ReachMDIn Extremely Rare Case, Doctors Remove Fetus from Brain of 1-Year-Old

The mass in the young child’s head was a “malformed monochorionic diamniotic twin,” which means that the fetuses shared the same placenta but had separate amniotic sacs in the womb. The phenomenon of one fetus becoming enveloped by another is known as “fetus in fetu” (“parasitic twin”), and it occurs in approximately 1 in 500,000 live births. The malformed fetus usually appears as a mass in the abdomen of the other fetus, wedged behind the abdominal wall tissues. In this case, however, the mass appeared in the “host” fetus’s head and most likely arose very early in development.