Fetus in fetu is a rare congenital condition. It refers to presence of one twin in the body of the other twin. The fetus inside the body of the twin is often malformed. The fetus is often located in the retroperitoneal region. In few cases, the parasitic fetus may be present in the scrotum, sacrum, head, mouth, or the vertebral column. Reference: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2810823/ Image via: https://www.sciencedirect.com/science/article/pii/S2213576615000238