Why a sacrococcygeal teratoma (SCT) occurs is not known, but it could be tissue from a twin that did not form correctly, or growth from abnormally placed germ cells, or stem cells.
This rare tumour at the base of the tailbone is usually discovered around week 16 of pregnancy. It also affects more girls than boys.
While they can be as large as the baby itself, SCTs are not malignant. Their severity depends on size and amount of blood flow. SCTs that are largely cystic, that is, filled with fluid, generally do not cause a problem. But those made up mostly of solid tissue that is fed by blood can strain the infant's heart.
Some types of tumours may intrude into the pelvis and affect other organs such as the intestines, or the nerves and muscles in the pelvis that govern bowel and bladder functions.
The tailbone is usually removed along with the tumour once the baby's condition has stabilised to prevent it re-growing. The long-term outcome is usually good if no problems have developed before and during birth. Close monitoring is needed for three years.