<unk>  are conjoined twins that are fused at the cranium. The union may occur on any portion of the cranium, but does not primarily involve either the face or the foramen magnum; their brains are usually separate, but they may share some brain tissue. Conjoined twins are genetically identical and always share the same sex. The thorax and abdomen are separate and each twin has its own umbilicus and umbilical cord. The condition is extremely rare, with an incidence of approximately 1 in 2.5 million live births. An estimated 50<unk>  born around the world every year as of 2021, with only 15 twins surviving beyond the first 30 days of life. Relatively few<unk>  survive the perinatal period; approximately 40% of conjoined twins are stillborn and an additional 33% die within the immediate perinatal period, usually from organ abnormalities and failure. However, 25% of<unk>  survive and may be considered for a surgical separation; several such attempts occur yearly worldwide. Advances in neuroimaging, neuroanesthesia, and neurosurgery have proven that a successful outcome is possible. Among all conjoined twins,<unk>  account for a mere 2% to 6%.
Craniopagus twins
Monoamniotic twins
Conjoined twins
Monochorionic twins
Vanishing twin
Vestigial twin
Anencephaly
Locked twins