Although it is an exceptionally rare condition that occurs in approximately 1 in 500,000 pregnancies, the discovery of partially developed embryos inside living human beings is not completely unheard of. In May of 2008, doctors in Greece surgically removed an embryo from a 9-year-old girl's abdomen and in 1999, a 36-year-old Indian man's embryonic twin had grown so large inside his abdominal cavity that it was nearly as large as a full-term baby. Fetus in fetu embryos do not develop as normal pregnancies, but remain as partially-developed fetuses which can become parasitic tumors.ABC News:
Bearing Your Own Twin (May 16, 2008)
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