Twins physically joined in utero
Conjoined twins, popularly referred to as Siamese twins, are twins joined in utero. It is a very rare phenomenon, estimated to occur in anywhere between one in 49,000 births to one in 189,000 births, with a somewhat higher incidence in southwest Asia and Africa. Approximately half are stillborn, and an additional one-third die within 24 hours. Most live births are female, with a ratio of 3:1. Wikipedia