Foetus in foetu, stillborn conjoined twins, live-born conjoined twins
Foetus in foetu, or a parasitic twin, occurs approximately 7% of the time in cases of pregnancies with conjoined twins. Conjoined twins are formed when an egg about to split into identical twins does not fully separate, and foetus in foetu is simply a more extreme case of that. Unlike other cases of conjoined twins, the “secondary” body does not have a brain, heart, or any independent life, and is dependent upon the fully-formed twin to sustain it.
The conjoined twins shown here appear to be thoraco-omphalopagus (upper) and omphalopagus (or xiphopagus). Prior to modern Caesarian-sections, the life of the mother was almost always lost in cases of conjoined twins. However, when the twins were not in need of serious life-support measures (such as in most omphalopagus and xiphopagus twins), they were sometimes able to be kept alive by wet nurses. There are illustrated accounts of full-grown conjoined twins going all the way back to the 15th century, and they likely existed in some very limited capacity prior to that time.
Monstrorum historia memorabilis. Johann Schenk [Ioanne Schenckio], 1609.