Sacrococcygeal teratoma (SCT) is the most common congenital germ cell tumor, with an incidence of 1 in 35,000-40,000 live births  and a female predominance (3:1-4:1 ratio). [1, 2] The tumor arises from embryologically multipotent cells from the Hensen node, which is located in the coccyx. [3, 4] Most SCTs are now diagnosed prenatally because of the widespread use of routine obstetric ultrasonography. 
Patients in whom SCT is diagnosed postnatally typically do well after early surgical resection, and the main cause of mortality in these patients (though rare) is attributed to malignancy. However, mortality associated with prenatally diagnosed SCT is in the range of 30-50% [6, 7, 8] and is attributed to tumor morphology and vascularity. Whereas some fetuses are born without complications, others can develop high-output cardiac failure, nonimmune hydrops fetalis and, ultimately, fetal demise.
This wide disease spectrum has prompted several fetal treatment centers to identify ultrasonographic predictors of survival for fetuses with SCT to help identify high-risk fetuses who may benefit from fetal intervention. The key to optimizing survival in these fetuses is intervention before the development of high-output cardiac failure, hydrops, and maternal mirror syndrome. Identifying fetuses at risk for hydrops and fetal demise isolates those who may be salvaged by reversing the pathophysiology—the premise behind fetal intervention. 
The vascular supply to an SCT commonly arises from the middle sacral artery, which can enlarge to the size of the common iliac artery and cause a vascular steal syndrome.  These large vascular tumors can lead to high-output cardiac failure as a consequence of arteriovenous shunting through the tumor, resulting in placentomegaly, hydrops, and, ultimately, fetal demise. 
Polyhydramnios is commonly seen because of increased fetal cardiac output, which often leads to preterm labor and premature rupture of membranes. Conversely, oligohydramnios can also occur if an intrapelvic portion of the tumor causes significant urinary obstruction. 
In severe cases, maternal mirror syndrome, in which the mother develops symptoms that mimic those of the hydropic fetus, may develop. Mothers develop symptoms similar to those of severe preeclampsia, such as hypertension, emesis, peripheral edema, pulmonary edema, and proteinuria. 
Fetal surgery is contraindicated after maternal mirror syndrome has developed; accordingly, prognostic indicators have been characterized do as to identify patients before terminal progression of this disease. Mothers who potentially have maternal mirror syndrome need to be very closely monitored and may require delivery or pregnancy termination for maternal safety.
SCTs are categorized according to the classification developed by the American Academy of Pediatrics, Surgical Section (AAPSS), as follows  :
Type I - Primarily external or has a minimal presacral component
Type II - Predominantly external but has a significant intrapelvic component
Type III - Predominantly intrapelvic with abdominal extension, with a small external component
Type IV - Entirely within the pelvis and abdomen
Whereas type I tumors, being primarily external to the fetus, are easily diagnosed prenatally and are amenable to fetal resection, type IV tumors can be difficult to diagnose and are not amenable to fetal resection. [5, 12] The AAPSS classification describes surgical anatomy and identifies tumors that are amenable to fetal resection, but it does not provide prognostic information,  nor does it identify fetuses who would benefit from fetal intervention.
Fetuses with SCT are considered for fetal resection or fetal intervention only in extreme cases on an individual basis.
Small tumors without significant vascularity are unlikely to affect the fetus significantly.  These fetuses are unlikely to develop high-output cardiac failure or hydrops and can be monitored throughout gestation with serial ultrasonography. Those with signs of placentomegaly and hydrops after lung maturity (usually after 32 weeks’ gestation) are delivered emergently. Only fetuses of less than 32 weeks’ gestation with signs of impending hydrops that have tumors amenable to surgical resection are considered for fetal intervention. [6, 12]
As with all invasive procedures, the risks and benefits of fetal intervention must be considered for each patient. However, consideration for the risk to and safety of the pregnant mother are unique to fetal surgery. Before fetal intervention is considered, a multidisciplinary team should counsel and evaluate each family. The evaluation should include the following  :
Detailed ultrasonography to confirm the diagnosis and to detect any other anatomic abnormalities
Ultrafast fetal magnetic resonance imaging (MRI) for additional anatomic information
Fetal echocardiography to rule out congenital heart disease and to assess fetal cardiac function
Amniocentesis for fetal karyotyping
In 2009, Wilson et al proposed the following criteria for surgical resection of sacrococcygeal teratoma  :
No maternal contraindications to fetal surgery (medical or surgical issues, body mass index [BMI] < 36, anesthesia risks)
Fetal gestational age 20-30 weeks
A favorable AAPSS stage and no additional anomalies
Impending hydrops (evidence of ascites, pleural effusion, and subcutaneous edema)
Normal fetal karyotype
Fetal cardiac output greater than 600-900 mL/kg/min (adjusted for gestational age)
Contraindications to fetal intervention for SCT include the following  :
Significant placentomegaly (placental thickness at cord insertion >35-45 mm with a gestational age < 30 weeks)
Maternal mirror syndrome
Other fetal anatomic abnormalities
Several institutions have reported outcomes with and without fetal intervention for prenatally diagnosed SCT. [14, 15, 16, 17, 18] Among patients with prenatally diagnosed SCT, 36-41% require fetal intervention. [15, 16]
Mortality due to SCT is mainly attributed to tumor morphology; small cystic SCTs rarely cause problems in utero. Rapid growth of large vascular tumors can rupture and hemorrhage during delivery, which is usually fatal. 
The overall survival rate of prenatally diagnosed SCT is 47-83%, [14, 16, 17] but the survival rate after fetal surgery is 50-75%. [14, 15, 16] It is important to note that survival after fetal intervention should be compared to survival for the subgroup of patients with hydrops and no intervention, in whom the survival rate approaches 0%. [19, 20]
About 40-50% of survivors with prenatally diagnosed SCT have long-term morbidity, which may include obstructive uropathy, bowel and bladder incontinence caused by damage to the sacral nerves due to the tumor or damage during SCT resection, and dissatisfaction with cosmetic outcomes. [14, 18]
Placentomegaly and hydrops are harbingers of fetal demise in SCT. 
A retrospective review of 17 fetuses with prenatally diagnosed SCT treated at the University of California, San Francisco (UCSF), between 1986 and 1998 evaluated the factors associated with hydrops.  There was a significant difference in tumor morphology (solid vs cystic) and vascularity in fetuses who developed hydrops compared with those without hydrops. In addition, fetuses who developed hydrops were diagnosed at an earlier gestational age (19 vs 25 weeks) and were delivered at an earlier gestational age (28 vs 38 weeks).
In this series, 12 fetuses developed hydrops, four of whom survived.  Of the four survivors, three underwent fetal intervention because they developed hydrops before viability, and one patient developed hydrops at 32 weeks’ gestation and was delivered immediately. All fetuses who did not develop hydrops survived.
This study showed that fetuses with predominantly solid and highly vascular tumors were at high risk for developing hydrops. These patients should undergo close follow-up throughout gestation with serial ultrasonography and echocardiography and may be considered for fetal intervention upon signs of impending hydrops.
A retrospective review of 23 patients evaluated at the Children’s Hospital of Philadelphia (CHOP) between 2003 and 2006 with prenatally diagnosed SCT showed that SCTs with a growth rate exceeding 150 cm3 per week are associated with increased perinatal mortality. 
In a study from UCSF that retrospectively reviewed 28 fetuses with prenatally diagnosed SCT between 1991 and 2005, solid tumor volume–to–head volume ratio (STV/HV) was identified as an ultrasonographic predictor of poor outcomes.  All patients with an STV/HV lower than 1 survived, whereas 61% of fetuses with an STV/HV higher than 1 died.
In addition, the study determined that 97.3% of fetuses with an STV/HV higher than 1 were associated with one or more abnormal ultrasonographic findings, such as polyhydramnios, hepatomegaly, placentomegaly, cardiomegaly, ascites, pericardial effusion, or integumentary edema.  With serial ultrasonography, increases in the STV/HV ratio can guide management in fetal SCT so that fetal intervention or early delivery  can be performed before hydrops develops.
In a study from the Fetal Center at Texas Children’s Hospital, tumor volume–to–fetal weight ratio (TFR) was a marker of poor outcome in 12 fetuses with SCT between 2004 and 2009.  With MRI or ultrasonography, tumor volume was determined by the prolate ellipsoid formula and fetal weight by the Hadlock formula. A TFR higher than 0.12 before 24 weeks’ gestation predicted poor outcomes (fetal hydrops, demise, or neonatal death) with 100% sensitivity, 83% specificity, a negative predictive value of 100%, and a positive predictive value of 80%.
Of the 12 fetuses with SCT in this series, 33% (4/12) developed hydrops.  All fetuses who developed hydrops had a TFR higher than 0.12 by 24 weeks’ gestation, and three fetuses died. One patient underwent fetal intervention after hydrops developed and survived. Thus, TFR may be used to identify fetuses with SCT who are at risk for poor outcomes before 24 weeks’ gestation and who may benefit from fetal intervention.
In a review of 79 fetuses with prenatally diagnosed SCT at three fetal centers from 1986 to 2011, receiver operating characteristic (ROC) analysis revealed that a TFR higher than 0.12 before 24 weeks' gestation was predictive of a poor prognosis, as was solid tumor morphology and the presence of hydrops.  However, none of these factors were found to be independent predictors of a poor prognosis on multivariate analysis.
In a retrospective review of 28 pathology-confirmed isolated SCT patients evaluated with at least two documented ultrasound scans and followed through hospital discharge between 2005 and 2012, a faster SCT growth rate — calculated as the difference between tumor volumes on a late-gestation sonogram and an early-gestation sonogram divided by the difference in time —was associated with adverse outcomes ( death, high-output cardiac failure, hydrops, and preterm delivery). 
SCT can create a low-resistance large arteriovenous shunt, which can progressively increase preload and afterload on the fetal heart, leading to volume overload, ventricular dilation, ventricular hypertrophy, and high-output cardiac failure.  A 10-year retrospective review of seven fetuses showed that the most important prognostic criteria for maternal and fetal complications due to prenatally diagnosed SCT included cardiomegaly, hydrops, and increased preload indexes of the fetal venous system. 
Studies on fetuses with SCT show that combined cardiac output increases dramatically before the development of hydrops. [27, 28] Fetuses with combined cardiac output that exceeded 700-800 mL/kg/min died in utero.  Rychik et al analyzed the acute cardiovascular effects of fetal surgery in four patients with SCT and saw a significant decrease in combined cardiac output (690 ±181 mL/kg/min vs 252 ±82 mL/kg/min) after fetal resection of SCT. 
In a retrospective review of 11 fetuses with SCT, those with poor outcomes (ie hydrops, fetal demise, neonatal death) had a cardiothoracic ratio higher than 0.5, a combined ventricular output exceeding 550 mL/kg/min, tricuspid or mitral valve regurgitation, or a mitral valve Z-score higher than 2.  Identifying these cardiovascular indicators of poor outcome helps identify patients at high risk for fetal demise and can prompt fetal surgical intervention before the development of hydrops.
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