Fetus in fetu, literally "fetus within fetus", is a rare condition arising, it is thought, from gestational maldevelopment. The term describes the parasitic implantation of one fetus within another.
It is difficult to conceive of how this might occur, but it likely involves a twin gestation within a single gestational sac wherein one embryo envelopes the other and the second embryo continues to develop for a time. Chromosomal analysis of the fetus in fetu shows an identical genetic makeup to the host fetus, thus lending support to the conjoined twin theory. Fetus in fetu is considered a form of the Terata Anacatadidyma (conjunction in the mid-portion of the body) subtype of conjoined twins, as the second fetus is connected to the first by its umbilical cord.
As one might imagine, the second fetus has almost always died by the time of birth, and the baby is left with a calcified tumor, often mistaken for a teratoma - another strange mass. The actual development of organs and a spinal cord differentiates a true case of fetus in fetu from a teratoma.
To date, there are less than 100 cases of fetus in fetu known to medical science by report in the medical literature. There are, however, known cases where the second fetus survived until the birth of the first, and was still living at the time of surgical removal - the usual necessary treatment for the condition. It is thought that there may be the potential of malignant recurrence or spread without surgical removal, but this may be due to teratomas misdiagnosed as fetus in fetu.
The usual area of implantation is on the retroperitoneum within the abdomen - often with the superior mesenteric artery providing the blood supply. Cases have been reported including implatation to the intestine, the liver, and the peritoneum. There has been at least one reported case of fetus in fetu occuring in the skull of the baby implanted (apparently) to the brain, and another within the scrotum.
Synonyms: Cryptodidymus, double monster, endocyme fetus, fetiform teratoma, fetal inclusion, included heteropagus twin, suppressed twin