Sacrococcygeal teratoma (SCT) is being more often detected due to availability of prenatal ultrasonography. Type IV SCT could be misdiagnosed as cloacal abnormalities due to the pelvic midline cystic mass associated with renal malformations and obstructive uropathy during the pregnancy. We discuss difficulties in the prenatal differential diagnosis of SCT and urogenital sinus in a 26-year-old pregnant woman, admitted to our prenatal diagnosis centre for a detailed US for a pre-sacral mass.