor was initially devascularized by ligation of both internal iliac arteries. Twenty-four hours later, the external portion of the mass was resected. The infant subsequently underwent resection of the intrapelvic portion of the tumor at 3 months of age, and did well.
Hedrick et al. (2004) reviewed their experiences with 30 cases of prenatally diagnosed SCT and reported 4 terminations, 5 fetal deaths, 7 neonatal deaths, and only 14 survivors (47%). Among the 26 patients continuing the pregnancy, 81% experienced obstetric complications including polyhydramnios ( n = 7), oligohydramnios (n = 4), preterm labor ( n = 13), pre-eclampsia (n = 4), gestational diabetes (n = 1), HELLP syndrome (n = 1), and hyperemesis (n = 1).
Sonographic features of SCT such as size, AAPSS classification, solid or cystic composition, or presence or absence of calcifications have not been predictive of either fetal survival or future malignant potential (Altman et al., 1974; Flake, 1993). One exception to this may be the unilocular cystic form of SCT, which has a relatively favorable prognosis because of benign histology and limited vascular and metabolic demand (Horger and McCarter, 1979; Mintz et al., 1983). The growth of the SCT in relation to the size of the fetus is also unpredictable and may increase, decrease, or stabilize as gestation proceeds. However, a rapid phase of tumor growth usually precedes the development of placentomegaly and hydrops. Highly vascular lesions are more likely to undergo rapid tumor growth and to be associated with the development of placentomegaly and hydrops. The prenatal mortality, unlike postnatal mortality, is not due to malignant degeneration, but to complications of tumor mass or tumor physiology (Flake et al., 1993). The tumor mass may result in malpresentation or dystocia, which in turn may result in tumor rupture and hemorrhage during delivery. Dystocia has been reported in 6% to 13% of cases in postnatal series (Giugiaro et al., 1977;Musci et al., 1983; Gross et al., 1987). SCTs may also spontaneously rupture in utero leading to significant fetal anemia or death (Sy et al., 2006). The most important benefit of prenatal diagnosis is prevention of dystocia by elective or emergency cesarean section. Tumor mass effect may also result in uterine instability and preterm delivery because of uterine distention (Flake et al., 1986; Bond et al., 1990). Massive polyhydramniosis is frequently seen in large fetal SCTs, which also predisposes to uterine irritability and preterm delivery.
SCT may occur in twins further complicating the prenatal management. In Hedrick et al.'s series, 10% of the cases occurred in twin gestations (Hedrick et al., 2004). The presence of SCT in a twin gestation increases the risk of preterm delivery. Because SCT is associated with an increased risk of fetal death, intrauterine demise of a monochorionic twin with SCT places the surviving unaffected co-twin at risk of adverse neurologic outcome (Ayzen et al., 2006).
The physiologic consequence of fetal SCT depends on the metabolic demands of the tumor, blood flow to the tumor, and the presence and degree of anemia. The features of the SCT—whether cystic or solid, size, and rate of growth—all affect the metabolic demands of fetal SCT. While classically thought to derive its blood supply from the middle sacral artery (Smith et al., 1961), these large tumors often parasitize blood supply from the internal and external iliac systems. This may result in vascular "steal" from the umbilical artery blood flow to the placenta. As an SCT outgrows its blood supply, tumor necrosis may occur leading to tumor rupture and hemorrhage. The high-output cardiac failure in fetal SCT can be diagnosed by fetal echocardiography and Doppler studies (Flake et al., 1986; Langer et al., 1989; Schmidt et al., 1989).When hydrops develops in fetuses with SCT, all have dilated ventricles and dilated inferior venae cavae due to increased venous return from the lower body (Flake, 1993). Serial sonographic examinations in fetal SCT often show progressive increases in combined ventricular output and descending aortic flow velocity. In general, placental blood flow is decreased by the vascular steal by the SCT (Schmidt et al., 1989; Flake, 1993)and may lead to the finding of end-diastolic flow reversals in the umbilical artery.
Benachi et al. (2006) have suggested a prenatal prognostic classification system based on tumor diameter, vascularity, and rapidity of growth. In a group of 44 fetal SCTs divided into group A (tumor <10 cm, absent or mild vascularity and slow growth), group B (tumor >10 cm, pronounced vascularity or high output cardiac failure and rapid growth), and group C (tumor >10 cm, predominantly cystic lesion with absent or mild vascularity and slow growth). Groups A and C did well with gestational age at delivery of 38 and 37 weeks, respectively while group B delivered prematurely at 31 weeks of gestation. There was no mortality in either group A or C but was 52% for group B. The newborns in group B also have a much longer length of stay postnatally (Benachi et al. 2006).Postnatal measurements of umbilical arterial blood gases before and after removal of a large SCT demonstrate that the tumor acts as a large arteriovenous shunt.