Conjoined twins are identical twins who are physically connected. Normally, identical twins form when an embryo splits in two shortly after fertilization. But with conjoined twins, this separation process ends before it is completed, according to the University of Maryland Medical Center.

The embryos of identical twins start to separate around eight to 12 days after forming. It’s thought that if this separation process starts later than this, between 13 and 15 days after fertilization, it results in conjoined twins, according to the Mayo Clinic.

There is also a theory that conjoined twins can result from an egg that first splits completely, but then joins back together, according to Seattle Children’s Hospital.

Conjoined twins occur in about one out of every 200,000 live births, and many do not survive the first day of life, the University of Maryland says. Researchers estimate that 40 to 60 percent of conjoined twins are stillborn

Craniopagus twins are rare, making up only 2 percent of conjoined twin cases, the University of Maryland says. The craniopagus-separation surgery performed on the McDonald twins was the 59th such surgery in the world since 1952, according to CNN.

note: please do take this seriously

source:http://www.newskingz.com/2016/10/24/causes-twins-coinjoined/