Fetus in fetu

Fetus in fetu is a rare developmental anomaly in which a malformed twin is enclosed within the body of its more developed twin, usually in the abdomen.

Parasitic twin: A twin that depends on its more developed co-twin because it lacks the organs or circulation needed to survive independently. Fetus in fetu is an exceptionally organized form of parasitic twinning.
Retroperitoneum: The anatomical space behind the abdominal lining that contains the kidneys, pancreas, and major vessels. Most reported fetus-in-fetu masses occur in this space.
George William Young: A British surgeon who published an early account of a fetus-in-fetu case in 1808. His report is often cited among the earliest published descriptions of the anomaly.
Teratoma: A germ-cell tumor that can contain mature or immature tissues from multiple embryonic tissue layers. A teratoma may resemble fetus in fetu but usually lacks an organized body axis.
Conjoined twins: Identical twins physically connected because their embryonic separation was incomplete. Unlike conjoined twins, a fetus in fetu is enclosed within its twin's body.
Prenatal ultrasonography: Medical imaging that uses ultrasound to examine a fetus during pregnancy. Imaging may reveal an enclosed mass before birth, sometimes with recognizable skeletal parts.
Johann Friedrich Meckel: A German anatomist who studied congenital malformations and established principles of comparative anatomy. Meckel helped shape early medical discussion of malformed and enclosed fetuses.
Twin reversed arterial perfusion sequence: A complication of monochorionic twin pregnancy in which one twin's circulation perfuses an acardiac, malformed co-twin. Both involve a malformed twin, but this sequence occurs through placental blood-flow reversal rather than enclosure.
Embryonic development: The process by which an embryo forms, grows, and differentiates into tissues and organs. The anomaly arises during early development, though its precise mechanism remains debated.
Computed tomography: An imaging method that uses X-rays and computer processing to create cross-sectional views of the body. CT can show the mass's bones and spatial relation to surrounding organs.

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