Sacrococcygeal teratomas are rare tumors that develop at the base of the spine by the tailbone (coccyx) known as the sacrococcygeal region. Although most of these tumors are non-cancerous (benign), they may grow quite large and once diagnosed, always require surgical removal. It is likely that all sacrococcygeal teratomas are present at birth (congenital) and most are discovered before birth by a routine prenatal ultrasound examination or an exam indicated for a uterus too large for dates. In rare cases, sacrococcygeal teratomas may be cancerous (malignant) at birth and many will become malignant if surgical resection is not performed. In extremely rare cases, sacrococcygeal tumors may be seen in adults. Most of these represent slow growing tumors that originated prenatally. In the majority of these cases, the tumor is benign, but may cause lower back pain and genitourinary and gastrointestinal symptoms. The cause of sacrococcygeal teratomas is unknown.