* Abbreviations: CVO: : combined ventricular output DVP: : deepest vertical pocket EXIT: : ex utero intrapartum therapy FTC: : Fetal Treatment Center MCA: : middle cerebral artery MRI: : magnetic resonance imaging NTD: : neural tube defect PPROM: : preterm premature rupture of membranes RFA: : radiofrequency ablation SCT: : sacrococcygeal teratoma TFR: : tumor volume–to–fetal weight ratio UCSF: : University of California, San Francisco A large sacrococcygeal teratoma (SCT) was detected on routine 18-week fetal ultrasonography in a 26-year-old, gravida 2, para 0 woman. She had a history of Helicobacter pylori gastric infection and an ovarian cyst, in addition to a previous spontaneous abortion at 15 weeks’ gestation. This pregnancy had been uncomplicated and prenatal laboratory testing was unremarkable. She was evaluated at an outside fetal care center at 21 weeks’ gestation. At that time, the fetal SCT measured 5.9×5.3 cm of mixed solid and cystic components, about 30% to 50% of the fetal size. Given the size of the SCT, with interval growth from initial diagnosis, she was referred to the University of California, San Francisco (UCSF) Fetal Treatment Center (FTC) to discuss the options for fetal surgical intervention. The woman was seen at the UCSF FTC at 23 weeks’ gestation. Imaging at that time included fetal ultrasonography and fetal echocardiography. Ultrasonography (Fig 1) demonstrated a largely exterior SCT with a small, presacral component. The tumor was largely solid with small cystic components and classified as an SCT type 1 lesion. The dimensions were 11.6×10.5×9.6 cm with an estimated volume of 611 mL, about 87% of the fetal size. There was no evidence of polyhydramnios, with the deepest vertical pocket (DVP) at 7.7 cm. Fetal echocardiography showed normal cardiac anatomy with significant dilation of the inferior vena cava and mild cardiomegaly with a cardiothoracic ratio of 0.36. The combined ventricular output (CVO) was significantly elevated at 817 mL/kg per minute (upper limit of normal is 500 mL/kg per minute). Diastolic flow velocity in the umbilical artery (systolic-to-diastolic velocity ratio of 6.2) was low, possibly secondary to a “steal phenomenon” to the tumor. The middle cerebral artery (MCA) diastolic flow velocity was also low, suggestive of cerebral vasodilation to maintain cerebral blood flow (brain-sparing). …