page_title: Craniopagus twins
section_title: Gestation and embryology
hierarchical_section_title: Craniopagus twins / Gestation and embryology
context_page_description: Craniopagus twins are conjoined twins that are fused at the cranium. Conjoined twins occur in about 10–20 babies in every million births in the United States. Among this small group, cephalic conjoining, or craniopagus twinning, represents the rarest of congenital abnormalities, accounting for 2–6% of all conjoined twins. Additionally, conjoined twins are genetically identical and always share the same sex. The union in craniopagus twins may occur on any portion of the cranium, but does not include either the face or the foramen magnum. The thorax and abdomen are separate and each twin has its own umbilicus and umbilical cord.
  The union may involve the entire diameter of the head or only a small portion. This suggests that although many different kinds of vulnerabilities are already known in the scientific community, there are an infinite number of variations that can occur. Most of these variations are based on the rotation of one twin's skull to the other and the different phenotype sub-groups of craniopagus twins are based on all these rotational conformations.
context_section_description: The exact nature of how conjoined twins develop inutero remains unclear. Embryologists have traditionally attributed identical twinning as "splitting or fission" of either the inner cell mass of pleuripotential cells or early embryonic disc at 13–14 days of gestation just before the primitive streak. Some theorists suggested that conjoined twins develop as a result of the failed fusion of a single fertilized ovum. However a new hypothesis suggests that cranial fusion occurs between two separate embryos before the end of the 4th week of gestation. This can happen because the cranial neuropore is still open, which is responsible for the ultimate fusion and formation of the brain stem and central nervous system. Furthermore, this secondary fusion of embryonic discs could implicate that intact skin will not fuse to other intact skin, including the ectoderm of the embryo. This means that two embryonic discs could only unite in locations where the ectoderm is absent. Moreover, the fusion occurs from neural folds of two separate, dorsally oriented embryonic discs, and the union can occur only after the ectoderm is disrupted to allow the neural and surface ectodermal layers to separate from each other. 
  The union in craniopagus twins may happen at any portion of the calvarium. The juncture can involve either the entire diameter of the head or any portion of the head and can be positioned at a multitude of rotational angles. In fact, craniopagus twins are rarely found in a symmetrical union. Apart from this, the vertebral axes may have a straight line. Despite this, the angle of the vertebrae is the ultimate dictator in how the individuals heads actually face. The majority of twins face either the same way or the exact opposite direction.
  Many reviews suggest a practical four-category system that breaks down the craniopagus twins on the basis of vertical or angular configuration or on the basis if there were shared dural venous sinuses. This scheme was applied to 64 cases and has adequately described sets of twins for over the last 86 years.
caption: Dural venous sinus
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