Fetus in fetu

Fetus in fetu describes an extremely rare abnormality that involves a fetus getting trapped inside of its twin. It continues to survive as a parasite even past birth until it grows so large that it starts to harm the host, at which point doctors usually intervene. Invariably the parasitic fetus is anencephalic (without a brain) and lacks internal organs, and as such is unable to survive on its own.

The condition causes the host to look like they are pregnant (since they technically are) and can occur in both males and females.