[citation needed] In normal monozygotic twin development, one egg is fertilized by a single sperm.

If the egg splits in the early blastocyst stage, two inner cell masses will be present, eventually leading to the twins sharing the same chorion and placenta, but with separate amnions.

[2] This is thought to be the most likely reason why conjoined twins occur,[2] and could possibly play a role in the development of craniopagus parasiticus.

[citation needed] One hypothesis is that craniopagus parasiticus starts with the development of two fetuses from a single zygote that fail to separate at the head region around the second week of gestation.

[1] Another is that it occurs later in development, around the fourth week of gestation, at which time the two embryos fuse together near the anterior open neuropore.

[3] In addition to craniopagus parasiticus, a few other conditions involve a child's, or animal's, birth with two heads or faces.

[6] Only four cases have been documented by modern medicine to have survived birth:[6] In the past, the use of terminology when describing parasitic twins has been somewhat inconsistent.