Fetus in fetu is a rare developmental abnormality in which a mass of tissue resembling a fetus forms inside the body of its twin.

There are two hypotheses for the origin of a “fetus in fetu”. One hypothesis is that the mass begins as a normal fetus but becomes enveloped inside its twin. The other hypothesis is that the mass is a highly developed teratoma.

The anomaly was first defined in early nineteenth century by Meckel. Despite its prevalence among infants and children, there have been reports of cases in which the anomaly had remained asymptomatic until later ages. This rare congenital anomaly was reported around 100 times since its first definition in the nineteenth century.